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One or more beam-driven TM monopole modes of an RF cavity, with data from a file.
Parallel capable? : yes
Parameter Name Units Type Default Description
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?
FACTOR   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   STRING NULL 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.
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

This element simulates a set of beam-driven monopole modes in a cavity using the fundamental theorem of beam loading and phasor rotation. It is similar to RFMODE, 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, defined as $V^2/(2*P)$. 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 column:

  1. beta -- Normalized load impedance (dimensionless). Floating point. If not given, the $\beta=0$ is assumed for all modes.

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 WAKE or ZLONGIT 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.

next up previous
Next: FTABLE Up: Element Dictionary Previous: FMULT
Robert Soliday 2014-06-26