The injector block specifies a particle source to be introduced through a simulation boundary. Each injector block specifies a source of a single species of particle defined by a density, centre of mass drift momentum, temperature and number of simulation particles per cell. The current version of the injectors is incompatible with the PER_SPECIES_WEIGHT compiler flag, and attempting to use an injector with a version of EPOCH compiled with this flag will fail.

Concepts

EPOCH can inject particles through any of the simulation boundaries. This plasma is either a drifting Maxwellian corresponding to a collisionally thermalized beam or a “flux Maxwellian” corresponding to a Maxwellian source accelerated by an electrostatic accelerator. It can have any temporal or transverse spatial profile of density, temperature or drift that you wish to specify.

EPOCH does not automatically make any assumption about the plasma that you wish to inject and does not correct for currents injected into the domain. Current due to an injected beam will be smoothly created as the particles enter the domain. If you wish to inject a neutral beam, you will have to use multiple injectors to inject electrons and ions so as to produce a neutral beam. Great care must be taken when introducing relativistic beams since the current due to a highly relativistic beam will not be the current due to the centre of mass velocity since EPOCH does not use the Maxwell-Jüttner distribution for loading particles.

Boundary conditions

The injectors only work properly with certain boundary conditions. For most purposes the “open” boundary condition is the only one that makes sense with injectors since particles are flowing freely through the boundary. Remember that in any version of EPOCH that supports injectors you can also use per species boundary conditions to allow you to have different boundary conditions for injected and bulk particles.

Moving window

Injectors and moving windows can be tricky to work with, so the default behaviour of EPOCH is to stop all injectors when the window starts to move. If you wish to override this behaviour then simply explicitly set t_end in the injector block to a value after the window starts to move. Setting

 t_end = t_end 

will cause the injectors to continue running until the end of the simulation even with the moving window. You must take great care when specifying injectors for a moving window because you will likely get gaps or bunches in particles injected through the x boundary and there will probably be some shearing of particles introduced through y and z boundaries. It is in general recommended that you specify a velocity profile for the moving window that stops at times when particles are to be injected and then starts again once the injection is complete.

Keys

• boundary - specifies which boundary to attach the particle source too. Same specification as the laser block, so permitted values are x_min, x_max, y_min, y_max, z_min and z_max
• species - specifies which species should be injected through the boundary. Just specify the name of the species required.
• t_start - Time at which to start the injector
• t_end - Time at which to end the injector
• npart_per_cell - target pseudo-particle density for the injector. Average number of particles injected will be this value or slightly higher if very few particles are specified
• number_density - Number density of the particle source in $m^{-3}$. Can be space varying along the boundary to which the injector is attached and time varying
• number_density_min - Minimum number density in $m^{-3}$ below which pseudo particles are not loaded. Use if the density has a profile to avoid injecting low weight particles in low density regions
• number_density_max - Maximum particle number density in $m^{-3}$. When the number density in a cell rises above number_density_max the injector clips the density to number_density_max allowing easy implementation of exponential rises to plateaus for time-varying injectors. Note that the number of particles per cell is kept fixed and the number density adjustment is achieved by modifying the particle weight. This flag has no effect for particles with per-species weighting. If the flag has a negative value then no clipping is performed. This is the default.
• temp_x - Temperature in x direction (K)
• temp_y - Temperature in y direction (K)
• temp_z - Temperature in z direction (K)
• temp - Sets an isotropic temperature distribution in Kelvin. If both temp and a specific temp_x, temp_y, temp_z parameter is specified then the last to appear in the deck has precedence. If neither are given then the injector will have a default temperature of zero Kelvin.
• temp_{x,y,z}_ev, temp_ev - These are the same as the temperature parameters described above except the units are given in electronvolts rather than Kelvin, i.e. using 1ev = 11604.5K .
• drift_x - Momentum drift in x direction in $kg.m/s$
• drift_y - Momentum drift in y direction in $kg.m/s$
• drift_z - Momentum drift in z direction in $kg.m/s$
• drift_{x,y,z} - Specifies a momentum space offset in $kg m/s$ to the distribution function for this injector. By default, the drift is zero.
• use_flux_maxwellian - Logical flag to determine whether to use an accelerated flux Maxwellian rather than a drifting Maxwellian. This calculates the flux due to passing a Maxwellian source into an electrostatic accelerator instead of a drifting Maxwellian. If your particle source is a lab accelerator then you may want to set this to true.

Example Deck

begin:injector
   boundary = x_min
   species = Electron
   number_density = dens
   temp_x = temp
   drift_x = drift_p
   npart_per_cell = 32
end:injector

Warnings

Currently injectors are a beta feature of EPOCH. We believe them to work correctly, but unusual results must be considered suspect. If you get unexpected results, please contact the EPOCH development team.