Lower hybrid current drive for the steady-state scenario

Abstract

Lower Hybrid Current Drive (LHCD) experiments performed at density close to that required for the steady state scenario are reported. On C-Mod, FTU and Tore Supra, a strong decay of the brehmsstrahlung emission is observed when the density is increased, much faster that the prediction of LHCD modelling. On JET, LH power deposition is also found to be sensitive to the plasma density: LH power modulation indicates that the power deposition moves to the very edge of the plasma (r/a ~ 0.9) when the density approaches the requirement of the JET SS scenario. From this experiment but also from the reconstruction of the electron cyclotron emission spectrum, the decrease of the LHCD efficiency with density is also found. From LHCD modelling of different JET pulses performed at different densities and wave parallel refraction indexes, it is concluded that the wave accessibility condition is not the key parameter for explaining the decrease of the efficiency. C-Mod, FTU and Tore Supra experiments indicate that the plasma edge parameters, namely density and temperature but also fluctuations, are affecting the efficiency via loss mechanisms which are likely to be collisional damping (C-Mod), parametric decay instabilities or wave scattering (FTU/ Tore Supra).

Verification of particle simulation of radio frequency waves in fusion plasmas

“…It has also been proposed 3 that the RF waves can be used for driving plasma flows and current in the field reversed configuration 4 . To effectively utilize the RF power we need a better understanding of the key physics of RF waves in plasmas, e.g., waveparticle interaction [5][6][7] , mode conversion 8,9 and nonlinear effects [10][11][12][13][14][15][16] Two computational methods have been widely used to study wave-particle interactions in fusion plasmas. The first solves the wave equation derived from the linearized Vlasov-Maxwell system (the full wave model).…”

Isotopic effect in experiments on lower hybrid current drive in the FT-2 tokamak

“…The existence of the density limit for LH current drive (LHCD) was attributed to various mechanisms: linear absorption by ions, which increases as the density tends to the resonance LH value; collision losses; scattering by drift turbulence; parametric instabilities; etc. The density limit effect has been studied and discussed for several past decades; however, it has not received comprehensive physical explanation [1].…”

“…The existence of the density limit for LH current drive (LHCD) was attributed to various mechanisms: linear absorption by ions, which increases as the density tends to the resonance LH value; collision losses; scattering by drift turbulence; parametric instabilities; etc. The density limit effect has been studied and discussed for several past decades; however, it has not received comprehensive physical explanation [1].In the present work the main attention is paid to investigation of current drive by lower hybrid waves on the FT-2 (R = 0.55 m, a = 0.08 m, B T ≤ 3 T, I p = 19÷40 kA, f 0 = 920 МHz) tokamak. The results of experiments carried out at the FT-2 tokamak allow us to conclude that the most probable reason for LHCD termination in both hydrogen and deuterium plasmas at a relatively low plasma current of I = 22 kA and, accordingly, a low electron temperature is an additional reduction in T e at the periphery of the plasma column during the LH pulse [2].…”

Preface to Special Topic: Advances in Radio Frequency Physics in Fusion Plasmas

“…One outstanding problem in understanding lower hybrid current drive physics, which is discussed in his review, is the so-called spectral gap problem 4 in which lower hybrid waves launched at high phase speeds, where the absorption by electron Landau damping should be negligible, are nevertheless observed to be strongly absorbed in the plasma, presumably because interactions with the plasma alter the launched wave spectrum. A second outstanding issue he covers in detail is that recent experiments [5][6][7] indicate that the amount of current drive that can be driven by the launched LH waves decreases more strongly at a lower density than predicted by previous theoretical models 8 based on parametric instabilities. Different phenomena taking place in the edge regions of the tokamak between the vessel wall and the well-confined core region of the plasma are believed to play a major role in modifying the launched LH power spectrum and limiting wave penetration.…”

“…Supporting papers on experimental and theoretical studies in this matter are included here by Dr. S. G. Baek from the PSFC-MIT and Dr. Joan Decker from the CEA-Cadarache. In particular, a number of mechanisms that could be relevant to understanding the spectral gap problem 4 and the observed density limit on the efficiency of LH current drive [4][5][6][7][8][9] are discussed in this set of papers. A paper by Dr. Vladimir Fuchs from the IPPPrague is also included that addresses LH wave interactions at the antenna mouth to explain the observation of relativistic electrons in the WEGA stellarator.…”