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TRWAKE

Transverse 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 functions
TCOLUMN   STRING NULL column in INPUTFILE containing time data
WXCOLUMN   STRING NULL column in INPUTFILE containing x Green function
WYCOLUMN   STRING NULL column in INPUTFILE containing y Green function
CHARGE $C$ double 0.0 beam charge (or use CHARGE element)
FACTOR   double 1 factor by which to multiply both wakes
XFACTOR   double 1 factor by which to multiply x wake
YFACTOR   double 1 factor by which to multiply y wake
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
DX $M$ double 0.0 misalignment
DY $M$ double 0.0 misalignment
TILT $RAD$ double 0.0 rotation about longitudinal axis
X_DRIVE_EXPONENT   long 1 Exponent applied to x coordinates of drive particles
Y_DRIVE_EXPONENT   long 1 Exponent applied to y coordinates of drive particles
X_PROBE_EXPONENT   long 0 Exponent applied to x coordinates of probe particles
Y_PROBE_EXPONENT   long 0 Exponent applied to y coordinates of probe particles

Transverse wake specified as a function of time lag behind the particle.
Parameter Name Units Type Default Description
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 transverse-wake Green functions, $W_x(t)$ and $W_y(t)$, versus time behind the particle. The units of the wakes are V/C/m, 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. Time values must be equally spaced.

The sign convention for $W_q$ ($q$ being $x$ or $y$) is as follows: a particle with $q>0$ will impart a positive kick ( $\Delta q^\prime >
0$) to a trailing particle following $t$ seconds behind if $W_q(t)>0$. A physical wake function should be zero at $t=0$ and also be initially positive as $t$ increases from 0.

Use of the CHARGE parameter on the TRWAKE 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 XFACTOR and YFACTOR parameters can be used to adjust the strength of the wakes if the location at which you place the TRWAKE element has different beta functions than the location at which the object that causes the wake actually resides.

The X_DRIVE_EXPONENT and Y_DRIVE_EXPONENT parameters can be used to change the dependence of the wake on the x and y coordinates, respectively, of the particles. Normally, these have the value 1, which corresponds to an ordinary dipole wake in a symmetric chamber.

If you have an asymmetric chamber, then you will have a transverse wake kick even if the beam is centered. (Of course, you'll need a 3-D wake code like GdfidL or MAFIA to compute this wake.) This part of the transverse wake is modeled by setting X_DRIVE_EXPONENT=0 and Y_DRIVE_EXPONENT=0. It will result in an orbit distortion, but conceivably could have other effects, such as emittance dilution. In this case, the units for the x and y wake must be $V/C$. A negative value of the wake corresponds to a kick toward negative x (or y).

In addition, a quadrupole wake can be modeled by setting X_DRIVE_EXPONENT=0, Y_DRIVE_EXPONENT=0, X_PROBE_EXPONENT=1, and Y_PROBE_EXPONENT=1. The kick to a particle now depends on it's displacement, not on the displacement of the leading particles. In this case, the units for the wakes must be $V/C/m$.

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 TRWAKE element is controlled using the BUNCHED_BEAM_MODE parameter


next up previous
Next: TUBEND Up: Element Dictionary Previous: TRFMODE
Robert Soliday 2014-03-21