define_cathode

• description: Permits specification of the size, current density, time profile, and other parameters of particle emission from an annulus. Starting in version 2.3, multiple cathodes may be defined.

• example:

  &define_cathode
      z_position = 0, outer_radius = 0.003,
      current_density = 20e4, 
      start_time = 0, stop_time = 200e-12,
      number_per_step = 8, 
  &end
  

• synopsis and defaults:

  &define_cathode
      double z_position = 0;
      double inner_radius = 0;
      double outer_radius = 0;
      double current_density = 0;
      double temperature = 0;
      double work_function = 0;
      long field_emission = 0;
      double field_emission_beta = 1;
      long determine_temperature = 0;
      double electrons_per_macroparticle = 0;
      double start_time = 0;
      double stop_time = 0;
      long autophase = 0;
      double time_offset = 0;
      double number_per_step = 0;
      double initial_pz = 0;
      double initial_omega = 0;
      double stiffness = 1;
      long discretize_radii = 0;
      long random_number_seed = 987654321;
      long distribution_correction_interval = 0;
      long spread_over_dt = 0;
      long zoned_emission = 1;
      long halton_radix_dt = 0;
      long halton_radix_r = 0;
      STRING profile = NULL;
      STRING profile_time_name = NULL;
      STRING profile_factor_name = NULL;
  &end
  

• details:
  • z_position: Longitudinal position of the cathode in meters.
  • inner_radius, outer_radius: Inner and outer radius of the edges of the cathode, in meters.
  • current_density: Base current density, in Amperes/${\rm m^2}$.
  • 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.
  • 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.
  • number_per_step: How many macroparticles to emit per step. Incompatible with specifying electrons_per_macroparticle.
  • start_time, stop_time: Start and stop time for emission, in seconds.
  • autophase: Flag requesting that cathode emission start only when the field at the cathode has the proper phase to accelerate the beam. The total time for emission is still determined by the difference between the start and stop time.
  • time_offset: Only relevant when autophase = 1. Specifies a time offset relative to the emission start time determined by autophasing.
  • initial_pz: The initial longitudinal momentum of emitted particles, in dimensionless units (i.e., normalized to $m_e c$).
  • initial_omega: The initial angular velocity of particles, in radians per second.
  • stiffness: The beam stiffness, i.e., the particle mass, in electron masses.
  • discretize_radii: Flag requesting that particles be emitted only from radii $n*\Delta r$, where n is an integer and $\Delta r$ is the radial grid spacing. This can be useful for certain types of diagnostic runs, but should not be used with space charge.
  • 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.
  • distribution_correction_interval: The number of steps between corrections to the emitted particle distribution. Can be used to compensate for nonuniform emission that occurs due to use of random numbers in the emission algorithm. If used, it should be set to 1. Cannot be used when the temperature is nonzero (Richardson-Schottky emission law).
  • spread_over_dt: Flag requesting that emitted particles have their effective emission times spread out over the simulation time step, $\Delta t$. The particle velocities are adjusted appropriately using the instantaneous $E_z$ and $E_r$ fields only.
  • zoned_emission: Flag requesting that emission calculations take place separately for each annular zone of width $\Delta r$ (the radial grid spacing). Reduces the possibility that random number effects will result in a nonuniform current density.
  • halton_radix_t: 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.
  • 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 $[0, 1]$.