Radio frequency wave interactions with a plasma sheath: The role of wave and plasma sheath impedances

Abstract

Radio frequency (RF) sheaths form near surfaces where plasma and strong RF fields coexist. The effect of these RF sheaths on wave propagation near the boundary can be characterized by an effective sheath impedance that includes both resistive and capacitive contributions describing RF sheath rectification and RF power absorption in the sheath [J. R. Myra and D. A. D'Ippolito, Phys. Plasmas 22, 062507 (2015)]. Here we define a dimensionless parameter, the ratio of incoming wave impedance to the sheath impedance, which determines the characteristics of the interaction, ranging from quasi-conducting to quasi-insulating, or in the case of matched impedances, to either perfect absorption or a sheath-plasma resonance. A semianalytical analysis is carried out for electrostatic slow waves in the ion cyclotron range of frequencies (ICRF). For the propagating slow wave case, where the incident wave is partially reflected, the fraction of power dissipated in the sheath is calculated. For the evanescent slow wave case, which admits a sheath-plasma resonance, an amplification factor is calculated. Using the impedance ratio approach, RF sheath interactions are characterized for a range of RF wave and plasma parameters including plasma density, magnetic field angle with respect to the surface, wave frequency and wave-vector components tangent to the surface. For a particularly interesting example case, results are compared with the rfSOL code [H. Kohno and J. R. Myra, Comput. Phys. Commun. 220, 129 (2017)]. Finally electromagnetic effects, absent from the semi-analytical analysis, are assessed.

Filament-assisted mode conversion in magnetized plasmas

“…In visible light, filaments in NSTX show up as bright field-aligned spirals. They carry a large fraction of the antenna power along the parallel direction away from the antenna 20 , eventually depositing it on the divertors, where part of it is likely absorbed via sheath rectification 21,22 , a process whose efficiency is enhanced by the increased E in the wave-filament bound state.…”

A tutorial on radio frequency sheath physics for magnetically confined fusion devices

“…Some structure is seen at the ion plasma frequencyω = 1 and at the ion cyclotron frequencyω =Ω = 0.1. Reprinted by permission from figure 2 of Myra & Kohno (2019b). density waveform in the present formalism) onto the fundamental Fourier component of the applied voltage at the RF frequency.…”

“…The basic concepts and sheath interaction regimes were calculated in a semi-analytical one-dimensional model for the electrostatic SW in Myra & Kohno (2019b). Consider a plane wave incident on a flat surface on which there is an RF sheath.…”

“…Geometry of a plane wave interaction with a sheath. Reprinted by permission from figure 1 of Myra & Kohno (2019b).…”

“…If, in addition to being large, ρ 1 = −ρ 2 as it does when b and n are parallel (or anti-parallel), the sheath BC is in the quasi-insulating limit, A r = 1 and the sheath reflects the wave just as if it were a perfect insulator. In general, the ρ j are complex numbers and it can be shown (Myra & Kohno 2019b) that for propagating waves (i.e. k j pure real), the fraction of incident power per unit area that is absorbed by the sheath is (6.6) and that the power per unit area absorbed by the sheath is P rf /A = f P |Φ rf | 2 Re(y w1 )/2.…”