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Table of Contents

1. Elements

1.1. Element Input Format

All physical elements are defined by statements of the form

label:keyword, attribute,..., attribute

where

label

Is the name to be given to the element (in the example QF), it is an identifier see Identifiers or Labels.

keyword

Is a keyword see Identifiers or Labels, it is an element type keyword (in the example QUADRUPOLE),

attribute

normally has the form

attribute-name=attribute-value
attribute-name

selects the attribute from the list defined for the element type keyword (in the example L and K1). It must be an identifier see Identifiers or Labels.

attribute-value

gives it a value see Command Attribute Types (in the example 1.8 and 0.015832).

Omitted attributes are assigned a default value, normally zero.

Example:

QF: QUADRUPOLE, L=1.8, K1=0.015832;

1.2. Common Attributes for all Elements

The following attributes are allowed on all elements:

TYPE

A string value see String Attributes. It specifies an "engineering type" and can be used for element selection.

APERTURE

A string value see String Attributes which describes the element aperture. All but the last attribute of the aperture have units of meter, the last one is optional and is a positive real number. Possible choices are

  • APERTURE="SQUARE(a,f)" has a square shape of width and height a,

  • APERTURE="RECTANGLE(a,b,f)" has a rectangular shape of width a and height b,

  • APERTURE="CIRCLE(d,f)" has a circular shape of diameter d,

  • APERTURE="ELLIPSE(a,b,f)" has an elliptical shape of major a and minor b.

    The option SQUARE(a,f) is equivalent to RECTANGLE(a,a,f) and CIRCLE(d,f) is equivalent to ELLIPSE(d,d,f). The size of the exit aperture is scaled by a factor f. For f < 1 the exit aperture is smaller than the entrance aperture, for f = 1 they are the same and for f > 1 the exit aperture is bigger.

    Dipoles have GAP and HGAP which define an aperture and hence do not recognise APERTURE. The aperture of the dipoles has rectangular shape of height GAP and width HGAP. In longitudinal direction it is bent such that its center coincides with the circular segment of the reference particle when ignoring fringe fields. Between the beginning of the fringe field and the entrance face and between the exit face and the end of the exit fringe field the rectangular shape has width and height that are twice of what they are inside the dipole.

    Default aperture for all other elements is a circle of 1.0m.

L

The length of the element (default: 0m).

ELEMEDGE

The edge of an element is specified in s coordinates in meters. This edge corresponds to the origin of the local coordinate system and is the physical start of the element. (Note that in general the fields will extend in front of this position.) The physical end of the element is determined by ELEMEDGE and its physical length. (Note again that in general the fields will extend past the physical end of the element.)

X

X-component of the position of the element relative to the position of the first beamline which it is part of and which uses absolute positioning.

Y

Y-component of the position of the element relative to the position of the first beamline which it is part of and which uses absolute positioning.

Z

Z-component of the position of the element relative to the position of the first beamline which it is part of and which uses absolute positioning.

THETA

Angle of rotation of the element about the y-axis relative to the orientation of the first beamline which it is part of and which uses absolute positioning.

PHI

Angle of rotation of the element about the x-axis relative to the orientation of the first beamline which it is part of and which uses absolute positioning.

PSI

Angle of rotation of the element about the z-axis relative to the orientation of the first beamline which it is part of and which uses absolute positioning.

DX

Error on x-component of position of element. Doesn’t affect the design trajectory.

DY

Error on y-component of position of element. Doesn’t affect the design trajectory.

DZ

Error on z-component of position of element. Doesn’t affect the design trajectory.

DTHETA

Error on angle THETA. Doesn’t affect the design trajectory.

DPHI

Error on angle PHI. Doesn’t affect the design trajectory.

DPSI

Error on angle PSI. Doesn’t affect the design trajectory.

WAKEF

Attach wakefield that was defined using the WAKE command.

PARTICLEMATTERINTERACTION

Attach a handler for particle-matter interaction, see Chapter Particle Matter Interaction.

OUTFN

The file name into which the element should write the collected data. The user must only provide the output name without the extension. The extension will be set according to the OPTION statements. If this attribute is empty, the file will be named as the element label.

DELETEONTRANSVERSEEXIT

Particles that exit elements on their sides get deleted in OPAL-t. The user can control this behavior by setting this attribute. If its value is TRUE then particles that exit on the sides are deleted (default), with FALSE they are kept.

All elements can have arbitrary additional attributes which are defined in the respective section.

1.3. Drift Spaces

label:DRIFT, TYPE=string, APERTURE=string, L=real;

A DRIFT space has no additional attributes. Examples:

DR1:DRIFT, L=1.5;
DR2:DRIFT, L=DR1->L, TYPE=DRF;

The length of DR2 will always be equal to the length of DR1. The reference system for a drift space is a Cartesian coordinate system. This is a restricted feature of OPAL-t. In OPAL-t drifts are implicitly given, if no field is present.

1.4. Bending Magnets

Bending magnets refer to dipole fields that bend particle trajectories. Currently OPAL supports the following different bend elements: RBEND, (valid in OPAL-t, see RBend (OPAL-t)), SBEND (valid in OPAL-t, see SBend (OPAL-t)), RBEND3D, (valid in OPAL-t, see RBend3D (OPAL-t)) and SBEND3D (valid in OPAL-cycl, see SBend3D (OPAL-cycl)).

Describing a bending magnet can be somewhat complicated as there can be many parameters to consider: bend angle, bend radius, entrance and exit angles etc. Therefore we have divided this section into several parts:

  1. RBend (OPAL-t) and SBend (OPAL-t) describe the geometry and attributes of the OPAL-t bend elements RBEND and SBEND.

  2. RBend and SBend Examples (OPAL-t) describes how to implement an RBEND or SBEND in an OPAL-t simulation.

  3. SBend3D (OPAL-cycl) is self contained. It describes how to implement an SBEND3D element in an OPAL-cycl simulation.

Figure 1 illustrates a general rectangular bend (RBEND) with a positive bend angle \alpha. The entrance edge angle, E_{1}, is positive in this example. An RBEND has parallel entrance and exit pole faces, so the exit angle, E_{2}, is uniquely determined by the bend angle, \alpha, and E_{1} (E_{2}=\alpha - E_{1}). For a positively charge particle, the magnetic field is directed out of the page.

rbend
Figure 1. Illustration of a general rectangular bend (RBEND) with a positive bend angle \alpha.

1.4.1. RBend (OPAL-t)

An RBEND is a rectangular bending magnet. The key property of an RBEND is that it has parallel pole faces. Figure 1 shows an RBEND with a positive bend angle and a positive entrance edge angle.

L

Physical length of magnet (meters, see Figure 1).

GAP

Full vertical gap of the magnet (meters).

HAPERT

Non-bend plane aperture of the magnet (meters). (Defaults to one half the bend radius.)

ANGLE

Bend angle (radians). Field amplitude of bend will be adjusted to achieve this angle. (Note that for an RBEND, the bend angle must be less than \frac{\pi}{2} + E1, where E1 is the entrance edge angle.)

K0

Field amplitude in y direction (Tesla). If the ANGLE attribute is set, K0 is ignored.

K0S

Field amplitude in x direction (Tesla). If the ANGLE attribute is set, K0S is ignored.

K1

Field gradient index of the magnet, K_1=-\frac{R}{B_{y}}\frac{\partial B_y}{\partial x}, where R is the bend radius as defined in Figure 1. Not supported in OPAL-t any more. Superimpose a Quadrupole instead.

E1

Entrance edge angle (radians). Figure 1 shows the definition of a positive entrance edge angle. (Note that the exit edge angle is fixed in an RBEND element to \mathrm{E2} = \mathrm{ANGLE} - \mathrm{E1}).

DESIGNENERGY

Energy of the reference particle (MeV). The reference particle travels approximately the path shown in Figure 1.

FMAPFN

Name of the field map for the magnet. Currently maps of type 1DProfile1 can be used. The default option for this attribute is FMAPN = 1DPROFILE1-DEFAULT see_Default Field Map (OPAL-t). The field map is used to describe the fringe fields of the magnet see 1DProfile1.

1.4.2. RBend3D (OPAL-t)

An RBEND3D3D is a rectangular bending magnet. The key property of an RBEND3D is that it has parallel pole faces. Figure 1 shows an RBEND3D with a positive bend angle and a positive entrance edge angle.

L

Physical length of magnet (meters, see Figure 1).

GAP

Full vertical gap of the magnet (meters).

HAPERT

Non-bend plane aperture of the magnet (meters). (Defaults to one half the bend radius.)

ANGLE

Bend angle (radians). Field amplitude of bend will be adjusted to achieve this angle. (Note that for an RBEND3D, the bend angle must be less than \frac{\pi}{2} + E1, where E1 is the entrance edge angle.)

K0

Field amplitude in y direction (Tesla). If the ANGLE attribute is set, K0 is ignored.

K0S

Field amplitude in x direction (Tesla). If the ANGLE attribute is set, K0S is ignored.

K1

Field gradient index of the magnet, K_1=-\frac{R}{B_{y}}\frac{\partial B_y}{\partial x}, where R is the bend radius as defined in Figure 1. Not supported in OPAL-t any more. Superimpose a Quadrupole instead.

E1

Entrance edge angle (radians). Figure 1 shows the definition of a positive entrance edge angle. (Note that the exit edge angle is fixed in an RBEND3D element to \mathrm{E2} = \mathrm{ANGLE} - \mathrm{E1}).

DESIGNENERGY

Energy of the reference particle (MeV). The reference particle travels approximately the path shown in Figure 1.

FMAPFN

Name of the field map for the magnet. Currently maps of type 1DProfile1 can be used. The default option for this attribute is FMAPN = 1DPROFILE1-DEFAULT see Default Field Map (OPAL-t). The field map is used to describe the fringe fields of the magnet 1DProfile1.

Figure 2 illustrates a general sector bend(SBEND) with a positive bend angle \alpha. In this example the entrance and exit edge angles E_{1} and E_{2} have positive values. For a positively charge particle, the magnetic field is directed out of the page.

sbend
Figure 2. Illustration of a general sector bend(SBEND) with a positive bend angle \alpha

1.4.3. SBend (OPAL-t)

An SBEND is a sector bending magnet. An SBEND can have independent entrance and exit edge angles. Figure 2 shows an SBEND with a positive bend angle, a positive entrance edge angle, and a positive exit edge angle.

L

Chord length of the bend reference arc in meters (see Figure 2), given by: L = 2 R \sin\left(\frac{\alpha}{2}\right)

GAP

Full vertical gap of the magnet (meters).

HAPERT

Non-bend plane aperture of the magnet (meters). (Defaults to one half the bend radius.)

ANGLE

Bend angle (radians). Field amplitude of the bend will be adjusted to achieve this angle. (Note that practically speaking, bend angles greater than \frac{3 \pi}{2} (270 degrees) can be problematic. Beyond this, the fringe fields from the entrance and exit pole faces could start to interfere, so be careful when setting up bend angles greater than this. An angle greater than or equal to 2 \pi (360 degrees) is not allowed.)

K0

Field amplitude in y direction (Tesla). If the ANGLE attribute is set, K0 is ignored.

K0S

Field amplitude in x direction (Tesla). If the ANGLE attribute is set, K0S is ignored.

K1

Field gradient index of the magnet, K_1=-\frac{R}{B_{y}}\frac{\partial B_y}{\partial x}, where R is the bend radius as defined in Figure 2. Not supported in OPAL-t any more. Superimpose a Quadrupole instead.

E1

Entrance edge angle (rad). Figure 2 shows the definition of a positive entrance edge angle.

E2

Exit edge angle (rad). Figure 2 shows the definition of a positive exit edge angle.

DESIGNENERGY

Energy of the bend reference particle (MeV). The reference particle travels approximately the path shown in Figure 2.

FMAPFN

Name of the field map for the magnet. Currently maps of type 1DProfile1 can be used. The default option for this attribute is FMAPN = 1DPROFILE1-DEFAULT see_Default Field Map (OPAL-t). The field map is used to describe the fringe fields of the magnet see 1DProfile1.

1.4.4. RBend and SBend Examples (OPAL-t)

Describing an RBEND or an SBEND in an OPAL-t simulation requires effectively identical commands. There are only slight differences between the two. The L attribute has a different definition for the two types of bends sees RBend (OPAL-t) and SBend (OPAL-t), and an SBEND has an additional attribute E2 that has no effect on an RBEND, see SBend (OPAL-t). Therefore, in this section, we will give several examples of how to implement a bend, using the RBEND and SBEND commands interchangeably. The understanding is that the command formats are essentially the same.

When implementing an RBEND or SBEND in an OPAL-t simulation, it is important to note the following:

  1. Internally OPAL-t treats all bends as positive, as defined by Figure 1 and Figure 2. Bends in other directions within the x/y plane are accomplished by rotating a positive bend about its z axis.

