Coupled oscillations of the cathode temperature and the sheath in self-sustained arcs

Previous works showed that externally heated cathodes with fixed thermionic emission can have a sheath that attracts ions or repels ions [1,2]. Here, we show that when the thermionic emission is self-sustained by plasma-induced heating, a persistent oscillation between sheath structures is possible. Starting with a classical ion-accelerating sheath, the ion bombardment heating of the cathode raises the thermionic emission until a potential well forms at the surface. Charge-exchange ions accumulate in the well and restructure the sheath until ion bombardment is minimized. The cathode then undergoes net cooling from the loss of thermoelectrons, reducing the emitted flux and restoring a classical cathode sheath. Characteristics of the oscillation will be demonstrated in 1D kinetic continuum plasma simulations under development. Although the simulations are not a complete model of arc physics, the resulting oscillation likely has implications on stability, current flow, and cathode erosion in certain regimes of arcs. [1] F. Greiner, et al., Phys. Plasmas 2, 1810 (1995). [2] M. D. Campanell and M. V. Umansky, PSST 26, 124002 (2017).