Nano-Tendril Bundles Behavior Under Plasma-Relevant Electric Fields

The appearance of unipolar arcs is a negative phenomenon that occurs among plasma-wall interaction processes in thermonuclear devices. Nano-tendril bundles (NTBs) can be formed on a plasma-facing surface of tungsten after irradiation by helium plasma that contains impurities of neon or nitrogen. These structures can increase the probability of unipolar arcing because of significant electron emission under a relatively small applied electric field of several kV/mm. The electron emission can appear due to sufficient field enhancement factor that can be up to several thousand. Furthermore, NTBs can be easily overheated, which leads to initiation of the thermofield emission, consequently triggering explosive emission because of intensive heating by Joule source, even without any additional external power load. This paper shows the results of the field enhancement factor estimations, as well as the critical field strength for NTBs when they can collapse because of explosive electron emission caused by Joule heating. The results were obtained by numerical simulations in COMSOL Multiphysics (c) and experimentally in a vacuum diode.