  2. If the ANGLE attribute is set to a non-zero value, the K0 and K0S attributes will be ignored.

  3. When using the ANGLE attribute to define a bend, the actual beam will be bent through a different angle if its mean kinetic energy doesn’t correspond to the DESIGNENERGY.

  4. Internally the bend geometry is setup based on the ideal reference trajectory, as shown in Figure 1 and Figure 2.

  5. If the default field map, 1DPROFILE-DEFAULT see Default Field Map (OPAL-t), is used, the fringe fields will be adjusted so that the effective length of the real, soft edge magnet matches the ideal, hard edge bend that is defined by the reference trajectory.

For the rest of this section, we will give several examples of how to input bends in an OPAL-t simulation. We will start with a simple example using the ANGLE attribute to set the bend strength and using the default field map see Default Field Map (OPAL-t) for describing the magnet fringe fields see 1DProfile1:

Bend: RBend, ANGLE = 30.0 * Pi / 180.0,
         FMAPFN = "1DPROFILE1-DEFAULT",
         ELEMEDGE = 0.25,
         DESIGNENERGY = 10.0,
         L = 0.5,
         GAP = 0.02;

This is a definition of a simple RBEND that bends the beam in a positive direction 30 degrees (towards the negative x axis as if Figure 1). It has a design energy of 10 MeV, a length of 0.5 m, a vertical gap of 2 cm and a 0^{\circ} entrance edge angle. (Therefore the exit edge angle is 30^{\circ}.) We are using the default, internal field map "1DPROFILE1-DEFAULT" see Default Field Map (OPAL-t) which describes the magnet fringe fields see 1DProfile1. When OPAL is run, you will get the following output (assuming an electron beam) for this RBEND definition:

RBend > Reference Trajectory Properties
RBend > ===============================
RBend >
RBend > Bend angle magnitude:    0.523599 rad (30 degrees)
RBend > Entrance edge angle:     0 rad (0 degrees)
RBend > Exit edge angle:         0.523599 rad (30 degrees)
RBend > Bend design radius:      1 m
RBend > Bend design energy:      1e+07 eV
RBend >
RBend > Bend Field and Rotation Properties
RBend > ==================================
RBend >
RBend > Field amplitude:         -0.0350195 T
RBend > Field index (gradient):  0 m^-1
RBend > Rotation about x axis:   0 rad (0 degrees)
RBend > Rotation about y axis:   0 rad (0 degrees)
RBend > Rotation about z axis:   0 rad (0 degrees)
RBend >
RBend > Reference Trajectory Properties Through Bend Magnet with Fringe Fields
RBend > ======================================================================
RBend >
RBend > Reference particle is bent: 0.523599 rad (30 degrees) in x plane
RBend > Reference particle is bent: 0 rad (0 degrees) in y plane

The first section of this output gives the properties of the reference trajectory like that described in Figure 1. From the value of ANGLE and the length, L, of the magnet, OPAL calculates the 10 MeV reference particle trajectory radius, R. From the bend geometry and the entrance angle (0^{\circ} in this case), the exit angle is calculated.

The second section gives the field amplitude of the bend and its gradient (quadrupole focusing component), given the particle charge (-e in this case so the amplitude is negative to get a positive bend direction). Also listed is the rotation of the magnet about the various axes.

Of course, in the actual simulation the particles will not see a hard edge bend magnet, but rather a soft edge magnet with fringe fields described by the RBEND field map file FMAPFN see 1DProfile1. So, once the hard edge bend/reference trajectory is determined, OPAL then includes the fringe fields in the calculation. When the user chooses to use the default field map, OPAL will automatically adjust the position of the fringe fields appropriately so that the soft edge magnet is equivalent to the hard edge magnet described by the reference trajectory. To check that this was done properly, OPAL integrates the reference particle through the final magnet description with the fringe fields included. The result is shown in the final part of the output. In this case we see that the soft edge bend does indeed bend our reference particle through the correct angle.

What is important to note from this first example, is that it is this final part of the bend output that tells you the actual bend angle of the reference particle.

In this next example, we merely rewrite the first example, but use K0 to set the field strength of the RBEND, rather than the ANGLE attribute:

Bend: RBend, K0 = -0.0350195,
         FMAPFN = "1DPROFILE1-DEFAULT",
         ELEMEDGE = 0.25,
         DESIGNENERGY = 10.0E6,
         L = 0.5,
         GAP = 0.02;

The output from OPAL now reads as follows:

RBend > Reference Trajectory Properties
RBend > ===============================
RBend >
RBend > Bend angle magnitude:    0.523599 rad (30 degrees)
RBend > Entrance edge angle:     0 rad (0 degrees)
RBend > Exit edge angle:         0.523599 rad (30 degrees)
RBend > Bend design radius:      0.999999 m
RBend > Bend design energy:      1e+07 eV
RBend >
RBend > Bend Field and Rotation Properties
RBend > ==================================
RBend >
RBend > Field amplitude:         -0.0350195 T
RBend > Field index (gradient):  0 m^-1
RBend > Rotation about x axis:   0 rad (0 degrees)
RBend > Rotation about y axis:   0 rad (0 degrees)
RBend > Rotation about z axis:   0 rad (0 degrees)
RBend >
RBend > Reference Trajectory Properties Through Bend Magnet with Fringe Fields
RBend > ======================================================================
RBend >
RBend > Reference particle is bent: 0.5236 rad (30.0001 degrees) in x plane
RBend > Reference particle is bent: 0 rad (0 degrees) in y plane

The output is effectively identical, to within a small numerical error.

Now, let us modify this first example so that we bend instead in the negative x direction. There are several ways to do this:

1.

Bend: RBend, ANGLE = -30.0 * Pi / 180.0,
         FMAPFN = "1DPROFILE1-DEFAULT",
         ELEMEDGE = 0.25,
         DESIGNENERGY = 10.0E6,
         L = 0.5,
         GAP = 0.02;

2.

Bend: RBend, ANGLE = 30.0 * Pi / 180.0,
         FMAPFN = "1DPROFILE1-DEFAULT",
         ELEMEDGE = 0.25,
         DESIGNENERGY = 10.0E6,
         L = 0.5,
         GAP = 0.02,
         ROTATION = Pi;

3.

Bend: RBend, K0 = 0.0350195,
         FMAPFN = "1DPROFILE1-DEFAULT",
         ELEMEDGE = 0.25,
         DESIGNENERGY = 10.0E6,
         L = 0.5,
         GAP = 0.02;

4.

Bend: RBend, K0 = -0.0350195,
         FMAPFN = "1DPROFILE1-DEFAULT",
         ELEMEDGE = 0.25,
         DESIGNENERGY = 10.0E6,
         L = 0.5,
         GAP = 0.02,
         ROTATION = Pi;

In each of these cases, we get the following output for the bend (to within small numerical errors).

RBend > Reference Trajectory Properties
RBend > ===============================
RBend >
RBend > Bend angle magnitude:    0.523599 rad (30 degrees)
RBend > Entrance edge angle:     0 rad (0 degrees)
RBend > Exit edge angle:         0.523599 rad (30 degrees)
RBend > Bend design radius:      1 m
RBend > Bend design energy:      1e+07 eV
RBend >
RBend > Bend Field and Rotation Properties
RBend > ==================================
RBend >
RBend > Field amplitude:         -0.0350195 T
RBend > Field index (gradient):  -0 m^-1
RBend > Rotation about x axis:   0 rad (0 degrees)
RBend > Rotation about y axis:   0 rad (0 degrees)
RBend > Rotation about z axis:   3.14159 rad (180 degrees)
RBend >
RBend > Reference Trajectory Properties Through Bend Magnet with Fringe Fields
RBend > ======================================================================
RBend >
RBend > Reference particle is bent: -0.523599 rad (-30 degrees) in x plane
RBend > Reference particle is bent: 0 rad (0 degrees) in y plane

In general, we suggest to always define a bend in the positive x direction (as in Figure 1) and then use the ROTATION attribute to bend in other directions in the x/y plane (as in examples 2 and 4 above).

As a final RBEND example, here is a suggested format for the four bend definitions if one where implementing a four dipole chicane:

Bend1: RBend, ANGLE = 20.0 * Pi / 180.0,
              E1 = 0.0,
              FMAPFN = "1DPROFILE1-DEFAULT",
              ELEMEDGE = 0.25,
              DESIGNENERGY = 10.0E6,
              L = 0.25,
              GAP = 0.02,
              ROTATION = Pi;

Bend2: RBend, ANGLE = 20.0 * Pi / 180.0,
              E1 = 20.0 * Pi / 180.0,
              FMAPFN = "1DPROFILE1-DEFAULT",
              ELEMEDGE = 1.0,
              DESIGNENERGY = 10.0E6,
              L = 0.25,
              GAP = 0.02,
              ROTATION = 0.0;

Bend3: RBend, ANGLE = 20.0 * Pi / 180.0,
              E1 = 0.0,
              FMAPFN = "1DPROFILE1-DEFAULT",
              ELEMEDGE = 1.5,
              DESIGNENERGY = 10.0E6,
              L = 0.25,
              GAP = 0.02,
              ROTATION = 0.0;

Bend4: RBend, ANGLE = 20.0 * Pi / 180.0,
              E1 = 20.0 * Pi / 180.0,
              FMAPFN = "1DPROFILE1-DEFAULT",
              ELEMEDGE = 2.25,
              DESIGNENERGY = 10.0E6,
              L = 0.25,
              GAP = 0.02,
              ROTATION = Pi;

Up to now, we have only given examples of RBEND definitions. If we replaced "RBend" in the above examples with "SBend", we would still be defining valid OPAL-t bends. In fact, by adjusting the L attribute according to RBend (OPAL-t) and SBend (OPAL-t), and by adding the appropriate definitions of the E2 attribute, we could even get identical results using `SBEND`s instead of `RBEND`s. (As we said, the two bends are very similar in command format.)

Up till now, we have only used the default field map. Custom field maps can also be used. There are two different options in this case see 1DProfile1:

  1. Field map defines fringe fields and magnet length.

  2. Field map defines fringe fields only.

The first case describes how field maps were used in previous versions of OPAL (and can still be used in the current version). The second option is new to OPAL OPALversion 1.2.00 and it has a couple of advantages:

  1. Because only the fringe fields are described, the length of the magnet must be set using the L attribute. In turn, this means that the same field map can be used by many bend magnets with different lengths (assuming they have equivalent fringe fields). By contrast, if the magnet length is set by the field map, one must generate a new field map for each dipole of different length even if the fringe fields are the same.

  2. We can adjust the position of the fringe field origin relative to the entrance and exit points of the magnet see 1DProfile1. This gives us another degree of freedom for describing the fringe fields, allowing us to adjust the effective length of the magnet.

We will now give examples of how to use a custom field map, starting with the first case where the field map describes the fringe fields and the magnet length. Assume we have the following 1DProfile1 field map:

1DProfile1 1 1 2.0
 -10.0  0.0  10.0 1
  15.0  25.0 35.0 1
  0.00000E+00
  2.00000E+00
  0.00000E+00
  2.00000E+00

We can use this field map to define the following bend (note we are now using the SBEND command):

Bend: SBend, ANGLE = 60.0 * Pi / 180.0,
             E1 = -10.0 * Pi / 180.0,
             E2 = 20.0  Pi / 180.0,
             FMAPFN = "TEST-MAP.T7",
             ELEMEDGE = 0.25,
             DESIGNENERGY = 10.0E6,
             GAP = 0.02;

Notice that we do not set the magnet length using the L attribute. (In fact, we don’t even include it. If we did and set it to a non-zero value, the exit fringe fields of the magnet would not be correct.) This input gives the following output:

SBend > Reference Trajectory Properties
SBend > ===============================
SBend >
SBend > Bend angle magnitude:    1.0472 rad (60 degrees)
SBend > Entrance edge angle:     -0.174533 rad (-10 degrees)
SBend > Exit edge angle:         0.349066 rad (20 degrees)
SBend > Bend design radius:      0.25 m
SBend > Bend design energy:      1e+07 eV
SBend >
SBend > Bend Field and Rotation Properties
SBend > ==================================
SBend >
SBend > Field amplitude:         -0.140385 T
SBend > Field index (gradient):  0 m^-1
SBend > Rotation about x axis:   0 rad (0 degrees)
SBend > Rotation about y axis:   0 rad (0 degrees)
SBend > Rotation about z axis:   0 rad (0 degrees)
SBend >
SBend > Reference Trajectory Properties Through Bend Magnet with Fringe Fields
SBend > ======================================================================
SBend >
SBend > Reference particle is bent: 1.0472 rad (60 degrees) in x plane
SBend > Reference particle is bent: 0 rad (0 degrees) in y plane

Because we set the bend strength using the ANGLE attribute, the magnet field strength is automatically adjusted so that the reference particle is bent exactly ANGLE radians when the fringe fields are included. (Lower output.)

