TWISS—Sets Twiss parameter values.

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


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		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	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)


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.

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.

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.

10.121 TWISS—Sets Twiss parameter values.