10.115 TWISS—Sets Twiss parameter values.

Sets Twiss parameter values.
Parallel capable? : yes
GPU capable? : no
Back-tracking capable? : no






Parameter Name UnitsType Default

Description






BETAX M double1

horizontal beta function






ALPHAX double0.0

horizontal alpha function






ETAX M double0.0

horizontal eta function






ETAXP double0.0

slope of horizontal eta function






BETAY M double1

vertical beta function






ALPHAY double0.0

vertical alpha function






ETAY M double0.0

vertical eta function






ETAYP double0.0

slope of vertical eta function






FROM_BEAM short 0

compute transformation from tracked beam properties instead of Twiss parameters?






FROM_0VALUES short 0

if non-zero, transformation is from the ”0” values provided in the element definition






COMPUTE_ONCE short 0

compute transformation only for first beam or lattice functions?






APPLY_ONCE short 1

apply correction only on first pass through for each beam?






VERBOSE short 0

if non-zero, print extra information about transformations






DISABLE short 0

if non-zero, element is ignored






BETAX0 M double1

initial horizontal beta function (if FROM_0VALUES nonzero)






ALPHAX0 double0.0

initial horizontal alpha function (if FROM_0VALUES nonzero)






ETAX0 M double0.0

initial horizontal eta function (if FROM_0VALUES nonzero)






TWISS continued

Sets Twiss parameter values.






Parameter NameUnitsType Default

Description






ETAXP0 double0.0

initial slope of horizontal eta function (if FROM_0VALUES nonzero)






BETAY0 M double1

initial vertical beta function (if FROM_0VALUES nonzero)






ALPHAY0 double0.0

initial vertical alpha function (if FROM_0VALUES nonzero)






ETAY0 M double0.0

initial vertical eta function (if FROM_0VALUES nonzero)






ETAYP0 double0.0

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.

TWLA