Effects of electron emission from biased electrodes on sheath dynamics under fusion-relevant conditions

Abstract

The study of plasma-material interactions in nuclear fusion devices seeks to provide insightinto the complex interplay between a high temperature fusion plasma and the solid walls of a containment vessel. There are many consequences of PMI that lead to decreased performance of a fusion plasma and degradation of wall materials that severely limits their operational lifetime. Providing a better understanding of PMI helps to develop strategies to mitigate these effects, and in turn bolster the performance of fusion devices. Fundamentally, these processes occur in a region of positively charged, low densityplasma that forms near the wall referred to as the sheath. The sheath is a small region, limited to be on the order of the Debye length. It is therefore difficult to resolve the physics that occurs in the sheath experimentally, motivating the use of computational methods. In this study, a subset of PMI known as electron emission is examined by coupling analytical models of ion- and electron-induced emission to continuum kinetic simulations of plasma sheaths. The framework for simulating the effects of electron emission on the sheath can begenerally applied to many plasma systems. In this work the plasma sheath that forms in the Z-pinch fusion concept is chosen to explore how the addition of a strong applied bias potential to the walls changes the emission of electrons, structure of the sheath, and fluxes to the wall. Initial conditions are taken from experimental work on the Fusion Z-pinch Experiment (FuZE). The materials studied in the presented work are graphite and tungsten, which are used in FuZE and in fusion devices more broadly. The sheath simulations reveal that the yield of ion-induced emitted electrons increaseslinearly with the applied bias. When electron-induced emission is included at high bias, a space-charge limited sheath forms at the anode, while the sheath at the cathode remains classical. Increased electron emission is also found to increase the particle and heat flux to both electrodes. The total heat load to the wall is found to be on the order of megawatts, with the heat flux in the graphite case being more than double that of the tungsten case. Finally, a key performance metric for the Z-pinch, the pinch current, is found to also increase linearly with bias potential in correlation with the linear increase in emission. This relationship is supported by theoretical models and the predicted values for the current follow closely with experimental measurements on the FuZE device.