Ignition and Sustainment of Arcing on Nanostructured Tungsten under Plasma Exposure

Abstract

In this research, we initiated the laser-induced arc on the tungsten nanostructured ‘fuzz’ cathode surface which was simultaneously exposed to a stationary helium (He) plasma. The arc current-voltage characteristic had a linear relationship, which was mediated by the plasma resistance along the He plasma. The specific resistance obtained from the slope of the arc current-voltage relationship had a good agreement with the plasma resistance of He plasmas. By varying the sheath potential drop $(\mathbf{V}_{s})$ and the electron density $(\mathbf{n}_{e})$ and the electron temperature of the He plasmas, the arc ignition condition was investigated in detail. It was revealed that the arcing was likely to ignite when $\mathbf{V}_{s} 100 \mathbf{V}$ and $\mathbf{n}_{e} > 2\times 10^{18} \mathbf{m}^{-3}$ . The lower limit of Child-Langmuir sheath electric field, $\mathbf{E}_{s}$ , to ignite arcing was $\mathbf{E}_{s} > 2\times 10^{6}$ V/m. This result indicates the necessity to reduce the electron density and the electron temperature, which lowers $\mathbf{V}_{s}$ and $\mathbf{E}_{s}$ in front of the plasma-facing material surface to avoid arc ignitions in fusion devices.