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[[chp:lines]]
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Beam Lines
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----------
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The accelerator to be studied is known to _OPAL_ as a sequence of
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physical elements called a *beam line*. A beam line is built from
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simpler beam lines whose definitions can be nested to any level. A
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powerful syntax allows to repeat or to reflect pieces of beam lines.
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Formally a beam line is defined by a `LINE` command:
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....
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label:LINE=(member,...,member);
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....
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label see Section [label] gives a name to the beam line for later
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reference.
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Each `member` may be one of the following:
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* An element label,
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* A beam line label,
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* A sub-line, enclosed in parentheses,
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Beam lines can be nested to any level.
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[[simple-beam-lines]]
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Simple Beam Lines
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~~~~~~~~~~~~~~~~~
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The simplest beam line consists of single elements:
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....
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label:LINE=(member,...,member);
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....
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Example:
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....
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L:LINE=(A,B,C,D,A,D);
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....
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ORIGIN::
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Position vector of the origin of the line. All elements in this line
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that are placed using `ELEMEDGE` use this position as reference.
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ORIENTATION::
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Vector of Tait-Bryan angles [bib:tait-bryan] of the orientation of the
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line at the origin.
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[[sub-lines]]
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Sub-lines
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~~~~~~~~~
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Instead of referring to an element, a beam line member can refer to
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another beam line defined in a separate command. This provides a
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shorthand notation for sub-lines which occur several times in a beam
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line. Lines and sub-lines can be entered in any order, but when a line
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is used, all its sub-lines must be known.
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Example:
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....
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L:LINE=(A,B,S,B,A,S,A,B);
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S:LINE=(C,D,E);
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....
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This example produces the following expansion steps:
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1. Replace sub-line `S`:
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+
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....
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(A,B,(C,D,E),B,A,(C,D,E),A,B)
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....
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2. Omit parentheses:
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+
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....
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A,B,C,D,E,B,A,C,D,E,A,B
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....
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valuated to constants immediately. |