Effects of magnetized alpha particles on lower hybrid heating and current drive in a reactor grade plasma

Abstract

The results of computer simulations of lower hybrid current drive (LHCD) for ITER parameters are presented. The damping of LH waves by alpha particles due to the interaction at high cyclotron resonances is obtained by solving the 2-D Fokker-Planck equation with the corresponding quasi-linear term (FPA code). This code is incorporated into a spatially 1-D LHCD code so as to obtain radial profiles of the radiofrequency (RF) power absorbed by electrons (1-D Fokker-Planck code) and alpha particles as well as the radial profile of the driven current. One-pass absorption is assumed and verified by a detailed analysis of LH rays for an ITER-like magnetic configuration. Noticeable absorption by alpha paraticles is observed only below ∼5 GHz of the generator frequency and is still acceptable down to ∼3 GHz (19% of the input power Pin, with a 16% reduction of the total driven current for Pin = 50 MW). Two competing quasi-linear effects influence the damping rates: the RF field reduces the slopes of the alpha particle distribution function but, since the absorption is accompanied by acceleration of groups of alpha particles above their initial velocity, the RF field also increases the number of resonant particles. Both the global results and the damping rates are compared with those obtained for unmagnetized alpha particles and with the linear approximation. For frequencies above ∼3 GHz, the latter approach was found to significantly overestimate the absorption by alpha particles. Some further mechanisms not included in the code but having possible effects on the interaction are also discussed.

Studies of challenge in lower hybrid current drive capability at high density regime in experimental advanced superconducting tokamak

“…This was first pointed out by Wong & Ono (1984), supported by calculations by Fisch & Rax (1992a), and led to suggestions (Barbato & Santini 1991;Pavlo, Krlin & Tluchor 1991) that, to propagate LH waves close to the plasma centre while avoiding absorption of the LH wave by alpha particles, it would be necessary to use LH waves in a rather high frequency range, even higher than the 4.6 GHz used here, in EAST tokamak. The waveguides for launching such high frequency waves are technologically more challenging than those used at lower frequency.…”

“…Concerning the application of the LH wave to a reactor, whether in the higher frequency range or the lower, in which many alpha particles could be in the plasma centre, there is a concern that energetic alpha particles in the plasma centre could damp the lower hybrid wave. This was first pointed out by Wong & Ono (1984), supported by calculations by Fisch & Rax (1992 a ), and led to suggestions (Barbato & Santini 1991; Pavlo, Krlin & Tluchor 1991) that, to propagate LH waves close to the plasma centre while avoiding absorption of the LH wave by alpha particles, it would be necessary to use LH waves in a rather high frequency range, even higher than the 4.6 GHz used here, in EAST tokamak. The waveguides for launching such high frequency waves are technologically more challenging than those used at lower frequency.…”

Influence of antenna aiming on ECE in MAST

“…We also use this slab model to determine the position of the UHR nearest to each spot and to obtain here the solution of the electrostatic dispersion relation of the electron Bernstein waves (EBW). This solution serves as an initial condition for the standard ray tracing 6 describing the propagation of EBW in 3D.…”

ECE from MAST — Gaussian beams and antenna aiming problem

“…This solution serves as an initial condition for the standard ray tracing [6] describing the propagation of EBW in 3D.…”