TRWAKE

Transverse wake specified as a function of time lag behind the particle.

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	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
XPOWER		long	1	Power of x that x kick depends on.
YPOWER		long	1	Power of y that y kick depends on.

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 XPOWER and YPOWER parameters can be used to change the dependence of the wake on the x and y coordinates, respectively, of the particles. Normally, XPOWER=1 and YPOWER=1. This is an ordinary dipole wake in a (supposedly) 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 described with XPOWER=0 and YPOWER=0. It will result in an orbit distortion, but conceivably could have other effects, such as emittance dilution. If XPOWER=0 or YPOWER=0, the units for the x or y wake (respectively) must be $V/C$. A negative value of the wake corresponds to a kick toward negative x (or y).