Hilbert‐Expansion‐Based Fluid Models for Neutral Particles in the Plasma Edge With Self‐Collisions

ABSTRACT Neutral particles in the plasma edge of a fusion device are governed by a kinetic equation that describes how the particles are distributed in space, velocity, and time. The kinetic equation incorporates various interactions between the neutral particles and the plasma particles, such as recombination reactions, ionization reactions, and charge–exchange reactions. These so‐called neutral–plasma interactions can be described mathematically by linear operators. On the other hand, there are also (elastic) self‐collisions between the neutral particles themselves. These so‐called neutral–neutral interactions are often neglected as they turn the kinetic equation nonlinear. For realistic fusion devices, however, these interactions can have an important influence on simulation results. Here, we describe the kinetic equation including elastic neutral–neutral collisions and derive a reduced‐order model, also called a fluid model, that takes this nonlinear effect into account. The accuracy of the reduced‐order model is verified numerically.