Ray-tracing and Fokker–Planck modelling of the effect of plasma current on the propagation and absorption of lower hybrid waves

Abstract

Waves at a frequency close to the lower hybrid (LH) resonance are widely used in tokamaks for non-inductive current drive. Modelling of LH waves is usually carried out by combining a Ray-tracing (RT) code for computing the LH waves propagation to a solver of the Fokker-Planck (FP) equation which calculates an electron distribution function self-consistently with the waves absorption. The DELPHINE code has been developed along this approach with accurate treatment of the magnetic equilibrium and the fast electrons dynamics in momentum space. Using this code, the influence of the plasma current on the LH waves propagation and absorption is investigated in detail. High plasma current is found to broaden the absorbed LH spectrum towards high phase velocities, thus increasing the current drive efficiency of the waves. The shape of the current density profile also has an impact on the propagation of the waves and the resulting power deposition. In discharges where the current profile is dominated by LH current drive (LHCD), this dependence leads to the auto-regulation of the LHCD via the current density profile. The RT/FP technique reproduces at least qualitatively some of the experimental trends, though inconsistencies still remain. Perspectives for improving the relevance of the modelling are discussed.

Simulation of the hybrid and steady state advanced operating modes in ITER

“…It integrates, in a modular structure, a 1-D transport solver with general 2-D magnetic equilibria, self-consistently computed by the HELENA code [11], including several heat, particle and impurities transport models, as well as heat, particle, current and momentum sources. The source modules used in the simulations included here are the following: SINBAD [12] for NBI heating and current drive, PION [13] for ICRH, REMA [14] for EC ray tracing, with a linear estimate of the ECCD efficiency [15], Delphine [16] for LH ray tracing (including 2D Fokker-Planck evaluation of LHCD efficiency), and SPOT (an orbit following Monte Carlo code) for the alpha particles distribution function [17].…”

Pushing Particles with Waves: Current Drive and <i>α</i>-Channeling

“…There have been a number of studies to optimize LHCD in and of itself, while avoiding the α-particles [64][65][66][67][68][69][70][71][72][73][74][75][76]. However, little effort was made to utilize the α-particle energy, even though, historically, the α-channeling paradigm was born out of the worry that lower hybrid waves would be damped by the α-particles.…”

The alpha channeling effect

“…It is worth noting that, while there have been a number of studies to optimize LHCD [47,48,49,50,51,52,53,54,55,56,57,58,59], the joint optimization of LHCD and α-channeling was considered only recently [60], where it was pointed out that launching the LH wave from the tokamak high-field side facilitated the joint optimization. This study was stimulated by the recent proposal using high-field launch so that the LH wave would penetrate more deeply the plasma core, with the waveguide better protected from the plasma [61,62].…”