Now we will illustrate the case where the magnet length is set by the L attribute and only the fringe fields are described by the field map. We change the TEST-MAP.T7 file to:

1DProfile1 1 1 2.0
 -10.0  0.0  10.0 1
 -10.0  0.0  10.0 1
  0.00000E+00
  2.00000E+00
  0.00000E+00
  2.00000E+00

and change the bend input to:

Bend: SBend, ANGLE = 60.0 * Pi / 180.0,
             E1 = -10.0 * Pi / 180.0,
             E2 = 20.0  Pi / 180.0,
             FMAPFN = "TEST-MAP.T7",
             ELEMEDGE = 0.25,
             DESIGNENERGY = 10.0E6,
             L = 0.25,
             GAP = 0.02;

This results in the same output as the previous example, as we expect.

SBend > Reference Trajectory Properties
SBend > ===============================
SBend >
SBend > Bend angle magnitude:    1.0472 rad (60 degrees)
SBend > Entrance edge angle:     -0.174533 rad (-10 degrees)
SBend > Exit edge angle:         0.349066 rad (20 degrees)
SBend > Bend design radius:      0.25 m
SBend > Bend design energy:      1e+07 eV
SBend >
SBend > Bend Field and Rotation Properties
SBend > ==================================
SBend >
SBend > Field amplitude:         -0.140385 T
SBend > Field index (gradient):  0 m^-1
SBend > Rotation about x axis:   0 rad (0 degrees)
SBend > Rotation about y axis:   0 rad (0 degrees)
SBend > Rotation about z axis:   0 rad (0 degrees)
SBend >
SBend > Reference Trajectory Properties Through Bend Magnet with Fringe Fields
SBend > ======================================================================
SBend >
SBend > Reference particle is bent: 1.0472 rad (60 degrees) in x plane
SBend > Reference particle is bent: 0 rad (0 degrees) in y plane

As a final example, let us now use the previous field map with the following input:

Bend: SBend, K0 = -0.1400778,
             E1 = -10.0 * Pi / 180.0,
             E2 = 20.0  Pi / 180.0,
             FMAPFN = "TEST-MAP.T7",
             ELEMEDGE = 0.25,
             DESIGNENERGY = 10.0E6,
             L = 0.25,
             GAP = 0.02;

Instead of setting the bend strength using ANGLE, we use K0. This results in the following output:

SBend > Reference Trajectory Properties
SBend > ===============================
SBend >
SBend > Bend angle magnitude:    1.0472 rad (60 degrees)
SBend > Entrance edge angle:     -0.174533 rad (-10 degrees)
SBend > Exit edge angle:         0.349066 rad (20 degrees)
SBend > Bend design radius:      0.25 m
SBend > Bend design energy:      1e+07 eV
SBend >
SBend > Bend Field and Rotation Properties
SBend > ==================================
SBend >
SBend > Field amplitude:         -0.140078 T
SBend > Field index (gradient):  0 m^-1
SBend > Rotation about x axis:   0 rad (0 degrees)
SBend > Rotation about y axis:   0 rad (0 degrees)
SBend > Rotation about z axis:   0 rad (0 degrees)
SBend >
SBend > Reference Trajectory Properties Through Bend Magnet with Fringe Fields
SBend > ======================================================================
SBend >
SBend > Reference particle is bent: 1.04491 rad (59.8688 degrees) in x plane
SBend > Reference particle is bent: 0 rad (0 degrees) in y plane

In this case, the bend angle for the reference trajectory in the first section of the output no longer matches the reference trajectory bend angle from the lower section (although the difference is small). The reason is that the path of the reference particle through the real magnet (with fringe fields) no longer matches the ideal trajectory. (The effective length of the real magnet is not quite the same as the hard edged magnet for the reference trajectory.)

We can compensate for this by changing the field map file TEST-MAP.T7 file to:

1DProfile1 1 1 2.0
 -10.0  -0.03026  10.0 1
 -10.0  0.03026  10.0 1
  0.00000E+00
  2.00000E+00
  0.00000E+00
  2.00000E+00

We have moved the Enge function origins see 1DProfile1 outward from the entrance and exit faces of the magnet see 1DProfile1 by 0.3026 mm. This has the effect of making the effective length of the soft edge magnet longer. When we do this, the same input:

Bend: SBend, K0 = -0.1400778,
             E1 = -10.0 * Pi / 180.0,
             E2 = 20.0  Pi / 180.0,
             FMAPFN = "TEST-MAP.T7",
             ELEMEDGE = 0.25,
             DESIGNENERGY = 10.0E6,
             L = 0.25,
             GAP = 0.02;

produces

SBend > Reference Trajectory Properties
SBend > ===============================
SBend >
SBend > Bend angle magnitude:    1.0472 rad (60 degrees)
SBend > Entrance edge angle:     -0.174533 rad (-10 degrees)
SBend > Exit edge angle:         0.349066 rad (20 degrees)
SBend > Bend design radius:      0.25 m
SBend > Bend design energy:      1e+07 eV
SBend >
SBend > Bend Field and Rotation Properties
SBend > ==================================
SBend >
SBend > Field amplitude:         -0.140078 T
SBend > Field index (gradient):  0 m^-1
SBend > Rotation about x axis:   0 rad (0 degrees)
SBend > Rotation about y axis:   0 rad (0 degrees)
SBend > Rotation about z axis:   0 rad (0 degrees)
SBend >
SBend > Reference Trajectory Properties Through Bend Magnet with Fringe Fields
SBend > ======================================================================
SBend >
SBend > Reference particle is bent: 1.0472 rad (60 degrees) in x plane
SBend > Reference particle is bent: 0 rad (0 degrees) in y plane

Now we see that the bend angle for the ideal, hard edge magnet, matches the bend angle of the reference particle through the soft edge magnet. In other words, the effective length of the soft edge, real magnet is the same as the hard edge magnet described by the reference trajectory.

1.4.5. Bend Fields from 1D Field Maps (OPAL-t)

Enge func plot
Figure 3. Plot of the entrance fringe field of a dipole magnet along the mid-plane, perpendicular to its entrance face. The field is normalized to 1.0. In this case, the fringe field is described by an Enge function see Enge function with the parameters from the default 1DProfile1 field map described in Default Field Map (OPAL-t). The exit fringe field of this magnet is the mirror image.

So far we have described how to setup an RBEND or SBEND element, but have not explained how OPAL-t uses this information to calculate the magnetic field. The field of both types of magnets is divided into three regions:

  1. Entrance fringe field.

  2. Central field.

  3. Exit fringe field.

This can be seen clearly in [fig_rbend_field_profile].

The purpose of the 1DProfile1 field map see 1DProfile1 associated with the element is to define the Enge functions (Enge function) that model the entrance and exit fringe fields. To model a particular bend magnet, one must fit the field profile along the mid-plane of the magnet perpendicular to its face for the entrance and exit fringe fields to the Enge function:

  F(z) = \frac{1}{1 + e^{\sum\limits_{n=0}^{N_{order}} c_{n} (z/D)^{n}}}

where D is the full gap of the magnet, N_{order} is the Enge function order and z is the distance from the origin of the Enge function perpendicular to the edge of the dipole. The origin of the Enge function, the order of the Enge function, N_{order}, and the constants c_0 to c_{N_{order}} are free parameters that are chosen so that the function closely approximates the fringe region of the magnet being modeled. An example of the entrance fringe field is shown in Figure 3.

Let us assume we have a correctly defined positive RBEND or SBEND element as illustrated in Figure 1 and Figure 2. (As already stated, any bend can be described by a rotated positive bend.) OPAL-t then has the following information:

\begin{aligned}
B_0 &= \text{Field amplitude (T)} \\
R &= \text{Bend radius (m)} \\
n &= -\frac{R}{B_{y}}\frac{\partial B_y}{\partial x} \text{ (Field index, set using the parameter } \mathrm{K1} \text{)} \\
F(z) &= \left\{
\begin{array}{lll}
    & F_{entrance}(z_{entrance}) \\
    & F_{center}(z_{center}) = 1 \\
    & F_{exit}(z_{exit})
\end{array}
\right.\end{aligned}

Here, we have defined an overall Enge function, F(z), with three parts: entrance, center and exit. The exit and entrance fringe field regions have the form of Enge function with parameters defined by the 1DProfile1 field map file given by the element parameter FMAPFN. Defining the coordinates:

\begin{aligned}
y &\equiv \text{Vertical distance from magnet mid-plane} \\
\Delta_x &\equiv \text{Perpendicular distance to reference trajectory (see Figures)} \\
\Delta_z &\equiv \left\{
\begin{array}{lll}
    & \text{Distance from entrance Enge function origin perpendicular to magnet entrance face.} \\
    & \text{Not defined, Enge function is always 1 in this region.} \\
    & \text{Distance from exit Enge function origin perpendicular to magnet exit face.}
\end{array}
\right.\end{aligned}

using the conditions

\begin{aligned}
\nabla \cdot \vec{B} &= 0 \\
\nabla \times \vec{B} &= 0
\end{aligned}

and making the definitions:

\begin{aligned}
F'(z) &\equiv   \frac{\mathrm{d} F(z)}{\mathrm{d} z} \\
F''(z) &\equiv  \frac{\mathrm{d^{2}} F(z)}{\mathrm{d} z^{2}} \\
F'''(z) &\equiv \frac{\mathrm{d^{3}} F(z)}{\mathrm{d} z^{3}}
\end{aligned}

we can expand the field off axis, with the result:

\begin{aligned}
B_x(\Delta_x, y, \Delta_z) &= -\frac{B_0 \frac{n}{R}}{\sqrt{\frac{n^2}{R^2} +  \frac{F''(\Delta_z)}{F(\Delta_z}}} e^{-\frac{n}{R} \Delta_x} \sin \left[ \left( \sqrt{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}} \right) y \right] F(\Delta_z) \\ \\
B_y(\Delta_x, y, \Delta_z) &= B_0 e^{-\frac{n}{R} \Delta_x} \cos \left[ \left( \sqrt{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}} \right) y \right] F(\Delta_z) \\ \\
B_z(\Delta_x, y, \Delta_z) &= B_0 e^{-\frac{n}{R} \Delta_x} \left\{\frac{F'(\Delta_z)}{\sqrt{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}}} \sin \left[ \left( \sqrt{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}} \right) y \right] \right. \\ \\
&- \frac{1}{2 \sqrt{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}}} \left(F'''(\Delta_z) - \frac{F'(\Delta_z) F''(\Delta_z)}{F(\Delta_z)} \right) \left[ \frac{\sin \left[ \left( \sqrt{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}} \right) y \right]}{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}} \right. \\ \\
&- \left. \left. y \frac{\cos \left[ \left( \sqrt{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}} \right) y \right]}{\sqrt{\frac{n^2}{R^2} + \frac{F''(\Delta_z)}{F(\Delta_z)}}} \right] \right\}\end{aligned}

These expression are not well suited for numerical calculation, so, we expand them about y to O(y^2) to obtain:

  • In fringe field regions:

\begin{aligned}
B_x(\Delta_x, y, \Delta_z) &\approx -B_0 \frac{n}{R} e^{-\frac{n}{R} \Delta_x} y \\
B_y(\Delta_x, y, \Delta_z) &\approx B_0 e^{-\frac{n}{R} \Delta_x} \left[ F(\Delta_z) - \left( \frac{n^2}{R^2} F(\Delta_z) + F''(\Delta_z) \right) \frac{y^2}{2} \right] \\
B_z(\Delta_x, y, \Delta_z) &\approx B_0 e^{-\frac{n}{R} \Delta_x} y F'(\Delta_z)
\end{aligned}
  • In central region:

\begin{aligned}
B_x(\Delta_x, y, \Delta_z) &\approx -B_0 \frac{n}{R} e^{-\frac{n}{R} \Delta_x} y \\
B_y(\Delta_x, y, \Delta_z) &\approx B_0 e^{-\frac{n}{R} \Delta_x} \left[ 1 -  \frac{n^2}{R^2} \frac{y^2}{2} \right] \\
B_z(\Delta_x, y, \Delta_z) &\approx 0
\end{aligned}

These are the expressions OPAL-t uses to calculate the field inside an RBEND or SBEND. First, a particle’s position inside the bend is determined (entrance region, center region, or exit region). Depending on the region, OPAL-t then determines the values of \Delta_x, y and \Delta_z, and then calculates the field values using the above expressions.

