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FTRFMODE

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





This element 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 $V^2/(2*P)/y$. 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 $\beta=0$ 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.

N.B.: Unlike TRFMODE, FTRFMODE does not include the longitudinal field that, strictly speaking, must also be excited. Generally this is a very small effect. It will be added in a future version.

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).


next up previous
Next: HISTOGRAM Up: Element Dictionary Previous: FRFMODE
Robert Soliday 2007-04-02