Dust particle surface potential in fusion plasma with supra-thermal electrons

Abstract

By solving the current balance equations, the effect of the supra-thermal electrons on the surface potential of a negatively charged dust particle in a fusion plasma is studied based on the orbital motion limited theory. A non-Maxwellian plasma is modeled by employing a q-non-extensive velocity distribution, where a decreased q-parameter (q < 1) yields increased population of supra-thermal electrons. It is found that with the decrease in the q-parameter, the surface potentials of tungsten and carbon dust particles become more negative. When the ion temperature or ion flow velocity is increased, the dust surface potential changes non-monotonically, which is first decreasing and then increasing. For a small q-parameter, the non-monotonic variation is more pronounced and the minimum value of surface potential moves toward higher values of the ion temperature or ion flow velocity. Due to the dependence of the dust particle surface potential on the supra-thermal electrons, the increased proportion of supra-thermal electrons causes the increase in the ion drag force and the decrease in the dust particle lifetime. Here, lifetime refers to the time taken for the dust surface temperature to increase from its initial value to the sublimation temperature (for carbon) or the melting temperature (for tungsten). The decrease in lifetime caused by supra-thermal electrons is significant as the dust particle radius is increased.