1.4.6. Default Field Map (OPAL-t)

Rather than force users to calculate the field of a dipole and then fit that field to find Enge coefficients for the dipoles in their simulation, we have a default set of values we use from [2] that are set when the default field map, 1DPROFILE1-DEFAULT is used:

\begin{aligned}
  c_{0} &= 0.478959 \\
  c_{1} &= 1.911289 \\
  c_{2} &= -1.185953 \\
  c_{3} &= 1.630554 \\
  c_{4} &= -1.082657 \\
  c_{5} &= 0.318111\end{aligned}

The same values are used for both the entrance and exit regions of the magnet. In general they will give good results. (Of course, at some point as a beam line design becomes more advanced, one will want to find Enge coefficients that fit the actual magnets that will be used in a given design.)

The default field map is the equivalent of the following custom 1DProfile1 (see 1DProfile1 for an explanation of the field map format) map:

1DProfile1 5 5 2.0
 -10.0 0.0 10.0 1
 -10.0 0.0 10.0 1
  0.478959
  1.911289
 -1.185953
  1.630554
 -1.082657
  0.318111
  0.478959
  1.911289
 -1.185953
  1.630554
 -1.082657
  0.318111

As one can see, the default magnet gap for 1DPROFILE1-DEFAULT is set to 2.0 cm. This value can be overridden by the GAP attribute of the magnet (see RBend (OPAL-t) and SBend (OPAL-t)).

1.4.7. SBend3D (OPAL-cycl)

The SBend3D element enables definition of a bend from 3D field maps. This can be used in conjunction with the RINGDEFINITION element to make a ring for tracking through OPAL-cycl.

label: SBEND3D, FMAPFN=string, LENGTH_UNITS=real, FIELD_UNITS=real;
FMAPFN

The field map file name.

LENGTH_UNITS

Units for length (set to 1.0 for units in mm, 10.0 for units in cm, etc).

FIELD_UNITS

Units for field (set to 1.0 for units in T, 0.001 for units in mT, etc).

Field maps are defined using Cartesian coordinates but in a polar geometry. The following conventions have to be fulfilled:

  1. 3D Field maps have to be generated in the vertical direction (z coordinate in OPAL-cycl) from z = 0 upwards. Maps cannot be generated symmetrically about z = 0 towards negative z values.

  2. The field map file must be in the form with columns ordered as follows: [x, z, y, B_{x}, B_{z}, B_{y}].

  3. Grid points of the position and field strength have to be written on a grid in (r, z, \theta) with the primary direction corresponding to the azimuthal direction, secondary to the vertical direction and tertiary to the radial direction.

SBEND3D assumes a dipole symmetry. In a dipole symmetry, fields below the symmetry plane Z=0 have the same field in the direction perpendicular to the symmetry plane, B_{z}, but field components parallel to the symmetry plane have the opposite direction (sign).

Below are two examples of a SBEND3D which loads a field map file named “90degree_Dipole_Magnet.out” defining a hard edge model of 90 degree dipole magnet with homogenous magnetic field. The first 8 lines are presumed to be header material and are ignored. Positions have units of m and fields units of Tesla. The corresponding 3D magnetic field map is shown in the following figure in the Cartesian coordinate system (x, y, z). A horizontal cross section of the 3D magnetic field map when z = 0 is also shown.

Dipole: SBEND3D, FMAPFN="90degree_Dipole_Magnet.out", LENGTH_UNITS=1000.0, FIELD_UNITS=-10.0;

The first few lines of the field map file are as follows:

	4550000	4550000	4550000	1
X [LENGTH_UNITS]
Z [LENGTH_UNITS]
Y [LENGTH_UNITS]
BX [FIELD_UNITS]
BZ [FIELD_UNITS]
BY [FIELD_UNITS]
0
4.3586435e-01   5.0000000e-02   1.2803431e+00   0.0000000e+00   1.6214000e+00   0.0000000e+00
4.2691532e-01   5.0000000e-02   1.2833548e+00   0.0000000e+00   1.6214000e+00   0.0000000e+00
4.1794548e-01   5.0000000e-02   1.2863039e+00   0.0000000e+00   1.6214000e+00   0.0000000e+00

This is a restricted feature for OPAL-cycl.

sbend3d
Figure 4. A hard edge model of 90 degree dipole magnet with homogeneous magnetic field. The right figure is showing the horizontal cross section of the 3D magnetic field map when z = 0

1.5. Quadrupole

label:QUADRUPOLE, TYPE=string, APERTURE=real-vector,
      L=real, K1=real, K1S=real;

The reference system for a quadrupole is a Cartesian coordinate system This is a restricted feature for OPAL-t.

A QUADRUPOLE has the following real attributes:

K1

The normal quadrupole component K_1=\frac{\partial B_y}{\partial x}. The default is 0 \mathrm{Tm^{-1}}. The component is positive, if B_y is positive on the positive x-axis. This implies horizontal focusing of positively charged particles which travel in positive s-direction.

K1S

The skew quadrupole component. K_{1s}=-\frac{\partial B_x}{\partial x}. The default is 0 \mathrm{Tm^{-1}}. The component is negative, if B_x is positive on the positive x-axis.

DK1

The normalised quadrupole coefficient error.

DK1S

The normalised skew quadrupole coefficient error.

Example:

QP1: Quadrupole, L=1.20, ELEMEDGE=-0.5265, K1=0.11;

1.6. Sextupole

label: SEXTUPOLE, TYPE=string, APERTURE=real-vector,
       L=real, K2=real, K2S=real;

A SEXTUPOLE has the following real attributes:

K2

The normal sextupole component K_2=\frac{\partial{^2} B_y}{\partial x^2}. The default is 0 \mathrm{T m^{-2}}. The component is positive, if B_y is positive on the x-axis.

K2S

The skew sextupole component K_{2s}=-\frac{\partial{^2}B_x}{\partial x^{2}}. The default is 0 \mathrm{T m^{-2}}. The component is negative, if B_x is positive on the x-axis.

DK2

The normalised sextupole coefficient error.

DK2S

The normalised skew sextupole coefficient error.

Example:

S:SEXTUPOLE, L=0.4, K2=0.00134;

The reference system for a sextupole is a Cartesian coordinate system

1.7. Octupole

label:OCTUPOLE, TYPE=string, APERTURE=real-vector,
      L=real, K3=real, K3S=real;

An OCTUPOLE has the following real attributes:

K3

The normal octupole component K_3=\frac{\partial{^3} B_y}{\partial x^3}. The default is 0 \mathrm{Tm^{-3}}. The component is positive, if B_y is positive on the positive x-axis.

K3S

The skew octupole component K_{3s}=-\frac{\partial{^3}B_x}{\partial x^{3}}. The default is 0 \mathrm{Tm^{-3}}. The component is negative, if B_x is positive on the positive x-axis.

DK3

The normalised octupole coefficient error.

DK3S

The normalised skew octupole coefficient error.

Example:

O3:OCTUPOLE, L=0.3, K3=0.543;

The reference system for an octupole is a Cartesian coordinate system

1.8. General Multipole

The MULTIPOLE element defines a thick multipole. If the length is non-zero, the strengths are per unit length. If the length is zero, the strengths are the values integrated over the length. With zero length no synchrotron radiation can be calculated.

A MULTIPOLE in OPAL-t is of arbitrary order.

label:MULTIPOLE, TYPE=string, APERTURE=real-vector,
      L=real, KN=real-vector, KS=real-vector;
KN

A real vector see Arrays, containing the normal multipole coefficients, K_n=\frac{\partial{^n} B_y}{\partial x^n}. (default is 0 \mathrm{Tm^{-n}}). A component is positive, if B_y is positive on the positive x-axis.

KS

A real vector see Arrays, containing the skew multipole coefficients, K_{n~s}=-\frac{\partial{^n}B_x}{\partial x^{n}}. (default is 0 \mathrm{Tm^{-n}}). A component is negative, if B_x is positive on the positive x-axis.

DKN

A real vector see Arrays, containing the normal normalised multipole strength errors. (default is 0 \mathrm{Tm^{-n}}).

DKS

A real vector see Arrays, containing the skew normalised multipole strength errors. (default is 0 \mathrm{Tm^{-n}}).

The number of poles of each component is (2 n + 2).

Superposition of many multipole components is permitted. The reference system for a multipole is a Cartesian coordinate system

The following example is equivalent to the quadruple example in Quadrupole.

M27:MULTIPOLE, L=1, ELEMEDGE=3.8, KN={0.0,0.11};

A multipole has no effect on the reference orbit, i.e. the reference system at its exit is the same as at its entrance. Use the dipole component only to model a defective multipole.

1.9. General Multipole (will replace General Multipole when implemented)

A MULTIPOLET is in OPAL-t a general multipole with extended features. It can represent a straight or curved magnet. In the curved case, the user may choose between constant or variable radius. This model includes fringe fields. The detailed description can be found at: https://gitlab.psi.ch/OPAL/src/uploads/0d3fc561b57e8962ed79a57cd6115e37/8FBB32A4-7FA1-4084-A4A7-CDDB1F949CD3_psi.ch.pdf.

Label:MULTIPOLET, L=real, ANGLE=real, VAPERT=real, HAPERT=real,
      LFRINGE=real, RFRINGE=real, TP=real-vector, VARRADIUS=bool;
L

Physical length of the magnet (meters), without end fields. (Default: 1 m)

ANGLE

Physical angle of the magnet (radians). If not specified, the magnet is considered to be straight (ANGLE=0.0). This is not the total bending angle since the end fields cause additional bending. The radius of the multipole is set from the LENGTH and ANGLE attributes.

VAPERT

Vertical (non-bend plane) aperture of the magnet (meters). (Default: 0.5 m)

HAPERT

Horizontal (bend plane) aperture of the magnet (meters). (Default: 0.5 m)

LFRINGE

Length of the left fringe field (meters). (Default: 0.0 m)

RFRINGE

Length of the right fringe field (meters). (Default: 0.0 m)

TP

A real vector see Arrays, containing the multipole coefficients of the field expansion on the mid-plane in the body of the magnet: the transverse profile T(x) = B_0 + B_1 x + B_2 x^2 + \ldots is set by TP=B_0, B_1, B_2 (units: T \cdot m^{-n}). The order of highest multipole component is arbitrary, but all components up to the maximum must be given, even if they are zero.

MAXFORDER

The order of the maximum function f_n used in the field expansion (default: 5). See the scalar magnetic potential below. This sets for example the maximum power of z in the field expansion of vertical component B_z to 2 \cdot \text{MAXFORDER} .

EANGLE

Entrance edge angle (radians).

ROTATION

Rotation of the magnet about its central axis (radians, counterclockwise). This enables to obtain skew fields. (Default 0.0 rad)

VARRADIUS

This is to be set TRUE if the magnet has variable radius. More precisely, at each point along the magnet, its radius is computed such that the reference trajectory always remains in the centre of the magnet. In the body of the magnet the radius is set from the LENGTH and ANGLE attributes. It is then continuously changed to be proportional to the dipole field on the reference trajectory while entering the end fields. This attribute is only to be set TRUE for a non-zero dipole component. (Default: FALSE)

VARSTEP

The step size (meters) used in calculating the reference trajectory for VARRARDIUS = TRUE. It specifies how often the radius of curvature is re-calculated. This has a considerable effect on tracking time. (Default: 0.1 m)

Superposition of many multipole components is permitted. The reference system for a multipole is a Cartesian coordinate system for straight geometry and a (x,s,z) Frenet-Serret coordinate system for curved geometry. In the latter case, the axis \hat{s} is the central axis of the magnet.

The following example shows a combined function magnet with a dipole component of 2 Tesla and a quadrupole gradient of 0.1 Tesla/m.

M30:MULTIPOLET, L=1, RFRINGE=0.3, LFRINGE=0.2, ANGLE=PI/6, TP={2.0, 0.1}, VARRADIUS=TRUE;

The field expansion used in this model is based on the following scalar potential:

 V = z f_0(x,s) + \frac{z^3}{3!} f_1(x,s) + \frac{z^5}{5!} f_2(x,s) + \ldots

Mid-plane symmetry is assumed and the vertical component of the field on the mid-plane is given by the user under the form of the transverse profile T(x). The full expression for the vertical component is then

B_z = f_0 = T(x) \cdot S(s)

where S(s) is the fringe field. This element uses the Tanh model for the end fields, having only three parameters (the centre length s_0 and the fringe field lengths \lambda_{left}, \lambda_{right}):

 S(s) = \frac{1}{2} \left[ \tanh \left( \frac{s + s_0}{\lambda_{left}} \right) -
 \tanh \left( \frac{s - s_0}{\lambda_{right}} \right) \right]

Starting from Maxwell’s laws, the functions f_n are computed recursively and finally each component of the magnetic field is obtained from V using the corresponding geometries.

1.10. Solenoid

label:SOLENOID, TYPE=string, APERTURE=real-vector,
      L=real, KS=real;

A SOLENOID has two real attributes:

KS

The solenoid strength K_s=\frac{\partial B_s}{\partial s}, default is 0 \mathrm{Tm^{-1}}. For positive KS and positive particle charge, the solenoid field points in the direction of increasing s.

