Fast wave evanescence in filamentary boundary plasmas

Abstract

Radio frequency waves for heating and current drive of plasmas in tokamaks and other magnetic confinement devices must first traverse the scrape-off-layer (SOL) before they can be put to their intended use. The SOL plasma is strongly turbulent and intermittent in space and time. These turbulent properties of the SOL, which are not routinely taken into account in wave propagation codes, can have an important effect on the coupling of waves through an evanescent SOL or edge plasma region. The effective scale length for fast wave (FW) evanescence in the presence of short-scale field-aligned filamentary plasma turbulence is addressed in this paper. It is shown that although the FW wavelength or evanescent scale length is long compared with the dimensions of the turbulence, the FW does not simply average over the turbulent density; rather, the average is over the exponentiation rate. Implications for practical situations are discussed.

Full wave simulations of fast wave efficiency and power losses in the scrape-off layer of tokamak plasmas in mid/high harmonic and minority heating regimes*

“…Experimental studies on erosion and impurity generation in the presence of ICRF have been performed on different machines [4,7,10,8,9]. In the recent years, a large effort has been made to understand the main physical mechanisms behind the interaction between RF waves and the SOL, such as parametric decay [11,3,12,13], sheath effects [14,15], etc.. Recently, experimental studies employing high harmonic fast wave (HHFW) heating on the National Spherical Torus eXperiment (NSTX) [16], a low aspect ratio tokamak, have shown that substantial HHFW power loss (up to 60% of the HHFW power coupled from the antenna) can occur along the open field lines in the SOL [6,17,18,19,20].…”

Recent progress in modeling ICRF-edge plasma interactions with application to ASDEX Upgrade

“…Previously, efforts were devoted to understanding the mechanism of RF wave scattering by density filaments [86,90,95,[148][149][150]. Recently, a 3D COMSOL model with a realistic AUG antenna geometry has been built to study the influence of filaments on RF wave propagation [98].…”

Redirection of radio-frequency power flow by filaments

“…In reality, the antenna faces the SOL with a structure of filaments typically spaced poloidally by few centimeters [8,22]. In the current simulations, we use only one filament for the following reasons: firstly, the simulation region is only representative of a certain poloidal range of the antenna, and only parallel power redirection at a certain poloidal location of the antenna is studied; secondly, tests show that a local power redirection is mainly affected by a local filament.…”

“…Filaments, usually with a size of a few centimeters, can have a significant influence not only on the ECRF (wavelength ~millimeter) and LH waves (wavelength ~centimeter), but also on the ICRF waves (wavelength ~decimeter). Previously, a lot of efforts were devoted to understand the RF wave scattering by density filaments [3][4][5][6][7][8]. Myra developed theories of scattering of ICRF fast wave and LH slow wave by a cylindrical filament [5].…”