|JX||double||1||x damping partition number|
|JY||double||1||y damping partition number|
|JDELTA||double||2||momentum damping partition number|
|EXREF||double||0.0||reference equilibrium x emittance|
|EYREF||double||0.0||reference equilibrium y emittance|
|SDELTAREF||double||0.0||reference equilibrium fractional momentum spread|
|DDELTAREF||double||0.0||reference fractional momentum change per turn due to SR (negative value)|
|PREF||double||0.0||reference momentum (to which other reference values pertain)|
|FRACTION||double||1||fraction of implied SR effect to simulate with each instance|
|DAMPING||long||1||include damping, less rf effects?|
|QEXCITATION||long||1||include quantum excitation?|
|LOSSES||long||1||include average losses?|
|CUTOFF||double||100||cutoff (in sigmas) for gaussian random numbers|
|INCLUDE_OFFSETS||long||1||include orbit offsets in tracking (see below)?|
|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 is intended for storage ring modeling only and provides a fast alternative to element-by-element modeling of synchrotron radiation. It should be used with care because the results will not necessarily be self-consistent. This is particularly an issue when there is dispersion at the location of the
There are several types of storage ring simulation in which one may want to use this element:
The major parameters (
PREF) can be supplied explicitly by the user, or filled in by elegant
twiss_output command is given with
In explicit initialization, the user supplies the quantities EXREF, EYREF, SDELTAREF, DDELTAREF, and PREF. These are, respectively, the reference values for the x-plane emittance, y-plane emittance, fractional momentum spread, energy loss per turn, and momentum. The first four values pertain to the reference momentum. JX, JY, and JDELTA may also be given, although the defaults work for typical lattices.
In automatic initialization, the user turns on the radiation integral
feature in twiss_output, causing elegant to automatically
compute the above quantities. This will occur only if PREF=0.
The COUPLING parameter can be used to change the partitioning of
quantum excitation between the horizontal and vertical planes.
Because the radiation integrals computation in
twiss_output pertains to the
horizontal plane only, the user must supply either
non-zero vertical emittance is desired.
The user may elect to turn off some aspects of the synchrotron radiation model. These should be changed from the default values with care!
DAMPING-- Default is 1. If set to 0, then no radiation damping effects will be included. More precisely, it is equivalent to setting
JX=JY=JDELTA=1. Damping still occurs at any rf cavities (since elegant works in trace space).
QEXCITATION-- Default is 1. If set to 0, then no quantum excitation effects are included, which is to say that all particles will experience the same perturbation.
LOSSES-- Default is 1. If set to 0, no average energy losses are included.
There are a number of caveats that must be observed when using this element.
SREFFECTSelement, the closed orbit will change because of the average momentum change, but it will disagree with tracking results. The reason is that in tracking
SREFFECTSmust displace the beam to the new equilibrium orbit, because otherwise there will be additional betatron motion excited and the wrong equilibrium emittance will be obtained. (Since the
SREFFECTSelement is already adding the betatron motion excitation for the entire ring, elegant is forced to offset each particle by to suppress any additional excitation.)
This issue can be resolved by placing the
SREFFECTS element next to the
rf cavity and setting
INCLUDE_OFFSETS=0. Since the average momentum change is
zero from the two elements, no additional betatron motion will be generated.
Optionally, one can also use many
SREFFECTS elements at equivalent locations in
the lattice, which will decrease the magnitude of the effect.
QEXCITATION=0. Putting the rf cavity (if any) right next to the
SREFFECTSelement is a good idea to avoid spurious excitation of betatron motion.
SREFFECTSis a good idea.
output_at_each_step=0. The user is free to subsequently give
output_at_each_step=1to obtain the results on the closed orbit.