Establish the physical adequacy of collisional cold-plasma modeling at low-density ICRF antenna edges

Establish whether macroscopic rectified sheaths in low-density ICRF antenna structures interact non-conservatively with electrons through Fermi acceleration or stochastic heating, thereby raising the electron temperature above its thermal-equilibrium value and making collisional cold-plasma, or even Maxwellian hot-plasma, descriptions inadequate.

Background

The paper shows that physical collisions regularize the lower-hybrid and resonance-cone singularities predicted by collisionless cold-plasma theory, but the resulting electric-field peaks remain confined to very short length scales. The authors emphasize that the neutral density controls these regularizing scales and that the neutral density in the antenna private scrape-off layer is poorly constrained.

A separate unresolved issue concerns the validity of the collisional cold-plasma description itself. At low density, rectified sheaths can become macroscopic and may interact non-conservatively with electrons, potentially causing stochastic heating or Fermi acceleration. If this raises the electron temperature substantially above its thermal-equilibrium value, both collisional cold-plasma and possibly Maxwellian hot-plasma models could become inappropriate for describing ICRF-frequency behavior inside antenna structures.