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When electrodes are biased above the plasma potential, electrons acceleratedthrough the associated electron sheath can dramatically increase the ionizationrate of neutrals near the electrode surface. It has previously been observedthat if the ionization rate is great enough, a double layer separates aluminous high-potential plasma attached to the electrode surface (called ananode spot or fireball) from the bulk plasma. Here, results of the first 2Dparticle-in-cell simulations of anode spot formation are presented along with atheoretical model describing the formation process. It is found that ionizationleads to the buildup of an ion-rich layer adjacent to the electrode, forming anarrow potential well near the electrode surface that traps electrons born fromionization. Anode spot onset occurs when a quasineutral region is establishedin the potential well and the density in this region becomes large enough toviolate the steady-state Langmuir condition, which is a balance betweenelectron and ion fluxes across the double layer. A model for steady-stateproperties of the anode spot is also presented, which predicts values for theanode spot size, double layer potential drop, and form of the sheath at theelectrode by considering particle, power, and current balance. Thesepredictions are found to be consistent with the presented simulation andprevious experiments.

Theory and simulation of anode spots in low pressure plasmas