The simulation is only concerned with the near surface region (up to about ~1 cm from the material surface). Because electrons are much lighter than ions, they diffuse from the plasma to the walls much quicker than ions. This creates a charge seperation and sets up an electric field near the surface which tends to repel electrons and attract ions. This region is called a "sheath" and has a thickness on the order of a couple Debye lengths. The Debye length is defined as the distance over which a plasma is not neutral, i.e. it has some localization of charge. Over a distance greater than the Debye length, the plasma looks to be neutral because there are as many positively charged ions as negatively charged electrons.
In addition to this original sheath region, called the Debye sheath in the figure below, a second sheath region comes about due to the obliquely incident magnetic field and the much larger Larmor radius (the radius of gyration about a field line) of the ions than electrons. This region is called the magnetic sheath and has a thickness on the order of the ion Larmor radius.
Past this sheath, the plasma is considered to be in its normal bulk state at a plasma potential of 3kTe relative to the grounded vessel walls. The voltage drop of 3kTe occurs over the magnetic and Debye sheath region, typically with 75% of the voltage drop occuring over the magnetic sheath region. The electron density of the plasma, ne, is at its bulk value of ne0 outside the sheath regions and decays exponentially as it approaches the surface. All of these effects are shown below in a schematic of the region being simulated in our code.
