Damping of lower hybrid waves in large spectral gap configurations

Abstract

Extensive experimental data support reliable power deposition and current drive by lower-hybrid (LH) waves in conditions where a large spectral gap exists between the nominal parallel index of refraction prescribed by the antenna characteristics and phasing, and that required for significant Landau damping to take place. We argue that only a significant modification of the initial spectrum at the plasma edge could explain experimental observations. Based on this assumption, a new prescription for reliable simulations of LH current drive using ray-tracing and Fokker-Planck modelling is proposed. A remarkable agreement between experimental observations in the Tore Supra tokamak and simulations is obtained for relevant parametric scans, including electron density and LH waveguide phasing. In an effort to investigate the possible role of fluctuations, it is shown that the spectral gap can be bridged dynamically in the presence of a fluctuating LH spectrum.

Impact of SOL plasma profiles on lower hybrid current drive: Experimental evidence, mitigation and modeling approaches

“…For example, the same author relocated 50 % of injected LH power in high N || components in Ref 16. In the last RF conference, J. Decker studied spectrum spreading on Tore Supra and concluded that shifting approximately the half of the injected power yield a robust agreement with experiment [17]. The reason why we need significantly smaller fractions is not known yet.…”

Measurements of the parallel wavenumber of lower hybrid waves in the scrape-off layer of a high-density tokamak

“…The n || is an important physics parameter determining wave propagation and absorption -not only Landau absorption but also additional parasitic absorption mechanisms, such as collisional damping [17] and parametric instabilities [18]. The n || quantification is also important from a viewpoint of modeling the LH physics as several recent modeling works propose the importance of the ponderomotive force [10,19], and the redistribution of the power spectrum on the first pass before crossing the separatrix [4,20,21].…”

“…On tokamaks, lower hybrid (LH) waves are used to drive toroidal plasma current [1], and their application on reactor-relevant tokamaks as a current drive actuator is an active research area [2][3][4][5][6]. On the Alcator C-Mod tokamak [7], the lower hybrid current drive (LHCD) system [8] at f 0 = 4.6 GHz operates at ITER-and reactor-relevant conditions with relatively low temperatures (T e0 ≈ 2 keV in L-mode plasmas, as opposed to 15 keV in ITER).…”

Development of long pulse RF heating and current drive for H-mode scenarios with metallic walls in WEST

“…The LH power deposition and current density profiles have been modelled with the recent "Tail LH" model in LUKE [32], which has proven to reproduce well the experimental results, e.g. hard X-ray emission profile shapes, on Tore Supra [32] and EAST [33].…”

“…hard X-ray emission profile shapes, on Tore Supra [32] and EAST [33]. For the "Tail LH" modelling of the WEST scenarios, 50% of the LH power in the original launched wave spectrum is transferred to high N // in order to bridge the spectral gap.…”