4.9 define_emitter

• description: Permits specification of properties of an emitting region on the surface. Similar to define_cathode, but without some features. Unlike define_cathode, the emitter can be any surface, not just a plane of constant z.
• example:
  &define_emitter,  
      material_id = 10  
      current_density = 20e4,  
      start_time = 0, stop_time = 200e-12,  
      electrons_per_macroparticle = 1e6  
  &end

• synopsis and defaults:
  &define_emitter  
      long material_id = -1;  
      double current_density;  
      double temperature;  
      long determine_temperature;  
      double work_function ; /* in eV */  
      long add_thermal_velocities = 0;  
      long field_emission = 0;  
      double field_emission_beta = 1;  
      double electrons_per_macroparticle;  
      double start_time;  
      double stop_time;  
      double initial_p;  
      long random_number_seed;  
      int32_t halton_radix_dt = 0;  
      int32_t halton_radix_r = 0;  
      int32_t halton_radix_z = 0;  
      STRING profile = NULL;  
      STRING profile_time_name = NULL;  
      STRING profile_factor_name = NULL;  
      STRING emission_log = NULL;  
  #end  
  

• details:
  • material_id — A positive integer giving the material for which this command specifies emission properties. This will be used together with the material_id parameter of the point namelists in the geometry file to determine the appropriate emission properties for each segment of the cavity boundary.
  • current_density: Base current density, in Amperes/m².
  • temperature: Temperature of the cathode in degrees Kelvin. If zero, then emission is constant at the rate given by current_density. Otherwise, used together with the work function (given by the work_function parameter) and the Richardson-Schottky emission model to determine emission at each time step based on the electric field.
  • work_function: Work function of the cathode material in eV. Must be nonzero if the temperature is nonzero.
  • determine_temperature: If nonzero, then attempts to determine the temperature required to give the current density given by current_density. You must give the work_function. Results are approximate because of the Richardson-Schottky law.
  • add_thermal_velocities : If nonzero, thermal velocities are added at the time of emission, assuming a Maxwellian velocity distribution.
  • field_emission: If nonzero, then the cathode emits only by field emission. The treatment of field emission is from section 6.13 of The Handbook of Accelerator Physics and Engineering.. In field emission mode, spiffe splits the cathode into many subcathodes, each one radial grid space in extent. The field emission current density is computed for each subcathode separately, so that the results are correct in the case where the field varies over the cathode.
  • field_emission_beta: Gives the field enhancement factor for computing field emission current density. The value of the electric field is multiplied by this factor before being used to compute the field emission current density. Typical values are between 1 and 100. In this mode, you must specify electrons_per_macroparticle.
  • electrons_per_macroparticle: Number of electrons represented by each macroparticle.
  • start_time, stop_time: Start and stop time for emission, in seconds.
  • initial_p: The initial momentum of emitted particles, in dimensionless units (i.e., normalized to m_ec).
  • random_number_seed: The seed for the particle emission random number generator. A large, odd integer is recommended. If 0 is given, the seed is picked based on the computer clock.
  • spread_over_dt: Flag requesting that emitted particles have their effective emission times spread out over the simulation time step, Δt. The particle velocities are adjusted appropriately using the instantaneous E_z and E_r fields only.
  • halton_radix_dt: Halton radix (a small prime number) to be used for quiet-start generation of time values.
  • halton_radix_r: Halton radix (a small prime number) to be used for quiet-start generation of radius values.
  • halton_radix_z: Halton radix (a small prime number) to be used for quiet-start generation of longitudinal values.
  • profile: The name of an SDDS-protocol file containing a time-profile with which to modulate the base current density.
  • profile_factor_name, profile_time_name: The columns giving the current-density adjustment factor and the corresponding time when it is valid for the file named by profile. The adjustment factor should be on [0,1].
  • emission_log: The name of an SDDS-protocol file to which data will be written for each emitted particle.