The reference system for a solenoid is a Cartesian coordinate system Using a solenoid in OPAL-t mode, the following additional parameters are defined:

FMAPFN

Field maps must be specified.

Example:

SP1: Solenoid, L=1.20, ELEMEDGE=-0.5265, KS=0.11,
     FMAPFN="1T1.T7";

1.11. Cyclotron

label:CYCLOTRON, TYPE=string, CYHARMON=int,
      PHIINIT=real, PRINIT=real, RINIT=real,
      SYMMETRY=real, RFFREQ=real, FMAPFN=string;

A CYCLOTRON object includes the main characteristics of a cyclotron, the magnetic field, and also the initial condition of the injected reference particle, and it has currently the following attributes:

TYPE

The data format of field map. Currently the following formats are implemented: RING (PSI format), CARBONCYCL, CYCIAE, AVFEQ, FFA and BANDRF. For the details of their data format, please read Field Maps.

CYHARMON

The harmonic number of the cyclotron h.

RFFREQ

The RF system f_{rf} (unit:MHz, default: 0). The particle revolution frequency f_{rev} = f_{rf} / h.

FMAPFN

File name for the magnetic field map. BSCALE: Scale factor for the magnetic field map.

SYMMETRY

Defines symmetrical fold number of the B field map data.

GEOMETRY

Defines the boundary geometry in order to use it for particle termination (see Chapter Geometry). The particles hitting on the GEOMETRY will be deleted, and they are recorded in the HDF5 file <GEOM>.h5 (or ASCII if ASCIIDUMP is true).

FMLOWE

Minimal energy [GeV] the fieldmap can accept. Used in GAUSSMATCHED distribution.

FMHIGHE

Maximal energy [GeV] the fieldmap can accept. Used in GAUSSMATCHED distribution.

RINIT

The initial radius of the reference particle (unit: mm, default: 0)

PHIINIT

The initial azimuth of the reference particle (unit: degree, default: 0)

ZINIT

The initial axial position of the reference particle (unit: mm, default: 0)

PRINIT

Initial radial momentum of the reference particle P_r=\beta_r\gamma (default : 0)

PZINIT

Initial axial momentum of the reference particle P_z=\beta_z\gamma (default : 0)

MINZ

The minimal vertical extent of the machine (unit: mm, default : -10000.0)

MAXZ

The maximal vertical extent of the machine (unit: mm, default : 10000.0)

MINR

Minimal radial extent of the machine (unit: mm, default : 0.0)

MAXR

Minimal radial extent of the machine (unit: mm, default : 10000.0)

During the tracking, the particle (r, z, \theta) will be deleted if MINZ < z < MAXZ or MINR < r < MAXR, and it will be recorded in the HDF5 file OUTFN.h5 (or ASCII if ASCIIDUMP is true), (see Common Attributes). Example:

ring: Cyclotron, TYPE="RING", CYHARMON=6, PHIINIT=0.0,
      PRINIT=-0.000240, RINIT=2131.4, SYMMETRY=8.0,
      RFFREQ=50.650, FMAPFN="s03av.nar",
      MAXZ=10, MINZ=-10, MINR=0, MAXR=2500;

If TYPE is set to BANDRF, the 3D electric field map of RF cavity will be read from external H5Hut file and the following extra arguments need to specified:

RFMAPFN

The file name(s) for the electric field map(s) in H5Hut binary format.

RFPHI

The initial phase(s) of the electric field map(s) (rad)

RFFREQ

The frequencies of the electric field maps. 0 indicates a constant field.

ESCALE

The scale factor(s) for the electric field map(s)

SUPERPOSE

An option whether the electric field map(s) is superposed (see also below).

Example for single electric field map:

COMET: Cyclotron, TYPE="BANDRF", CYHARMON=2, PHIINIT=-71.0,
PRINIT=pr0, RINIT=r0, SYMMETRY=1.0, FMAPFN="Tosca_map.txt",
RFPHI=Pi, RFFREQ=72.0,  RFMAPFN="efield.h5part",
ESCALE=1.06E-6;

We can have more than one RF field maps.

Example for multiple RF field maps:

COMET: Cyclotron, TYPE="BANDRF", CYHARMON=2, PHIINIT=-71.0,
PRINIT=pr0, RINIT=r0, SYMMETRY=1.0, FMAPFN="Tosca_map.txt",
RFPHI={Pi,0,Pi,0}, RFFREQ={72.0,72.0,72.0,72.0},
RFMAPFN={"e1.h5part","e2.h5part","e3.h5part","e4.h5part"},
ESCALE={1.06E-6, 3.96E-6,1.3E-6,1.E-6}, SUPERPOSE={true,false,false,true};

If SUPERPOSE is set to true and if a particle is located in the field region, the field is always applied. If SUPERPOSE is set to false, then only one field map with SUPERPOSE false is applied, the one which has highest priority, is used to do interpolation for the particle tracking. The priority ranking is decided by their sequence in the list of RFMAPFN argument, i.e., "e1.h5part" has the highest priority and "e4.h5part" has the lowest priority.

Another method to model an RF cavity is to read the RF voltage profile in the RFCAVITY element see RF Cavities (OPAL-t and OPAL-cycl) and make a momentum kick when a particle crosses the RF gap. In the center region of the compact cyclotron, the electric field shape is complicated and may make a significant impact on transverse beam dynamics. Hence a simple momentum kick is not enough and we need to read 3D field map to do precise simulation.

In addition, a trim-coil field model is also implemented to do fine tuning on the magnetic field. The trimcoils can be added with:

TRIMCOIL

Array of the trim coil names

A TRIMCOIL object can be defined in two ways:

TYPE

Type specifies PSI-BFIELD, PSI-PHASE or PSI-BFIELD-MIRRORED trim coil descriptions. The general PSI-BFIELD and PSI-PHASE descriptions are based on rational functions with polynomials in the nominator and the denominator. The function describes the magnetic field [T] resp. the phase shift as function of the radius [mm]. Separate functions can be given for the radial and azimuthal direction. These functions are multiplied together for the function. If a function in a direction is not specified it is the identity 1. The PSI-BFIELD-MIRRORED type is described in http://accelconf.web.cern.ch/AccelConf/ipac2017/papers/thpab077.pdf.

RMIN

Inner radius of the trim coil [mm]

RMAX

Outer radius of the trim coil [mm]

PHIMIN

Minimal azimuth [deg] (default 0) (not for PSI-BFIELD-MIRRORED)

PHIMAX

Maximal azimuth [deg] (default 360) (not for PSI-BFIELD-MIRRORED)

BMAX

Maximal B field of the trim coils [T] (for PSI-BFIELD) or maximal phase shift (for PSI-PHASE)

COEFNUM

Coefficients of the numerator for the radial direction, first coefficient is zeroth order. If COEFNUMPHI is not specified, the numerator is 1 (not for PSI-BFIELD-MIRRORED).

COEFDENOM

Coefficients of the denominator for the radial direction, first coefficient is zeroth order. If COEFDENOM is not specified, the denominator is 1, and the description will be a normal polynom (not for PSI-BFIELD-MIRRORED).

COEFNUMPHI

Coefficients of the numerator for the azimuthal direction, first coefficient is zeroth order. If COEFNUMPHI is not specified, the numerator is 1. (not for PSI-BFIELD-MIRRORED).

COEFDENOMPHI

Coefficients of the denominator for the azimuthal direction, first coefficient is zeroth order. If COEFDENOMPHI is not specified, the denominator is 1, and the description will be a normal polynom (not for PSI-BFIELD-MIRRORED).

SLPTC

Slopes of the rising edge [1/mm] (for PSI-BFIELD-MIRRORED type only)

Example:

tc1:  TRIMCOIL, TYPE="PSI-BFIELD-MIRRORED", RMIN = 2022.09, RMAX = 2132.09, BMAX=2.0e-4, SLPTC=1;
tc15: TRIMCOIL, TYPE="PSI-BFIELD",          RMIN = 3000,    RMAX = 4500,    BMAX=13e-4,
      COEFNUM   = {-0.426038643356, 0.311242287271, -0.0484487029431},
      COEFDENOM = {19.3541404562, -22.2057165548, 9.99489842329, -2.00909633025, 0.14942099903};

Ring: CYCLOTRON, TYPE="RINGCYC", CYHARMON=6, PHIINIT=0.0, PRINIT=0.0,
      RINIT=2131, SYMMETRY=8.0, RFFREQ=50.65, BSCALE=1, FMAPFN="s03av.nar",
      TRIMCOIL={tc1, tc15};

This is a restricted feature: OPAL-cycl.

1.12. FFA Magnet

OPAL supports two analytical field models that describe FFA magnets. SCALINGFFAMAGNET generates a sector FFA magnet that scales radially. VERTICALFFAMAGNET generates a vertical FFA magnet that scales vertically.

1.12.1. Scaling FFA Magnet

The scaling FFA magnet is a fully scaling field model that includes scaling fringe fields. A scaling FFA magnet has a field profile like

B_\phi = \sum_{n=0} f_{2n+1}(\psi) h(r) \left(\frac{z}{r}\right)^{2n+1}

B_r = \sum_{n=0}  \left[ \frac{k-2n}{2n+1} f_{2n} - \tan(\delta) f_{2n+1} \right] h(r) \left(\frac{z}{r}\right)^{2n+1}

B_z = \sum_{n=0} f_{2n}(\psi) h(r) \left(\frac{z}{r}\right)^{2n}

where r and z are cylindrical polar coordinates, \psi = \phi - \tan(\delta) \ln(r/r_0) is the azimuthal angle in the spiral coordinate system, delta, r_0 and k are geometrical constants that define the magnet field dependence and B_0 is the dipole field strength of the magnet at radius r_0. In OPAL, f_0 is a tanh function and higher order terms are chosen so as to satisfy Maxwell’s equations.

B0

The nominal dipole field of the magnet [T].

R0

Radial scale [m].

FIELD_INDEX

The scaling magnet field index.

TAN_DELTA

Tangent of the spiral angle; set to 0 to make a radial sector magnet.

MAX_Y_POWER

The maximum power in y that will be considered in the field expansion.

END_LENGTH

The end length of the spiral FFA [m].

HEIGHT

Full height of the magnet. Particles moving more than height/2. off the midplane (either above or below) are out of the aperture [m].

CENTRE_LENGTH

The centre length of the spiral FFA [m].

RADIAL_NEG_EXTENT

Particles are considered outside the tracking region if radius is less than R0-RADIAL_NEG_EXTENT [m].

RADIAL_POS_EXTENT

Particles are considered outside the tracking region if radius is greater than R0+RADIAL_POS_EXTENT [m].

MAGNET_START

Determines the position of the central portion of the magnet field relative to the element start (default is 2*end_length). [m].

MAGNET_END

Offset to the end of the magnet, i.e. placement of the next element. Default is centre_length + 4*end_length.

AZIMUTHAL_EXTENT

The field will be assumed zero if particles are more than AZIMUTHAL_EXTENT from the magnet centre (psi=0). Default is CENTRE_LENGTH/2.+5.*END_LENGTH [m].

This is a restricted feature: OPAL-cycl.

1.12.2. Vertical FFA Magnet

The VERTICALFFAMAGNET is a fully scaling field model that includes scaling fringe fields. A vertical FFA magnet has a field profile like

B_x = \sum_n B_0 \exp(mz) \frac{1}{m} \partial_x f_n y^n

B_y = \sum_n B_0 \exp(mz) \frac{n+1}{m} f_{n+1} y^n

B_z = \sum_n B_0 \exp(mz) f_n y^n

where m and B_0 are magnet parameters, f_0 is a tanh function and higher order terms are chosen so as to satisfy Maxwell’s equations. The field parameters can be specified in the OPAL input file using the following parameters

B0

The nominal dipole field of the magnet at z = 0, B_0 [T].

FIELD_INDEX

The scaling magnet field index, m [m^-1].

MAX_Y_POWER

The maximum power in y that will be considered in the field expansion.

END_LENGTH

The end length of the VFFA [m].

CENTRE_LENGTH

The centre length of the VFFA [m].

WIDTH

The full width of the magnet. Particles moving more than WIDTH/2 horizontally, in either direction, are considered out of the tracking region [m].

HEIGHT_NEG_EXTENT

Particles are considered outside the tracking region if height is less than HEIGHT_NEG_EXTENT [m].

HEIGHT_POS_EXTENT

Particles are considered outside the tracking region if height is greater than HEIGHT_POS_EXTENT [m].

BB_LENGTH

The total length of the bounding box. The magnet will be placed symmetrically in the bounding box [m].

VERTICALFFAMAGNET is rectangular; the next element will be placed BB_LENGTH from the start position of the VERTICALFFAMAGNET.

This is a restricted feature: OPAL-cycl.

