# Neutral Particle Recycling¶

The native XGC neutral module is a Monte Carlo estimate of the neutral density, temperature, and associated reaction rates for ionization and charge exchange. The contribution from the neutrals to the ion and electron collision operators are thereby calculated.

## Input parameters¶

Click here for default values and descriptions.

## Monte Carlo calculation details¶

The neutrals translate in a 2D cartesian (R,Z) space against the toroidally-averaged background plasma. Sample contributions to density, energy, etc. are tallied every neu_dt seconds. For best accuracy, neu_dt ought to be at least as small as the time it takes for a fast neutral to traverse a mesh cell.

In each step, a random number is drawn and v*neu_dt (where v is the instantaneous neutral speed) is compared to the mean free path of ionization and charge exchange to determine if one of those reactions occur during that step.

When an ionization reaction occurs, the sample trajectory is terminated. When a charge exchange reaction occurs, the neutral attains an energy equal to the local ion temperature. The collision frequency for charge exchange in the native model is independent of neutral velocity (relative to the ion velocity).