On the origin of high harmonic fast wave edge losses in NSTX

Abstract

Many mechanisms have been proposed to explain the High Harmonic Fast Wave (HHFW) edge losses in the National Spherical Torus Experiment (NSTX). Here, we discuss a newly discovered edge loss mechanism which involves the resonant excitation of surface waves on filaments. We show that this edge loss mechanism can explain the main parameter dependencies observed in NSTX esperiments.

Radio-frequency sheath excitation at the extremities of scrape-off layer plasma filaments, mediated by resonant high harmonic fast wave scattering

“…In an infinite spatial domain with vanishing scattered fields at infinity transverse to B 0 , one can most easily solve the scattering problem in the spectral domain, by splitting the incident and scattered waves into independent cylindrical modes oscillating as exp(imθ + ik // z). This subsection briefly summarizes this method used Ram & Hizanidis (2016) and Tierens et al (2020aTierens et al ( , 2022a. Tierens et al (2020b) investigated numerically non-cylindrical FAMC modes in more realistic geometries and with more realistic density distributions.…”

“…One describes this matching by a 4 × 4 linear system of equations, whose unknows are the four complex amplitudes of the scattered modes and the right-hand side is a drive from the incident wave. Tierens et al (2022a) investigated analytically a simplified scattering problem under the following assumptions:…”

“…The FAMC modes belong to a broader zoology of ICRF surface waves (Myra & D'Ippolito 2010;Tierens et al 2020a). Using a spectral approach in the parallel direction, Tierens et al (2022a) estimated analytically their dispersion relation, as well as the fraction of the launched HHFW power that they could parasitically divert from the core plasma. Tierens et al (2022a) evidenced finite RF power damping in the filament volume even in the limit of vanishing anti-Hermitian part of the dielectric tensor.…”

“…Using a spectral approach in the parallel direction, Tierens et al (2022a) estimated analytically their dispersion relation, as well as the fraction of the launched HHFW power that they could parasitically divert from the core plasma. Tierens et al (2022a) evidenced finite RF power damping in the filament volume even in the limit of vanishing anti-Hermitian part of the dielectric tensor. Statistics of this power redirection, over a population of turbulent filaments observed on the National Spherical Torus eXperiment (NSTX) in Princeton, could reproduce several experimental trends about the missing HHFW power in the core of this device (Tierens et al 2022b).…”

“…Within a simplified analytic approach, the present paper explores how the nearly resonant FAMC modes can possibly excite sheath RF oscillations at the extremities of bounded filaments in the SOL of a tokamak. In the limit of vanishing collisionality, finite collisional damping is only possible if the nearly resonant mode can extend indefinitely in the parallel direction (Tierens et al 2022a). As the filaments guide the nearly resonant FAMC modes with weak parallel damping, the RF sheath excitation can be efficient even at large parallel distances from the HHFW launcher, and the sheath dissipation can possibly dominate the losses over the volume damping.…”