define_secondary_emission

• description: Permits specification of secondary emission yield function for the material surface. This refers to emission of one or more new particles when a particle impacts the surface. By default, no secondary emission is done.

• example:

  &define_secondary_emission
      input_file = ``secondary.sdds'',
      kinetic_energy_column = ``K'',
      yield_column = ``Yield''
  &end
  

• synopsis and defaults:

  &namelist secondary_emission
          STRING input_file = NULL;
          STRING kinetic_energy_column = NULL;
          STRING yield_column = NULL;
          long yield_limit = 0;
          double emitted_momentum = 0;
          long verbosity = 1;
          long material_id = 1;
          STRING log_file = NULL;
  &end
  

• details:
  • input_file -- Name of an SDDS file from which the secondary emission yield curve will be read.
  • kinetic_energy_column -- Name of the column in input_file giving values of the particle kinetic energy, in eV.
  • yield_column -- Name of the column in input_file giving values of the mean yield. This is a ratio, giving the mean number of new electrons per incident electron.
  • yield_limit -- If non-zero, this parameter limits from above the number of secondaries that can be emitted per primary particle. It can be helpful in preventing runaway, wherein the number of low-energy secondaries grows exponentially.
  • emitted_momentum -- $\beta\gamma$ value for newly-emitted particles. The orientation of the momentum is random.
  • verbosity -- Larger positive values result in more detailed printouts during the run.
  • material_id -- A positive integer giving the material for which this command specifies secondary emission properties. This will be used together with the material_id parameter of the point namelists in the geometry file to determine the appropriate secondary emission properties for each segment of the cavity boundary.
  • log_file -- A possibly incomplete name of an SDDS file to which secondary emission records will be written.

The algorithm is a simple one suggested by J. Lewellen (APS). We assume that the secondary emission yield is a function only of the incident particle's kinetic energy. Each time a particle is lost, the code determines where the particle intersected the metal boundary. The mean secondary yield is computed from the kinetic energy at the time of loss. The number of secondary particles emitted is chosen using a Poisson distribution with that mean. The secondary particles are placed ``slightly'' ($\Delta r/10^6$ or $\Delta z/10^6$) outside the metal surface.

To prevent runaway, the secondary yield curve should fall to zero for low energies. If you have problems with runaway, try setting the yield_limit parameter to a small positive integer. Runaway appears to be associated (at times) with the occasional production of large numbers of secondaries due to the tails of the Poisson distribution.