define_cathode

• description: Permits specification of the size, current density, time profile, and other parameters of particle emission from an annulus. At present, only one cathode 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 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;
      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/ .
  • 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 ).
  • 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 , where n is an integer and 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.
  • spread_over_dt: Flag requesting that emitted particles have their effective emission times spread out over the simulation time step, . The particle velocities are adjusted appropriately using the instantaneous and fields only.
  • zoned_emission: Flag requesting that emission calculations take place separately for each annular zone of width (the radial grid spacing). Reduces the possibility that random number effects will result in a nonuniform current density.
  • 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 .