- type: major action command.
- function: determine momentum aperture as a function of position in the lattice by tracking
- can use parallel resources (Pelegant)
- Command syntax, including use of equations and subcommands, is discussed in 7.2.
STRING output = NULL;
double x_initial = 0;
double y_initial = 0;
double delta_negative_start = 0.0;
double delta_positive_start = 0.0;
double delta_negative_limit = -0.10;
double delta_positive_limit = 0.10;
double delta_step_size = 0.01;
long steps_back = 1;
long splits = 2;
long split_step_divisor = 10;
long skip_elements = 0;
long process_elements = 2147483647;
double s_start = 0;
double s_end = DBL_MAX;
STRING include_name_pattern = NULL;
STRING include_type_pattern = NULL;
long fiducialize = 0;
long verbosity = 1;
long soft_failure = 0;
long output_mode = 0;
long forbid_resonance_crossing = 0;
- output — The (incomplete) name of a file to which the momentum aperture results will be
written. Recommended value: “%s.mmap”. The data are related to the momentum aperture
at the exit of the named elements.
- x_initial, y_initial — The initial x and y coordinate values for tracking. It is essential
that y_initial be nonzero if one wants to see losses due to vertical resonances.
- delta_negative_start, delta_positive_start — Starting values of scans in the negative
and positive directions.
- delta_negative_limit, delta_positive_limit — Limiting values of scans in the negative
and positive directions.
- delta_step_size — Initial size of steps in δ. This should be fairly large in order to save
- steps_back — Number of steps to back up after a particle is lost, relative to the last surviving
δ, before continuing with a smaller step size. If this is set to zero, there is a risk of finding a
too-large momentum aperture (a stable island).
- splits — Number of times to split the step size in order to refine the location of the maximum
surviving momentum offsets. When a particle is lost, the algorithm steps back to a momentum
offset where a particle survived, subdivides the step size, and continues searching.
- split_step_divisor — Factor by which to subdivide the step size for each split.
- skip_elements — Number of elements to skip before starting to compute momentum
- process_elements — Number of elements for which to compute momentum aperture.
- s_start, s_end — Limiting s coordinates of the elements from which tracking will start. The
default values will exclude no elements.
- include_name_pattern — If given, tracking will start only at the entrance to elements that
match the given wildcard pattern.
- include_type_pattern — If given, tracking will start only at the entrance to elements whose
type matches the given wildcard pattern.
- fiducialize — If given, an initially on-energy particle is tracked before the momentum
aperture search begins, in order to fiducialize the reference momentum. This is useful if there
are synchrotron radiation losses or energy gain due to cavities in the system.
- verbosity — Larger values result in more detailed printouts as calculations proceed. Mostly
- soft_failure — Normally, if elegant fails to find the momentum aperture, it aborts. If
soft_failure is non-zero, it instead assigns a momentum aperture equal to the search limit.
- output_mode — Normally, elegant puts the values for positive and negative momentum
aperture in different columns. Each element thus has a single row of data in the output
file. If output_mode=1, elegant instead puts the values for positive and negative apertures
in successive rows, with a reduced number of columns. This is mostly advantageous
for the parallel version, since it allows using twice as many simultaneous processors.
If output_mode=2, elegant tracks many more probe particles simultaneously, which is
better for massively parallel systems. The number of particles tracked is the number of
elements selected times the number of probe points between delta_negative_limit and
- forbid_resonance_crossing — Normally, elegant allows the momentum aperture search
to cross integer and half-integer resonances if no unstable particles are found. If this is
undesirable, this flag can be set to 1.
The idea for this command is from M. Belgroune et al., “Refined Tracking Procedure for the SOLEIL
Energy Acceptance Calculation,” Proceedings of PAC 2003, p 896, as implemented for TRACYII. In
particular, the energy aperture as a function of position around the ring is determined by tracking.
Starting at the beginning of the lattice and working downstream, particles are tracked starting from the
exit of each selected element. The betatron coordinates are initially zero (or very small), while the
momentum deviation is gradually increased until loss of the particle is observed. This defines the
momentum aperture at that location.
In elegant version 19.0 and later, the algorithm is as follows. For simplicity in wording, we’ll assume
the momentum deviations are positive values, although the method is applied separately for negative
values as well:
- Start with δ = 0, i.e., zero momentum offset.
- Track a particle to see if it gets lost. If so, proceed to step 4.
- Increase δ by step size Δδ and return to step 2.
- If no splitting steps remain, proceed to the next step. Otherwise:
- Change δ to δs -sbΔδ., where δs is the largest δ for which the particle survived, and sb
is the steps_back parameter.
- Divide the step size by split_step_divisor to get a new step size Δδ.
- Set δ = δ + Δδ.
- Decrement the “splits remaining” counter by 1.
- Continue from step 2.
- Stop. The momentum aperture is δs
This command can be used for both rings and transport lines. For rings it is most appropriate to have
an rf cavity (i.e., an RFCA element) in the lattice. One should also include radiation loss using either of two
- SREFFECTS element, with QEXCITATION=0. To set up this element more easily, one can include
a twiss_output command with radiation_integrals=1.
- Use CSBEND and KQUAD elements with SYNCH_RAD=1 and ISR=0.
When including radiation loss, one must be certain to set the frequency and phase of the rf cavity correctly.
The rf_setup command can be used for this purpose. It is also a good idea to track for several
synchrotron oscillation periods.
Note for Pelegant: Unlike for elegant, the data in the output file will not be sorted by s. To sort
the data, simply use sddssort from the commandline, e.g.,
sddssort -column=s output.mmap
Also, if it is desirable for the output from Pelegant to have exactly the same form as that from elegant,
then the script reorganizeMmap should be used. This script is provided with elegant and Pelegant