1.13. Ring Definition

label: RINGDEFINITION,
       RFFREQ=real, HARMONIC_NUMBER=real, IS_CLOSED=string, SYMMETRY=int,
       LAT_RINIT=real, LAT_PHIINIT=real, LAT_THETAINIT=real,
       BEAM_PHIINIT=real, BEAM_PRINIT=real, BEAM_RINIT=real;

A RingDefinition object contains the main characteristics of a generalized ring. The RingDefinition lists characteristics of the entire ring such as harmonic number together with the position of the initial element and the position of the reference trajectory.

The RingDefinition can be used in combination with SBEND3D, offsets and VARIABLE_RF_CAVITY elements to make up a complete ring.

RFFREQ

Nominal RF frequency of the ring [MHz].

HARMONIC_NUMBER

The harmonic number of the ring - i.e. number of bunches in a single pass.

SYMMETRY

Azimuthal symmetry of the ring. Ring elements will be placed repeatedly SYMMETRY times.

IS_CLOSED

Set to FALSE to disable checking for ring closure.

LAT_RINIT

Radius of the first element placement in the lattice [m].

LAT_PHIINIT

Azimuthal angle of the first element placed in the lattice [degree].

LAT_THETAINIT

Angle in the mid-plane relative to the ring tangent for placement of the first element [degree].

BEAM_RINIT

Initial radius of the reference trajectory [m].

BEAM_PHIINIT

Initial azimuthal angle of the reference trajectory [degree].

BEAM_PRINIT

Transverse momentum \beta \gamma for the reference trajectory.

In the following example, we define a ring with radius 2.35 m and 4 cells.

ringdef: RINGDEFINITION, HARMONIC_NUMBER=6, LAT_RINIT=2350.0, LAT_PHIINIT=0.0,
         LAT_THETAINIT=0.0, BEAM_PHIINIT=0.0, BEAM_PRINIT=0.0,
         BEAM_RINIT=2266.0, SYMMETRY=4.0, RFFREQ=0.2;

1.13.1. Local Cartesian Offset

The LOCAL_CARTESIAN_OFFSET enables the user to place an object at an arbitrary position in the coordinate system of the preceding element. This enables drift spaces and placement of overlapping elements.

END_POSITION_X

x position of the next element start in the coordinate system of the preceding element [m].

END_POSITION_Y

y position of the next element start in the coordinate system of the preceding element [m].

END_NORMAL_X

x component of the normal vector defining the placement of the next element in the coordinate system of the preceding element [m].

END_NORMAL_Y

y component of the normal vector defining the placement of the next element in the coordinate system of the preceding element [m].

1.13.2. Local Cylindrical Offset

The LOCAL_CYLINDRICAL_OFFSET enables the user to place an object at an arbitrary position in the coordinate system of the preceding element in cylindrical coordinates. This enables drift spaces and placement of overlapping elements.

THETA_IN

Angle between the previous element and the displacement vector [rad].

THETA_OUT

Angle between the displacement vector and the next element [rad].

LENGTH

Length of the offset [m].

1.14. Source (OPAL-t)

Its first purpose is to indicate that the particles are emitted from a gun. This is needed to place the elements in three-dimensional space. Its second purpose is to delete impacting particles that are propagating in reverse direction. This function is optional and can be controlled with the parameter TRANSPARENT. The particles hitting on the source are recorded in the OUTFN file (see Common Attributes). The SOURCE element only works in OPAL-t.

TRANSPARENT

Boolean to indicate whether impacting particles can propagate further. Its default is FALSE such that the particles are deleted.

1.15. RF Cavities (OPAL-t and OPAL-cycl)

For an RFCAVITY the three modes have four real attributes in common:

label:RFCAVITY, APERTURE=real-vector, L=real,
      VOLT=real, LAG=real;
L

The length of the cavity (default: 0 m)

VOLT

The peak RF voltage (default: 0 MV). The effect of the cavity is \delta E=\mathrm{VOLT}\cdot\sin(2\pi(\mathrm{LAG}-\mathrm{HARMON}\cdot f_0 t)).

LAG

The phase lag [rad] (default: 0). In OPAL-t this phase is in general relative to the phase at which the reference particle gains the most energy. This phase is determined using an auto-phasing algorithm (see Appendix Auto-phasing Algorithm). This auto-phasing algorithm can be switched off, see APVETO.

DLAG

The phase lag error [rad] (default: 0).

1.15.1. OPAL-t mode

Using a RF Cavity in OPAL-t mode, the following additional parameters are defined:

FMAPFN

Field maps in the T7 format can be specified.

TYPE

Type specifies STANDING [default] or SINGLEGAP structures.

FREQ

Defines the frequency of the RF Cavity in units of MHz. A warning is issued when the frequency of the cavity card does not correspond to the frequency defined in the FMAPFN file. The frequency of the cavity card overrides the frequency defined in the FMAPFN file.

APVETO

If TRUE this cavity will not be auto-phased. Instead the phase of the cavity is equal to LAG at the arrival time of the reference particle (arrival at the limit of its field not at ELEMEDGE).

Example standing wave cavity which mimics a DC gun:

gun: RFCavity, L=0.018, VOLT=-131/(1.052*2.658),
     FMAPFN="1T3.T7", ELEMEDGE=0.00,
     TYPE="STANDING", FREQ=1.0e-6;

Example of a two frequency standing wave cavity:

rf1: RFCavity, L=0.54, VOLT=19.961, LAG=193.0/360.0,
     FMAPFN="1T3.T7", ELEMEDGE=0.129, TYPE="STANDING",
     FREQ=1498.956;
rf2: RFCavity, L=0.54, VOLT=6.250, LAG=136.0/360.0,
     FMAPFN="1T4.T7", ELEMEDGE=0.129, TYPE="STANDING",
     FREQ=4497.536;

1.15.2. OPAL-cycl mode

Using a RF Cavity (standing wave) in OPAL-cycl mode, the following parameters are defined:

FMAPFN

Name of file which stores normalized voltage amplitude curve of cavity gap in ASCII format. (See data format in RF field)

VOLT

Peak value of voltage amplitude curve in MV.

TYPE

Defines Cavity type, SINGLEGAP represents cyclotron type cavity.

FREQ

Frequency of the RF Cavity in units of MHz.

RMIN

Radius of the cavity inner edge in mm.

RMAX

Radius of the cavity outer edge in mm.

ANGLE

Azimuthal position of the cavity in global frame in degree.

PDIS

Perpendicular distance (impact parameter) of cavity from center of cyclotron in mm. If its value is positive, the radius increases clockwise (larger radius has smaller azimuthal angle).

GAPWIDTH

Set gap width of cavity in mm.

PHI0

Set initial phase of cavity in degree.

Example of a RF cavity of cyclotron:

rf0: RFCavity, VOLT=0.25796, FMAPFN="Cav1.dat",
     TYPE="SINGLEGAP", FREQ=50.637, RMIN = 350.0,
     RMAX = 3350.0, ANGLE=35.0,   PDIS = 0.0,
     GAPWIDTH = 0.0, PHI0=phi01;

Figure 5 shows the simplified geometry of a cavity gap and its parameters.

Cavity
Figure 5. Schematic of the simplified geometry of a cavity gap and parameters

1.16. RF Cavities with Time Dependent Parameters

The VARIABLE_RF_CAVITY element can be used to define RF Cavities with Time Dependent Parameters in OPAL-cycl mode. Variable RF Cavities must be placed using the RingDefinition element.

FREQUENCY_MODEL

String naming the time dependence model of the cavity frequency, f [MHz].

AMPLITUDE_MODEL

String naming the time dependence model of the cavity amplitude, E_0 [MV/m].

PHASE_MODEL

String naming the time dependence model of the cavity phase offset, \phi [rad].

WIDTH

Full width of the cavity [m].

HEIGHT

Full height of the cavity [m].

L

Full length of the cavity [m].

The field inside the cavity is given by

\mathbf{E} = \big(0, 0, E_0(t)\sin[2\pi f(t) t+\phi(t)]\big)

with no field outside the cavity boundary. There is no magnetic field or transverse dependence on electric field.

1.16.1. Time Dependence

Polynomial Time Dependence

The POLYNOMIAL_TIME_DEPENDENCE element is used to define time dependent parameters in RF cavities in terms of a third order polynomial.

P0

Constant term in the polynomial expansion.

P1

First order term in the polynomial expansion [ns^{-1}].

P2

Second order term in the polynomial expansion [ns^{-2}].

P3

Third order term in the polynomial expansion [ns^{-3}].

The polynomial is evaluated as

g(t) = p_0 + p_1 t + p_2 t^2 + p_3 t^3.

An example of a Variable Frequency RF cavity of cyclotron with polynomial time dependence of parameters is given below:

Spline Time Dependence

The SPLINE_TIME_DEPENDENCE element is used to define time dependent parameters in RF cavities in terms of a first or third order spline fit.

ORDER

Order of the lookup - either 1 for linear interpolation, or 3 for cubic interpolation with quadratic smoothing. Other values make an error.

TIMES

Array of real times in ns. There must be at least ORDER+1 elements in the array and they must be strictly monotonically increasing.

VALUES

Array of real values. The length of VALUES must be the same as the length of TIMES.

1.16.2. Fringe Field

It is possible to model a soft-edged RF cavity with time dependent parameters using the VARIABLE_RF_CAVITY_FRINGE_FIELD element. This will place a full cavity including the field body and fringe fields. VARIABLE_RF_CAVITY_FRINGE_FIELD must be placed using the RingDefinition element.

FREQUENCY_MODEL

String naming the time dependence model of the cavity frequency, f [MHz].

AMPLITUDE_MODEL

String naming the time dependence model of the cavity amplitude, E_0 [MV/m].

PHASE_MODEL

String naming the time dependence model of the cavity phase offset, \phi [rad].

WIDTH

Full width of the cavity [m].

HEIGHT

Full height of the cavity [m].

L

Full length of the cavity bounding box [m].

CENTRE_LENGTH

Length of the cavity field flat top [m].

END_LENGTH

E-fold Length of the cavity field ends [m].

CAVITY_CENTRE

Position of the centre of the cavity relative to the start [m].

MAX_ORDER

Maximum power in vertical coordinate z to which the field will be evaluated.

REAL phi=2.*PI*0.25;

REAL rf_p0=0.00158279;
REAL rf_p1=9.02542e-10;
REAL rf_p2=-1.96663e-16;
REAL rf_p3=2.45909e-23;

RF_FREQUENCY: POLYNOMIAL_TIME_DEPENDENCE, P0=rf_p0, P1=rf_p1,   P2=rf_p2, P3=rf_p3;
RF_AMPLITUDE: POLYNOMIAL_TIME_DEPENDENCE, P0=1.0;
RF_PHASE: POLYNOMIAL_TIME_DEPENDENCE, P0=phi;

HARD_RF_CAVITY: VARIABLE_RF_CAVITY,
           PHASE_MODEL="RF_PHASE", AMPLITUDE_MODEL="RF_AMPLITUDE",
           FREQUENCY_MODEL="RF_FREQUENCY", L=0.100, HEIGHT=0.200, WIDTH=2.000;

SOFT_RF_CAVITY: VARIABLE_RF_CAVITY_FRINGE_FIELD,
           PHASE_MODEL="RF_PHASE", AMPLITUDE_MODEL="RF_AMPLITUDE",
           FREQUENCY_MODEL="RF_FREQUENCY", L=0.200, HEIGHT=0.200, WIDTH=2.000
           CENTRE_LENGTH=0.1, END_LENGTH=0.01, CAVITY_CENTRE=0.1, MAX_ORDER=4;

1.17. Traveling Wave Structure

FINSB RAC field
Figure 6. The on-axis field of an S-band (2997.924 MHz) TRAVELINGWAVE structure. The field of a single cavity is shown between its entrance and exit fringe fields. The fringe fields extend one half wavelength (\lambda/2) to either side.

An example of a 1D TRAVELINGWAVE structure field map is shown in Figure 6. This map is a standing wave solution generated by Superfish and shows the field on axis for a single accelerating cavity with the fringe fields of the structure extending to either side. OPAL-t reads in this field map and constructs the total field of the TRAVELINGWAVE structure in three parts: the entrance fringe field, the structure fields and the exit fringe field.

The fringe fields are treated as standing wave structures and are given by:

\begin{aligned}
    \mathbf{E_{entrance}}(\mathbf{r}, t) &= \mathbf{E_{from-map}}(\mathbf{r}) \cdot \mathrm{VOLT} \cdot \cos \left( 2\pi \cdot \mathrm{FREQ} \cdot t
        + \phi_{entrance} \right) \\
    \mathbf{E_{exit}}(\mathbf{r}, t) &= \mathbf{E_{from-map}}(\mathbf{r}) \cdot \mathrm{VOLT} \cdot \cos \left( 2\pi \cdot \mathrm{FREQ} \cdot t
        + \phi_{exit} \right)
  \end{aligned}

where VOLT and FREQ are the field magnitude and frequency attributes (see below). \phi_{entrance}= \mathrm{LAG}, the phase attribute of the element (see below). \phi_{exit} is dependent upon both LAG and the NUMCELLS attribute (see below) and is calculated internally by OPAL-t.

