This section configures the settings of neutrals with moving ions and must be present in the input file if any neutrals with moving ions were specified with the parameter num_neutral_mov_ions in the particles section. This module injects electrons from a tunnel-ionizable fixed gas background with moving ions (see also the neutral section). One of these sections must exist for every neutral with moving ions we intend to use, and must be followed by two sets of species, spe_bound and (optional) diag_species sections that refer to the species into which the ionization electrons and moving ions will be injected, respectively. It accepts the following data:

  • name, string, default = “Neutral”
  • neutral_gas, string, default = “H”
  • ion_param(3,ion_max), float, default = 0.0
  • den_min, float, default = 0.0
  • e_min, float, default = 1.0e-6
  • multi_max, integer, default = maximum ionization level available
  • multi_min, integer, default = 0
  • inject_line, logical, default = .true.
  • if_tunnel, logical, default = .true.
  • if_impact, logical, default = .false.

The associated species must define the number of particles per cell that will be used. The thermal and fluid velocities specified will be ignored. This module can optionally be followed by a profile section,, placed before the species section, that will define the density of the neutral background gas density.

name - specifies the name for the neutral.

neutral_gas - specifies the neutral gas to use in the simulation. Available gases are:

  • “H” - Hydrogen
  • “Li” - Lithium
  • “Na” - Sodium
  • “K” - Potassium
  • “Rb” - Rubidium
  • “Cs” - Cesium
  • “He” - Helium
  • “Ar” - Argon
  • “N” - Nitrogen
  • “O” - Oxygen
  • “C” - Carbon
  • “Ne” - Neon
  • “Xe” - Xenon
  • “custom” - Use a custom ionization model defined by the ion_param parameter.

ion_param - Allows the user to specify a custom neutral background gas. The ionization rate for each ionization level is given in $s^{-1}$ by [Bruhweiler D et al., Physics of Plasmas 10, 2022 (2003), eqs. (1) and (2)]:

$W[s^{-1}] = A \times E^{-C} \times e^{-B/E}$

Where $E$ is the absolute value of the electric field in GV/m, and the $A$, $B$ and $C$ parameters are defined for ionization level i as A = ion_param(1,i), B = ion_param(2,i) and C = ion_param(3,i). See the above mentioned paper for details on calculating these values for an arbitrary medium.

den_min - specifies the minimal density of neutrals to be considered for injection.

e_min - specifies the minimum field value, in GV/m to be considered for ionization. If the field is below this value, it will not ionize the gas.

multi_max - specifies the maximum number of ionization levels. It can be used to truncate ionization at a given level.

multi_min - specifies the minimum ionization level after which particles are injected (useful to assign a different species to a set of levels). Keyword should be set to the desired minimum ionization level minus one.

inject_line - specifies whether to inject particles uniformly along a line in the x1 direction (.true.) or to place them randomly in the cell (.false.).

if_tunnel - specifies whether or not tunnel ionization is turned on.

if_impact - specifies whether or not impact ionization is turned on.

Here’s an example of a neutral_mov_ions section using a Carbon background and allowing ionization only up to the 3rd level:

neutral_mov_ions
{
  neutral_gas = "C",
  multi_max = 3,
}