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TWISS

Sets Twiss parameter values.
Parallel capable? : yes
Parameter Name Units Type Default Description
BETAX $M$ double 1 horizontal beta function
ALPHAX   double 0.0 horizontal alpha function
ETAX $M$ double 0.0 horizontal eta function
ETAXP   double 0.0 slope of horizontal eta function
BETAY $M$ double 1 vertical beta function
ALPHAY   double 0.0 vertical alpha function
ETAY $M$ double 0.0 vertical eta function
ETAYP   double 0.0 slope of vertical eta function
FROM_BEAM   long 0 compute transformation from tracked beam properties instead of Twiss parameters?
FROM_0VALUES   long 0 if non-zero, transformation is from the "0" values provided in the element definition
COMPUTE_ONCE   long 0 compute transformation only for first beam or lattice functions?
APPLY_ONCE   long 1 apply correction only on first pass through for each beam?
VERBOSE   long 0 if non-zero, print extra information about transformations
DISABLE   long 0 if non-zero, element is ignored
BETAX0 $M$ double 1 initial horizontal beta function (if FROM_0VALUES nonzero)
ALPHAX0   double 0.0 initial horizontal alpha function (if FROM_0VALUES nonzero)
ETAX0 $M$ double 0.0 initial horizontal eta function (if FROM_0VALUES nonzero)

Sets Twiss parameter values.
Parameter Name Units Type Default Description
ETAXP0   double 0.0 initial slope of horizontal eta function (if FROM_0VALUES nonzero)
BETAY0 $M$ double 1 initial vertical beta function (if FROM_0VALUES nonzero)
ALPHAY0   double 0.0 initial vertical alpha function (if FROM_0VALUES nonzero)
ETAY0 $M$ double 0.0 initial vertical eta function (if FROM_0VALUES nonzero)
ETAYP0   double 0.0 initial slope of vertical eta function (if FROM_0VALUES nonzero)
GROUP   string NULL Optionally used to assign an element to a group, with a user-defined name. Group names will appear in the parameter output file in the column ElementGroup





This elements allows transformation of the twiss parameters of a beam with a first-order matrix. The matrix is computed in various ways based on initial and final twiss parameters. Depending on how you set it up, the final twiss parameters for your beam may not be the twiss parameters you specify.

The twiss parameter values BETAX, BETAY, etc. specified in the element definition specify the target values of the transformation. To completely specify the transformation, one must know the initial values as well.

Lattice-Function-Based Transformation

If FROM_BEAM is zero, which is the default, then the initial values are taken from the incoming lattice functions computed by twiss_output. This provides a way to transform the lattice functions between two parts of a transport line without designing intervening optics. A beam that is matched at the beginning of the transport line will remain matched. A beam that is mismatched at the beginning of the transport line will not be matched after the TWISS element.

By default, each time the twiss parameters are recomputed, the transformation is updated to maintain the desired lattice functions at the exit of the TWISS element. Setting COMPUTE_ONCE to a non-zero value specifies that elegant should compute the transformation matrix only once, i.e., for the first set of computed lattice functions.

By default, the transformation is applied to the beam only the first time it passes the element. Setting APPLY_ONCE to a zero will result in application of the transformation at each pass.

Beam-Ellipse-Based Transformation

If FROM_BEAM is non-zero, the the initial values for the transformation are computed from a beam. This provides a way to transform the beam ellipse to the desired twiss parameters irrespective of the lattice. The results from twiss_output will not necessarily be matched downstream of this element. Only if the beam ellipse and lattice ellipse are the same will this occur.

By default, each time a new beam is generated, the transformation will be updated to maintain the desired beam ellipse at the exit of the TWISS element. Setting COMPUTE_ONCE to a non-zero value specifies that elegant should compute the transformation matrix only once, i.e., for the first beam it sees.

By default, the transformation is applied to the beam only the first time it passes the element. Setting APPLY_ONCE to a zero will result in application of the transformation at each pass. This would make sense, for example, if the TWISS element was filling in for a section of a ring. It wouldn't make sense if the TWISS element was being used to match the beam from a transport line to a ring.


next up previous
Next: TWLA Up: Element Dictionary Previous: TUBEND
Robert Soliday 2014-06-26