Particles stored in trackOrbit.dat
According to the OPAL manual (pag. 40):
Multi-particle tracking mode
The intermediate phase space data of centeral particle (with ID of 0) and an off-centering particle (with ID of 1) are stored in an ASCII file.
Concerning the particle with ID0:
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particle position is not updated in case OFFSETY > 0 is set in the distribution definition. The tracking of this particle does not reflect the beam behavior (because of the offset)
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Is the general idea: ID0 particle = reference particle?
Concerning the particle with ID1:
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Distribution from file: the second particle in the distribution file is used as ID1
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Generated distribution: It seems that a random particle from the distribution is set as ID1
A possible suggestion:
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Distribution from file: the first and the second particle in the file are used as ID0 and ID1, respectively. The user is completely free to decide which particles track.
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Generated distribution (Option 1): ID0 is by default assigned to the reference particle (with updated offset). An option can be added to the DISTRIBUTION command where the user can specify which particle uses as ID1 (ie: DISPERSION, CENTROID or USERDEF or NULL = not stored). This means that the first two particles generated by OPAL are replaced with ID0 (reference) and ID1, if option NULL is not specified.
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Generated distribution (Option 2): An option can be added to the DISTRIBUTION command where the user can specify which particle uses as ID0 and ID1 (ie: DISPERSION, CENTROID or USERDEF or or NULL = not stored)
A possible problem could arise in case of multi-distribution or vector of distribution