fix ID ambipolar especies ion1 ion2 ...
fix 1 ambipolar e N+ O+ NO+
Enable the ambipolar approximation to be used in a simulation. The ambipolar approximation is a computationally efficient way to model low-density plasmas which contain positively-charged ions and negatively-charged electrons. In this model, electrons are not free particles which move independently. This would require a simulation with a very small timestep due to electon's small mass and high speed (1000x that of an ion or neutral particle).
Instead each ambipolar electron is assumed to stay "close" to its parent ion, so that the plasma gas appears macroscopically neutral. Each pair of particles thus moves together through the simulation domain, as if they were a single particle, which is how they are stored within SPARTA. This means a normal timestep can be used.
An overview of how to run simulations with the ambipolar approximation is given in the Section 6.11. This includes gas-phase collisions and chemistry as well as surface chemistry when particles collide with surface elements or the global boundary of the simulation box. The section also lists all the commands that can be used in an input script to invoke various options associated with the ambipolar approximation. All of them depend on this fix ambipolar command being defined.
This command defines especies which is the species ID associated with the ambipolar electrons. It also specifies one or more species IDs as ion1, ion2, etc for ambipolar ions. SPARTA checks that the especies has a negative charge (as read in by the species command), and the ions have positive charges. An error is flagged if that is not the case.
Internally, this fix defines two custom particle attributes. The first is named "ionambi" and is an integer vector (one integer per particle). It stores a value of 1 for ambipolar ions, or 0 otherwise. The second is named "velambi" and is a floating-point arrays (3 values per particle). It stores the velocity of the ambipolar electron associated with the ambipolar ion, or zeroes otherwise.
Restart, output info:
No information about this fix is written to binary restart files.
However, the values of the two custom particle attributes defined by this fix are written to the restart file. Namely the integer value "ionambi" and floating-point velocity values "velambi" for each particle. As explained on the read_restart doc page these values can be re-assigned to particles when a restart file is read, if a new fix ambipolar command is specified in the restart script before the first run command is used.
No global or per-particle or per-grid quantities are stored by this fix for access by various output commands.
However, the two custom particle attributes defined by this fix can be accessed by the dump particle command, as p_ionambi and p_velambi. That means those per-particle values can be written to particle dump files.
collide_modify ambipolar yes