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##

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`

.

&coupled_twiss_output
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;
&end

`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.

** Next:** divide_elements
** Up:** Namelist Command Dictionary
** Previous:** correct_tunes
Robert Soliday
2014-06-26