|EMITNX||double||-1||horizontal normalized emittance|
|EMITNY||double||-1||vertical normalized emittance|
|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 element allows changing the emittance of a beam during tracking. It is intended to be used to modify the emittance ``slightly'' to agree with, for example, experimental measurements.
The LCLS provides an example application: we track a beam from a photo-injector simulation through a laser/undulator beam heater and then through the entire linac. The beam emittance and twiss parameters are measured at a diagnostic downstream of the laser heater. We can insert an EMITTANCE element and a TWISS element at the location of the diagnostic to force the beam properties to the exact values that are measured. This compensates for imperfect modeling of the photo-injector while allowing us to conveniently model the system between the photo-injector and the point at which the emittance is measured.