The Beam Description File
The beam description file contains a table of beam parameters such as emittance and energy at certain positions along the bunch. The positions listed in the file must be ordered starting with the tail. In GENESIS 1.3 smaller number refers to a position towards the tail of the bunch. Note that you have to adjust the simulation time-window in GENESIS 1.3 (ZSEP, NSLICE and NTAIL) to the range, defined in the beam description file. Genesis 1.3 uses relative positions within the time-window, starting at zero for the first entry in the beam description file. This corresponds to NTAIL = 0.
Each line can contain up to 15 parameters: corresponding to the input parameters of the main input deck:
Besides ZPOS and TPOS, which refers to the position within the bunch, most of the remaining parameters correspond to the input parameter of the main input file. If BETAX is defined instead of RXBEAM the beam size is calculated, using the specified beam energy and emittance. The same is valid for BETAY.
If a time coordinate specifies the position (TPOS) instead of a space coordinate (ZPOS), Genesis 1.3 enforces the convention that time and space coordinate differ with a relative sign. For the right order in the beam description file the ZPOS values should increment, while the TPOS should decremnt
The format of the beam description file is similar to the magnetic field input file. All numbers are given in ASCII format. The input file can contain content lines, comment lines and information lines. Commands lines are not needed and thus not supported.
Empty lines and lines starting with a #-sign are treated as comment lines. They are ignored while processing the beam description file.
A question mark indicates an information line. The VERSION-keyword is the same as in the magnetic field input file, defining the version of the input file. Note that there is a difference between earlier versions (<1.0) and the current version 1.0 (see below). The SIZE parameter is optional. It should contain the number of information lines in the input file. At the end of processing the file GENESIS 1.3 compares the number of valid lines with the number defined by SIZE and prints out a warning message if the two numbers differs. If no SIZE statement is included the check is omitted.
The last information keyword is COLUMNS. It controls the number and order of columns in the beam description file by simply adding the parameter names after the COLUMNS keyword. Parameters, which are excluded in the beam description file, fall back to their default value, defined in the main input deck. Omitting the current results in a step profile. The column ZPOS is mandatory and GENESIS 1.3 with given an error message when it is not specified in the COLUMN information line. If no COLUMNS-statement is given all input parameter are required in the order as they appear in the list above.
As an example the following line describes a 100 m long, uniform beam with an energy chirp of 1% along the beam.
Any version before 1.0 required that the number of input lines, containing the beam parameters, must be listed in the beginning of the file as the first information line before the actual data lines started. To guarantee backwards compatibility this is still valid if the version is no defined to be 1.0 and the SIZE-statement is omitted. The example above would be then
Again the specified value (here 2) is only used to check with the number of data lines, which is processed by GENESIS 1.3 without any error.
The beam description file can also be used for a scan (NSCAN = 23,24). Each line correspond to a run of the scan. The meaning of the ZPOS column becomes obsolete. However it is still required for a correct parsing of the file.