- type: setup/action command.
- function: set up or execute computation of coupled twiss parameters and beam sizes
- sequence: must follow run_setup.
- Command syntax, including use of equations and subcommands, is discussed in 7.2.
STRING filename = NULL;
long output_at_each_step = 0;
long emittances_from_twiss_command = 1;
double emit_x = 0;
double emittance_ratio = 0.01;
double sigma_dp = 0;
long calculate_3d_coupling = 1;
long verbosity = 0;
long concat_order = 2;
- filename — The (incomplete) name of the SDDS file to which coupled twiss parameters and
beam sizes will be written. Suggested value: “%s.ctwi”.
- output_at_each_step — If nonzero, then this is a setup command and results in
computations occurring for each simulation step (e.g., for each perturbed machine if errors are
included). If zero, then this is an action command and computations are done immediately
(e.g., for the unperturbed machine). If you wish to compute Twiss parameters on a closed
orbit or after other calculations, be sure to set this control to a nonzero value.
- emittances_from_twiss_command — If nonzero, then the values of the horizontal emittance
and the momentum spread are taken from the uncoupled computation done with the
twiss_output command. In this case, the user must issue a twiss_output command prior
to the coupled_twiss_output. If zero, then the values of the horizontal emittance and the
momentum spread are taken from the parameters emit_x and sigma_dp, respectively.
- emit_x — Gives the horizontal emittance, if emittances_from_twiss_command=0.
- emittance_ratio — Gives the ratio of the x and y emittances. Used to determine the vertical
emittance from the horizontal emittance. Note that the computation is not self-consistent.
I.e., the user is free to enter any emittance ratio desired, whether it is consistent with the
machine optics or now.
- sigma_dp — Gives the momentum spread, if emittances_from_twiss_command=0.
This feature was added to elegant using code supplied by V. Sajaev, based on Ripkin’s method. The
code computes the coupled lattice functions, then uses the supplied emittance, emittance ratio, and
momentum spread to compute the beam sizes, bunch length (if rf is included), and beam