The field of the main accelerating structure is reconstructed from the center section of the standing wave solution shown in Figure 6 using

\begin{aligned}
    \mathbf{E} ( \mathbf{r},t ) &= \frac{\mathrm{VOLT}}{\sin (2 \pi \cdot \mathrm{MODE})} \\
    & \times \Biggl\{ \mathbf{E_{from-map}} (x,y,z) \cdot \cos \biggl( 2 \pi \cdot \mathrm{FREQ} \cdot t + \mathrm{LAG}+ \frac{\pi}{2} \cdot \mathrm{MODE} \Bigr) + \\
    & \mathbf{E_{from-map}}(x,y,z+d) \cdot \cos \biggl( 2 \pi \cdot \mathrm{FREQ} \cdot t + \mathrm{LAG} + \frac{3 \pi}{2} \cdot \mathrm{MODE} \Bigr) \Biggr\}
  \end{aligned}

where d is the cell length and is defined as \text{d} = \lambda \cdot \mathrm{MODE} . MODE is an attribute of the element (see below). When calculating the field from the map (\mathbf{E_{from-map}}(x,y,z)), the longitudinal position is referenced to the start of the cavity fields at \frac{\lambda}{2} (In this case starting at z = {5.0}cm). If the longitudinal position advances past the end of the cavity map (\frac{3\lambda}{2} = {15.0}cm in this example), an integer number of cavity wavelengths is subtracted from the position until it is back within the map’s longitudinal range.

A TRAVELINGWAVE structure has seven real attributes, one integer attribute, one string attribute and one Boolean attribute:

label:TRAVELINGWAVE, APERTURE=real-vector, L=real,
      VOLT=real, LAG=real, FMAPFN=string,
      ELEMEDGE=real, FREQ=real, NUMCELLS=integer,
      MODE=real;
L

The length of the cavity (default: 0 m). In OPAL-t this attribute is ignored, the length is defined by the field map and the number of cells.

VOLT

The peak RF voltage (default: 0 MV). The effect of the cavity is \delta E=\mathrm{VOLT}\cdot\sin(\mathrm{LAG}- 2\pi\cdot\mathrm{FREQ}\cdot t).

LAG

The phase lag [rad] (default: 0). In OPAL-t this phase is in general relative to the phase at which the reference particle gains the most energy. This phase is determined using an auto-phasing algorithm (see Appendix Auto-phasing Algorithm). This auto-phasing algorithm can be switched off, see APVETO.

DLAG

The phase lag error [rad] (default: 0).

FMAPFN

Field maps in the T7 format can be specified.

FREQ

Defines the frequency of the traveling wave structure in units of MHz. A warning is issued when the frequency of the cavity card does not correspond to the frequency defined in the FMAPFN file. The frequency defined in the FMAPFN file overrides the frequency defined on the cavity card.

NUMCELLS

Defines the number of cells in the tank. (The cell count should not include the entry and exit half cell fringe fields.)

MODE

Defines the mode in units of 2\pi, for example \frac{1}{3} stands for a \frac{2 \pi}{3} structure.

FAST

If FAST is true and the provided field map is in 1D then a 2D field map is constructed from the 1D on-axis field, see Fieldmaps Types and Format. To track the particles the field values are interpolated from this map instead of using an FFT based algorithm for each particle and each step. (default: FALSE)

APVETO

If TRUE this cavity will not be auto-phased. Instead the phase of the cavity is equal to LAG at the arrival time of the reference particle (arrival at the limit of its field not at ELEMEDGE).

Use of a traveling wave requires the particle momentum P and the particle charge CHARGE to be set on the relevant optics command before any calculations are performed.

Example of a L-Band traveling wave structure:

lrf0: TravelingWave, L=0.0253, VOLT=14.750,
      NUMCELLS=40, ELEMEDGE=2.73066,
      FMAPFN="INLB-02-RAC.Ez", MODE=1/3,
      FREQ=1498.956, LAG=248.0/360.0;

1.18. Monitor

A MONITOR detects all particles passing it and writes the position, the momentum and the time when they hit it into an H5hut file. Furthermore the exact position of the monitor is stored. It has always a length of 1 cm consisting of 0.5 cm drift, the monitor of zero length and another 0.5 cm drift. This is to prevent OPAL-t from missing any particle. The positions of the particles on the monitor are interpolated from the current position and momentum one step before they would passe the monitor.

The attribute OUTFN defines the file into which the monitor should write the collected data (see Common Attributes). The file is an H5hut file.

If the attribute TYPE is set to TEMPORAL then the data of all particles are written to the H5hut file when the reference particle hits the monitor.

This is a restricted feature for OPAL-t.

1.19. Collimators

Four types of collimators are defined:

ECOLLIMATOR

Elliptic aperture,

RCOLLIMATOR

Rectangular aperture.

FLEXIBLECOLLIMATOR

Description of shape and location of holes can be provided

CCOLLIMATOR

Radial rectangular collimator in cyclotrons

label:ECOLLIMATOR, TYPE=string, APERTURE=real-vector,
      L=real, XSIZE=real, YSIZE=real;

label:RCOLLIMATOR,TYPE=string, APERTURE=real-vector,
      L=real, XSIZE=real, YSIZE=real;

label:FLEXIBLECOLLIMATOR, APERTURE=real-vector,
      L=real, DESCRIPTION=string, FNAME=string, OUTFN=string;

Each type has the following general attributes (available for both OPAL-t and OPAL-cycl collimators):

OUTFN

The file name into which the collimator should write the collected data. If this attribute is empty, the file will be named as the element label. The file is an H5hut file (or ASCII if ASCIIDUMP is true).

PARTICLEMATTERINTERACTION

PARTICLEMATTERINTERACTION is an attribute of the element (see Chapter Particle Matter Interaction). TYPE=SCATTERING must be selected to include scattering interactions and energy loss calculation through the MATERIAL definition (see Available Materials in OPAL). If this is not set, the particle-matter interaction module will not be activated. Then, the particle hitting the collimator will be recorded and directly deleted from the simulation.

The reference system for a collimator is a Cartesian coordinate system.

1.19.1. OPAL-t mode

Optically a collimator behaves like a drift space, but during tracking, it also introduces an aperture limit. The aperture is checked at the entrance. If the length is not zero, the aperture is also checked at the exit and at every timestep. Lost particles are saved in an H5hut file defined by OUTFN. The ELEMEDGE defines the location of the collimator and L the length.

ECOLLIMATORs and RCOLLIMATORs detect all particles which are outside the aperture defined by XSIZE and YSIZE. The CCOLLIMATOR isn’t supported.

XSIZE

The horizontal half-aperture (default: unlimited).

YSIZE

The vertical half-aperture (default: unlimited).

For elliptic apertures, XSIZE and YSIZE denote the half-axes respectively, for rectangular apertures they denote the half-width of the rectangle.

Example:

Col:ECOLLIMATOR, L=1.0E-3, ELEMEDGE=3.0E-3, XSIZE=5.0E-4,
    YSIZE=5.0E-4, OUTFN="Coll.h5";

The FLEXIBLECOLLIMATOR can be used to model both simple, rectangular or elliptic collimators and more complex devices like pepper-pots. The configuration of holes can be described with a special language. This language knows the following commands

rectangle(width, height)

A rectangle that is centered at the origin of the 2D coordinate system. The arguments width and heigth can be mathematical expressions.

ellipse(width, height)

An ellipse that is centered at the origin of the 2D coordinate system. The arguments width and heigth can be mathematical expressions.

polygon(x_0, y_0; x_1, y_1; x_2, y_2[; x_3, y_3[;…​ x_N, y_N]])

A polygon with with vertices (x_0, y_0), (x_1, y_1), (x_2, y_2), …​, (x_N, y_N). The first vertex doens’t have to be repeated, instead (x_N, y_N) is connected with (x_0, y_0). The polygon is then triangulized for a fast detection of stopped particles. In order for the triangulization to work the edges of the polygon may not cross each other. All arguments of the command polygon can be mathematical expressions.

mask('filename.pbm', width, height)

A black and white bitmap file (Portable Bitmap format) can be provided to describe the collimator. White pixels stop particles. The first argument is the path to the pixmap file, the second and third are the width and height of the mask in meters. The arguments width and height can be mathematical expressions.

translate(command, shiftx, shifty)

Translates the holes that are define by the command by shiftx in the x-direction and shifty in the y-direction. The arguments shiftx and shifty can be mathematical expressions.

rotate(command, angle)

Rotates the holes that are defined by the command about the origin of the 2D coordinate system. The argument angle can be a mathematical expression.

union(command1, command2 [, command3 [, command4 […​]]])

Collects the holes that are defined the by the commands.

difference(command1, command2)

All particles that pass command1 and not command2 pass the difference.

difference
Figure 7. Illustration of a difference between to circles
symmetric_difference(command1, command2)

All particles that pass either command but not both at the same time.

symmetric difference
Figure 8. Illustration of a symmetric difference between to circles
intersection(command1, command2)

All particles that pass both commands at the same time.

intersection
Figure 9. Illustration of a intersection between to circles
repeat(command, N, shiftx, shifty)

Repeats the holes that are defined by the command translating each copy successively by shiftx in x-direction and shifty in y-direction. The arguments shiftx and shifty can be mathematical expressions.

repeat(command, N, angle)

Repeats the holes that are defined by the command rotating each copy successively. The argument angle can be a mathematical expression.

The supported mathematical constants and functions are listed in the following table.

Table 1. Mathematical constants and functions

e

pi

abs(x)

acos(x)

acosh(x)

asin(x)

asinh(x)

atan(x)

atanh(x)

cbrt(x)

ceil(x)

cos(x)

cosh(x)

deg2rad(x)

erf(x)

erfc(x)

exp(x)

exp2(x)

floor(x)

isinf(x)

isnan(x)

log(x)

log2(x)

log10(x)

rad2deg(x)

round(x)

sgn(x)

sin(x)

sinh(x)

sqrt(x)

tan(x)

tanh(x)

tgamma(x)

atan2(y,x)

max(x,y)

min(x,y)

A simple elliptic collimator with major and minor axis of 4 cm and 3 cm respectively can be defined using

ellipse(0.04, 0.03)

A regular pepper-pot with rectangular holes can be define like this

repeat( // repeat it in y-direction
       repeat( // repeat it in x-direction
              translate(
                        rotate(
                               rectangle(
                                         0.002,
                                         0.002
                                        ),
                               0.78539
                              ),
                        -0.028,
                        -0.028
                       ),
              16,
              0.004,
              0.0
             ),
       16,
       0.0,
       0.004
      )

The latter example will produce a holes as in the following picture

pepperpot
Figure 10. Pepper-pot with rectangle holes

In the FLEXIBLECOLLIMATOR command the description of the holes can be provided as a string (using DESCRIPTION; the string may not contain comments and newlines) or in a separate file (using FNAME; comments and newlines are allowed).

1.19.2. OPAL-cycl mode

Only CCOLLIMATOR is available for OPAL-cycl. This element is radial rectangular collimator which can be used to collimate the radial tail particles. When a particle hits this collimator, it will be absorbed or scattered. The algorithm is based on the Monte Carlo method. Please note when a particle is scattered, it will not be recorded as the lost particle. If this particle leaves the bunch, it will be removed during the integration afterwards, so as to maintain the accuracy of space charge solving. In addition to the general attributes of a collimator (OUTFN and PARTICLEMATTERINTERACTION), the parameters for describing a CCOLLIMATOR are the following:

XSTART

The x coordinate of the start point. [mm]

XEND

The x coordinate of the end point. [mm]

YSTART

The y coordinate of the start point. [mm]

YEND

The y coordinate of the end point. [mm]

ZSTART

The minimum vertical coordinate [mm]. Default value is -100mm.

ZEND

The maximum vertical coordinate. [mm]. Default value is 100mm.

WIDTH

The width of the collimator. [mm]

collimator
Figure 11. Collimator

Example:

REAL y1=-0.0;
REAL y2=0.0;
REAL y3=200.0;
REAL y4=205.0;
REAL x1=-215.0;
REAL x2=-220.0;
REAL x3=0.0;
REAL x4=0.0;

cmphys: PARTICLEMATTERINTERACTION, TYPE=SCATTERING, MATERIAL="Copper";

cma1: CCollimator, XSTART=x1, XEND=x2,YSTART=y1, YEND=y2,
      ZSTART=2, ZEND=100, WIDTH=10.0, PARTICLEMATTERINTERACTION=cmphys ;

cma2: CCollimator, XSTART=x3, XEND=x4,YSTART=y3, YEND=y4,
      ZSTART=2, ZEND=100, WIDTH=10.0, PARTICLEMATTERINTERACTION=cmphys;

The particles lost on the CCOLLIMATOR are recorded in the HDF5 file OUTFN.h5 (or ASCII if ASCIIDUMP is true).

