7.20 coupled_twiss_output

• 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.

• 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 tilt.