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

Transverse wake specified as a function of time lag behind the particle.
Parallel capable? : yes
GPU capable? : yes
Back-tracking capable? : yes






Parameter Name UnitsType Default

Description






INPUTFILE STRINGNULL

name of file giving Green functions






TCOLUMN STRINGNULL

column in INPUTFILE containing time data






WXCOLUMN STRINGNULL

column in INPUTFILE containing x Green function






WYCOLUMN STRINGNULL

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 0

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 RADdouble 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






TRWAKE continued

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






Parameter Name UnitsType 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.






ACAUSAL_ALLOWED long 0

If non-zero, then an acausal wake is allowed.






GROUP stringNULL

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, Wx(t) and Wy(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, and be expressed in seconds. A positive value of time represents the distance behind the exciting particle. Time values must be equally spaced.

The sign convention for Wq (q being x or y) is as follows: a particle with q > 0 will impart a positive kick (Δq > 0) to a trailing particle following t seconds behind if Wq(t) > 0. A physical wake function should be zero at t = 0 and also be initially positive as t increases from 0. Causality requires that Wq(t) = 0 for t < 0. Acasual wakes are supported, provided the user sets ACAUSAL_ALLOWED=0. The data file must contain a value of W(t) at t = 0, and should have equal spans of time to the negative and positive side of t = 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

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