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10.39 FTRFMODE—One or more beam-driven TM dipole modes of an RF cavity, with data from a file.

One or more beam-driven TM dipole modes of an RF cavity, with data from a file.
Parallel capable? : yes
GPU capable? : no
Back-tracking capable? : no

Parameter Name UnitsType Default

Description

FILENAME STRINGNULL

input file

BIN_SIZE S double 0.0

bin size for current histogram (use 0 for autosize)

N_BINS long 20

number of bins for current histogram

RIGID_UNTIL_PASS long 0

don’t affect the beam until this pass

USE_SYMM_DATA long 0

use ”Symm” columns from URMEL output file?

DX M double 0.0

misalignment

DY M double 0.0

misalignment

XFACTOR double 1

factor by which to multiply shunt impedances

YFACTOR double 1

factor by which to multiply shunt impedances

CUTOFF HZ double 0.0

If >0, cutoff frequency. Modes above this frequency are ignored.

OUTPUT_FILE STRINGNULL

Output file for voltage in each mode.

FLUSH_INTERVAL long 1

Interval in passes at which to flush output data.

RAMP_PASSES long 0

Number of passes over which to linearly ramp up the impedance to full strength.

RESET_FOR_EACH_STEP long 1

If nonzero, voltage and phase are reset for each simulation step.

LONG_RANGE_ONLY long 0

If nonzero, induced voltage from present turn does not affect bunch. Short range wake should be included via WAKE or ZLONGIT element.

N_CAVITIES long 1

effect is multiplied by this number, simulating N identical cavities

FTRFMODE continued

One or more beam-driven TM dipole modes of an RF cavity, with data from a file.

Parameter Name UnitsType Default

Description

BUNCHED_BEAM_MODE long 1

If non-zero, then do calculations bunch-by-bunch.

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

This element simulates a set of beam-driven dipole modes in a cavity using the fundamental theorem of beam loading and phasor rotation. It is similar to TRFMODE, but it allows faster simulation of more than one mode. Also, the mode data is specified in an SDDS file. This file can be generated using the APS version of URMEL, or by hand. It must have the following columns and units:

  1. Frequency — The frequency of the mode in Hz. Floating point.
  2. Q — The quality factor. Floating point.
  3. ShuntImpedance or ShuntImpedanceSymm — The shunt impedance in Ohms/m, defined as V 2∕(2 * P)∕x or V 2∕(2 * P)∕y (i.e., “circuit definition”). Floating point. By default, ShuntImpedance is used. However, if the parameter USE_SYMM_DATA is non-zero, then ShuntImpedanceSymm is used. The latter is the full-cavity shunt impedance that URMEL computes by assuming that the input cavity used is one half of a symmetric cavity.

The file may also have the following columns:

  1. beta — Normalized load impedance (dimensionless). Floating point. If not given, the β = 0 is assumed for all modes.
  2. xMode — If given, then only modes for which the value is nonzero will produce an x-plane kick. Integer. If not given, all modes affect the x plane.
  3. yMode — If given, then only modes for which the value is nonzero will produce an y-plane kick. Integer. If not given, all modes affect the y plane.

In many simulations, a transient effect may occur when using this element because, in the context of the simulation, the impedance is switched on instantaneously. This can give a false indication of the threshold for instability. The RAMP_PASSES parameter should be used to prevent this by slowly ramping the impedance to full strength. This idea is from M. Blaskiewicz (BNL).

Normally, the field dumped in the cavity by one particle affects trailing particles in the same turn. However, if one is also using a TRWAKE or ZTRANSVSE element to simulate the short-range wake of the cavity, this would be double-counting. In that case, one can use LONG_RANGE_ONLY=1 to suppress the same-turn effects of the RFMODE element.

GFWIGGLER

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