  When electrodes are biased above the plasma potential, electrons accelerated
through the associated electron sheath can dramatically increase the ionization
rate of neutrals near the electrode surface. It has previously been observed
that if the ionization rate is great enough, a double layer separates a
luminous high-potential plasma attached to the electrode surface (called an
anode spot or fireball) from the bulk plasma. Here, results of the first 2D
particle-in-cell simulations of anode spot formation are presented along with a
theoretical model describing the formation process. It is found that ionization
leads to the buildup of an ion-rich layer adjacent to the electrode, forming a
narrow potential well near the electrode surface that traps electrons born from
ionization. Anode spot onset occurs when a quasineutral region is established
in the potential well and the density in this region becomes large enough to
violate the steady-state Langmuir condition, which is a balance between
electron and ion fluxes across the double layer. A model for steady-state
properties of the anode spot is also presented, which predicts values for the
anode spot size, double layer potential drop, and form of the sheath at the
electrode by considering particle, power, and current balance. These
predictions are found to be consistent with the presented simulation and
previous experiments.
