Parallel capable? : yes

Parameter Name | Units | Type | Default | Description |

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) | |

COUPLING | double | 0.0 | x-y coupling | |

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 `SREFFECTS`

element.

There are several types of storage ring simulation in which one may want to use this element:

- Simulation of instabilities or other dynamics where radiation damping or quantum excitation is important.
- Simulation of dynamics with an rf cavity when the synchronous phase is significantly different from 180 degrees, so that average radiation losses must be included.
- Computation of dynamic and momentum aperture in the presence of radiation damping.

The major parameters (`JX`

, `JY`

, `EXREF`

, `SDELTAREF`

,
`DDELTAREF`

, and `PREF`

) can be supplied explicitly by the user, or filled in by `elegant`
if the `twiss_output`

command is given with `radiation_integrals=1`

.

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 `EYREF`

or `COUPLING`

if
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.*

- If there is dispersion at the location of the
`SREFFECTS`

element, the closed orbit will change because of the average momentum change, but it will disagree with tracking results. The reason is that in tracking`SREFFECTS`

must 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`SREFFECTS`

element 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. - When used for dynamic aperture and momentum aperture determination,
one should set
`QEXCITATION=0`

. Putting the rf cavity (if any) right next to the`SREFFECTS`

element is a good idea to avoid spurious excitation of betatron motion. - Nothing prevents including this element in a lattice when doing frequency map analysis, although
it probably doesn't make any sense. Only the average energy loss per turn will be included.
Again, putting an rf cavity right after
`SREFFECTS`

is a good idea. - In versions 19.0 and later,
`elegant`includes the effect of`SREFFECTS`on the closed orbit. This presents a dilemna when automatic initialization is used, because in order to perform automatic initialization,`elegant`has to compute the optics functions. However, it must determine the closed orbit to compute the optics functions. The solution to this is for the user to pre-compute the twiss parameters and radiation integrals using`twiss_output`

with`output_at_each_step=0`

. The user is free to subsequently give`twiss_output`

with`output_at_each_step=1`

to obtain the results on the closed orbit. - Computation of Twiss parameters does not fully include the
effects of synchrotron radiation losses when these are imposed using
`SREFFECTS`elements. If`PREF=0`(automatic initialization), these effects are completely missing. If`PREF`is non-zero, then`elegant`will use the`DDELTAREF`parameter to compute the energy offset from the element, and thus its effect on the beam trajectory.