1.20. Septum (OPAL-cycl)

This is a restricted feature for OPAL-cycl. The particles hitting on the septum are removed from the bunch and recorded in the OUTFN file. There are 5 parameters to describe a septum.

XSTART

The x coordinate of the start point. [mm]

XEND

The x coordinate of the end point. [mm]

YSTART

The y coordinate of the start point. [mm]

YEND

The y coordinate of the end point. [mm]

WIDTH

The width of the septum. [mm]

septum
Figure 12. Septum

Example:

eec2: Septum, xstart=4100.0, xend=4300.0,
ystart=-1200.0, yend=-150.0, width=0.05;

The particles lost on the SEPTUM are recorded in the HDF5 file OUTFN.h5 (or ASCII if ASCIIDUMP is true).

1.21. Probe (OPAL-cycl)

The particles hitting on the probe are recorded in the OUTFN file (see Common Attributes). There are 5 parameters to describe a probe.

XSTART

The x coordinate of the start point. [mm]

XEND

The x coordinate of the end point. [mm]

YSTART

The y coordinate of the start point. [mm]

YEND

The y coordinate of the end point. [mm]

STEP

The step size of the probe (for histogram and peak finder output). Default: 1 [mm]

probe
Figure 13. Probe

Example:

prob1: Probe, xstart=4166.16, xend=4250.0,
ystart=-1226.85, yend=-1241.3;

The particles probed on the PROBE are recorded in the HDF5 file OUTFN.h5 (or ASCII if ASCIIDUMP is true). Please note that these particles are not deleted in the simulation, however, they are recorded in the "loss" file.

The radius of the particles recorded in the PROBE is recorded in the histogram ".hist" and peak ".peaks" file. The histogram file contains data as recorded in actual probe measurements. The corresponding peaks file contains the peaks found in the probe histogram by the same peak finder used for the PSI measurements. Note that for probes in multiple quadrants the histogram and peaks file is often not meaningful since the absolute radius is stored.

1.22. Stripper (OPAL-cycl)

A stripper element strip the electron(s) from a particle. The particle hitting the stripper is recorded in the OUTFN file (see Common Attributes), which contains the time, coordinates and momentum of the particle at the moment it hit the stripper. The charge and mass are changed. It has the same geometry as the PROBE element. Please note that the stripping physics is not included yet.

There are 9 parameters to describe a stripper.

XSTART

The x coordinate of the start point. [mm]

XEND

The x coordinate of the end point. [mm]

YSTART

The y coordinate of the start point. [mm]

YEND

The y coordinate of the end point. [mm]

OPCHARGE

Charge number of the outcoming particle. Negative value represents negative charge.

OPMASS

Mass of the outcoming particles. [\mathrm{GeV/c^2}]

OPYIELD

Yield of the outcoming particle (the number of outcoming particles per incoming particle), the default value is 1.

STOP

If STOP is true, the particle is stopped and deleted from the simulation; Otherwise, the outcoming particle continues to be tracked along the extraction path.

Example: H_2^+ particle stripping

prob1: Stripper, xstart=4166.16, xend=4250.0,
ystart=-1226.85, yend=-1241.3,
opcharge=1, opmass=PMASS, opyield=2, stop=false;

No matter what the value of STOP is, the particles hitting on the STRIPPER are recorded in the HDF5 file OUTFN.h5 (or ASCII if ASCIIDUMP is true).

1.23. Degrader (OPAL-t)

Elliptical degrader with an overall length L. The particles lost on a degrader are recorded in the OUTFN file (see Common Attributes).

XSIZE

Major axis of the transverse elliptical shape, default value is 1e6.

YSIZE

Minor axis of the transverse elliptical shape, default value is 1e6.

PARTICLEMATTERINTERACTION

PARTICLEMATTERINTERACTION is an attribute of the element (see Chapter Particle Matter Interaction). TYPE=SCATTERING must be selected to include scattering interactions and energy loss calculation through the MATERIAL definition (see Available Materials in OPAL). If this is not set, the particle-matter interaction module will not be activated. The particle hitting degrader will be recorded and directly deleted from the simulation.

Example: Graphite degrader of 15 cm thickness.

DEGPHYS: PARTICLEMATTERINTERACTION, TYPE=SCATTERING, MATERIAL="Graphite";

DEG1: DEGRADER, L=0.15, ELEMEDGE=0.02, PARTICLEMATTERINTERACTION=DEGPHYS;

1.24. Correctors (OPAL-t)

Three types of correctors are available:

HKICKER

A corrector for the horizontal plane.

VKICKER

A corrector for the vertical plane.

KICKER

A corrector for both planes.

They act as

label:HKICKER, TYPE=string, APERTURE=real-vector,
      L=real, KICK=real;
label:VKICKER, TYPE=string, APERTURE=real-vector,
      L=real, KICK=real;
label:KICKER, TYPE=string, APERTURE=real-vector,
      L=real, HKICK=real, VKICK=real;

They have the following attributes:

L

The length of the closed orbit corrector (default: 0 m).

KICK

The kick angle in rad for either horizontal or vertical correctors (default: 0 rad).

HKICK

The horizontal kick angle in rad for a corrector in both planes (default: 0 rad).

VKICK

The vertical kick angle in rad for a corrector in both planes (default: 0 rad).

DESIGNENERGY

Fix the magnitude of the magnetic field using the given DESIGNENERGY and the angle (KICK, HKICK or VKICK). If the design energy isn’t set then the actual energy of the reference particle at the position of the corrector is used. The DESIGNENERGY is expected in MeV.

A positive kick increases p_{x} or p_{y} respectively. Use KICK for an HKICKER or VKICKER and HKICK and VKICK for a KICKER. Instead of using a KICKER or a VKICKER one could use an HKICKER and rotate it appropriately using PSI.

Correctors don’t change the reference trajectory. Otherwise they are implemented as RBEND with \mathrm{E1} = 0 and without fringe fields (hard edge model). They can be used to model earth’s magnetic field which is neglected in the design trajectory but which has a noticeable effect on the trajectory of a bunch at low energies.

Examples:

HK1:HKICKER, KICK=0.001;
VK3:VKICKER, KICK=0.0005;
KHV:KICKER, HKICK=0.001, VKICK=0.0005;

The reference system for an orbit corrector is a Cartesian coordinate system.

1.25. Vacuum

Vacuum element represents the conditions and parameters to consider interactions with the residual gas and the magnetic field. When the particle interacts, it is recorded in the file, which contains the time, coordinates and momentum of the particle at this moment. The particle could produce a new particle, changing the charge and mass.

In OPAL-cycl the vacuum region is defined according to the cyclotron boundaries (MINZ, MAXZ, MINR and MAXR, see CYCLOTRON element), whereas in OPAL-t is described by L and ELEMEDGE parameters (see Common Attributes).

There are 7 specific parameters to describe the vacuum space.

PRESSURE

The average pressure of the residual gas. [mbar]

TEMPERATURE

Temperature of residual gas. [K]

PMAPFN

File name of the mid-plane pressure map. This feature is only available for OPAL-cycl. The pressure data is stored in a sequence shown in 2D field map on the median plane with primary direction corresponding to the azimuthal direction, secondary direction to the radial direction (same file structure as Cyclotron TYPE=CARBONCYCL). If PMAPFN is specified, PRESSURE parameter is taken as default value for regions in the accelerator out of the limits of the pressure map.

PSCALE

Scale factor for the pressure field map (default: 1.0). Only available for OPAL-cycl.

GAS

Type of gas for residual vacuum: H2 or AIR

STOP

If STOP is true, the particle is stopped and deleted from the simulation. Otherwise, the outcoming particle (according to the cross section evaluation) continues to be tracked as SECONDARY particle (default: true).

PARTICLEMATTERINTERACTION

PARTICLEMATTERINTERACTION is an attribute of the element (see Chapter Particle Matter Interaction). TYPE=BEAMSTRIPPING must be selected to include stripping interactions with the residual gas and Lorentz stripping.

Example: Vacuum representation with H_2 residual gas.

bstp_phys: PARTICLEMATTERINTERACTION, TYPE=BEAMSTRIPPING;

vac: VACUUM, PRESSURE=1E-8, TEMPERATURE=300,
      GAS="H2", STOP=true, PARTICLEMATTERINTERACTION=bstp_phys;

No matter what the value of STOP is, the particles stripped are recorded in the HDF5 file (or ASCII if ASCIIDUMP is true).

1.26. Undulator (OPAL-t)

OPAL's undulator element comes with its own Finite-Difference Time-Domain full-wave solver, which accounts for space-charge and radiation effects in 3D. It was implemented by means of the MITHRA library, developed by Arya Fallahi.

To use the undulator element and its solver, one needs to compile OPAL in the following way:

  1. install the MITHRA 2.0 library,

  2. set the environment variable MITHRA_PREFIX=directory/where/you/store/mithra

  3. compile OPAL with the option cmake -DENABLE_OPAL_FEL=yes ..

When the head of the bunch crosses the start of an undulator in the beamline (defined by ELEMEDGE), the solver changes automatically from Poisson to full-wave, and changes back to Poisson once the bunch has passed through the whole undulator and its fringe fields.

The length of the undulator is defined as

\begin{aligned}
    &L = N\lambda + 2\,\text{fringe},\\
    &\text{fringe} = 2\lambda,
\end{aligned}

where \lambda is the undulator period and N its number of periods. Since the element’s length is entirely defined by the undulator periods, there is no LENGTH parameter to be specified for the undulator element.

The magnetic field is that of a planar undulator with flat pole faces:

\begin{aligned}
    &B_x = B_0\cosh(kr)\sin(kz)\cos(\alpha),\\
    &B_y = B_0\cosh(kr)\sin(kz)\sin(\alpha),\\
    &B_z = B_0\sinh(kr)\cos(kz),
\end{aligned}

where r = x\cos(\alpha) + y\sin(\alpha) is the radial distance from the undulator axis, k=2\pi/\lambda the wave-number, \alpha the angle between the magnetic field polarisation and the x-axis, and B_0 the maximum magnetic field value.

The fringe fields are defined as:

\begin{aligned}
    &B_x = B_0\cosh(kr)kze^{-(kz)^2/2}\cos(\alpha),\\
    &B_y = B_0\cosh(kr)kze^{-(kz)^2/2}\sin(\alpha),\\
    &B_z = B_0\sinh(kr)e^{-(kz)^2/2}.
\end{aligned}

The parameters that describe the undulator element and its associated full-wave solver are as follows:

K

The undulator strength parameter.

LAMBDA

The undulator period \lambda [m].

NUMPERIODS

Number of periods N.

ANGLE

Angle \alpha between the magnetic field polarisation and the x-axis [rad] (default: 0).

MESHLENGTH

Size in three dimensions (L_x, L_y, L_z) of the computational grid [m], which is in a frame of reference moving at the average speed of the bunch. It should be large enough to contain the whole bunch, as particles outside of it will not perceive any fields. As a rule of thumb, the grid should be 3 times longer than the bunch, since the bunch will slightly shift longitudinally when entering and exiting the undulator, and 10 times wider than the bunch, to avoid spurious radiation reflections, since the Absorbing Boundary Conditions (ABCs) cannot correctly absorb obliquely incident waves.

MESHRESOLUTION

Grid-spacing (\Delta_x, \Delta_y, \Delta_z) of the computational domain [m].

DTBUNCH

Time-step for the particle update [s]. By default it is equal to the field update time-step, which is automatically chosen by the algorithm in order to satisfy the stability conditon. DTBUNCH needs to be equal to or smaller than the field time-step.

TRUNORDER

Truncation order of the ABCs. Can be 1 or 2 (default: 2).

TOTALTIME

Total time to simulate using the full-wave solver [s]. By default this is set such that the whole passage through the undulator is simulated.

FNAME

File specifying which output the full-wave solver should provide. It is equivalent to the job-file in MITHRA, without the parameters MESH, bunch-initialization, and UNDULATOR. This file is optional.

Example of the wiggler element used in the AWA beamline:

UND: UNDULATOR, ELEMEDGE = 44.0e-2, K = 10.81, LAMBDA = 8.5e-2, NUMPERIODS = 10, ANGLE = PI/2,
     MESHLENGTH = { 12e-3, 12e-3, 4e-3 }, MESHRESOLUTION = { 1e-5, 1e-5, 8e-6},
     FNAME = "wiggler_sims_July_2020/mithra_output.job";

1.27. References

[2] J. E. Spencer and H. A. Enge, Split-pole magnetic spectrograph for precision nuclear spectroscopy, Nucl. Instrum. Methods 49, 181 (1967).