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WAKE

Longitudinal wake specified as a function of time lag behind the particle.
Parallel capable? : yes
Parameter Name Units Type Default Description
INPUTFILE   STRING NULL name of file giving Green function
TCOLUMN   STRING NULL column in INPUTFILE containing time data
WCOLUMN   STRING NULL column in INPUTFILE containing Green function
CHARGE $C$ double 0.0 beam charge (or use CHARGE element)
FACTOR $C$ double 1 factor to multiply wake by
N_BINS   long 128 number of bins for current histogram
INTERPOLATE   long 0 interpolate wake?
SMOOTHING   long 0 Use Savitzky-Golay filter to smooth current histogram?
SG_HALFWIDTH   long 4 Savitzky-Golay filter half-width for smoothing
SG_ORDER   long 1 Savitzky-Golay filter order for smoothing
CHANGE_P0   long 0 change central momentum?
ALLOW_LONG_BEAM   long 0 allow beam longer than wake data?
RAMP_PASSES   long 0 Number of passes over which to linearly ramp up the wake to full strength.
BUNCHED_BEAM_MODE   long 1 If non-zero, then do calculations bunch-by-bunch.
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





The input file for this element gives the longitudinal Green function, $W(t)$ versus time behind the particle. The units of the wake are V/C, so this element simulates the integrated wake of some structure (e.g., a cell or series of cells). If you have, for example, the wake for a cell and you need the wake for N cells, then you may use the FACTOR parameter to make the appropriate multiplication. The values of the time coordinate should begin at 0 and be equi-spaced. A positive value of time represents the distance behind the exciting particle.

A positive value of $W(t)$ results in energy loss. A physical wake function should be positive at $t=0$.

Use of the CHARGE parameter on the WAKE element is disparaged. It is preferred to use the CHARGE element as part of your beamline to define the charge.

Setting the N_BINS paramater to 0 is recommended. This results in auto-scaling of the number of bins to accomodate the beam. The bin size is fixed by the spacing of the time points in the wake.

The default degree of smoothing (SG_HALFWIDTH=4) may be excessive. It is suggested that users vary this parameter to verify that results are reliable if smoothing is employed (SMOOTHING=1).

The algorithm for the wake element is as follows:

  1. Compute the arrival time of each particle at the wake element. This is necessary because elegant uses the longitudinal coordinate $s = \beta c t$.
  2. Find the mean, minimum, and maximum arrival times ($t_{mean}$, $t_{min}$, and $t_{max}$, respectively). If $t_{max}-t_{min}$ is greater than the duration of the wakefield data, then elegant either exits (default) or issues a warning (if ALLOW_LONG_BEAM is nonzero). In the latter case, that part of the beam that is furthest from $t_{mean}$ is ignored for computation of the wake.
  3. If the user has specified a fixed number of bins (not recommended), then elegant centers those bins on $t_{mean}$. Otherwise, the binning range encompasses $t_{min}-\Delta t$ to $t_{max}+\Delta t$, where $\Delta t$ is the spacing of data in the wake file.
  4. Create the arrival time histogram. If any particles are outside the histogram range, issue a warning.
  5. If SMOOTHING is nonzero, smooth the arrival time histogram.
  6. Convolve the arrival time histogram with the wake function.
  7. Multiply the resultant wake by the charge and any user-defined factor.
  8. Apply the energy changes for each particle. This is done in such a way that the transverse momentum are conserved.
  9. If CHANGE_P0 is nonzero, change the reference momentum of the beamline to match the average momentum of the beam.

Bunched-mode application of the short-range wake is possible using specially-prepared input beams. See Section 6 for details. The use of bunched mode for any particular WAKE element is controlled using the BUNCHED_BEAM_MODE parameter.


next up previous
Next: WATCH Up: Element Dictionary Previous: VMON
Robert Soliday 2014-03-21