Effects of ion cyclotron harmonic damping on current drive in the lower hybrid frequency range

Abstract

We investigate the ion cyclotron harmonic damping effects on slow and East waves in the lower hybrid frequency range for tokamak reactor parameters. Inclusion of the higher order terms in the *iot plasma dielectric tensor introduces ion cyclotron harmonic damping; these terms also contribute to the real part of the dispersion relation and affect the wave trajectories. However, wave absorption by 15 keV deuterium and tritium ions can be avoided by choosing the slow wave frequency above the lower hybrid frequency and the fast wave frequency below the lower hybrid frequency. But preliminary estimates show that energetic alpha particles tend to absorb both the slow and the fast waves. This absorption may become a serious obstacle for fusion-reactor current drive in the lower hybrid frequency range.

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.…”

“…However, the calculations of the energetic alpha particle damping (Wong & Ono 1984;Barbato & Santini 1991;Pavlo et al 1991;Fisch & Rax 1992a) do not take into account that the wave-particle interaction occurs in a radial gradient of the alpha particles. Because the alpha particles in the centre are energetic, but cool at the periphery, the alpha particles can amplify the LH waves through the alpha channelling effect (Fisch & Rax 1992b).…”

Methods of Radio-Frequency Current Drive

“…In fact, it was predicted that alpha particles might damp the lower hybrid wave, interfering with the LHCD effect in working reactors. 28 This prediction was verified, with suggestions for mitigating the effect based upon radial diffusion of electrons. 29 However, it remained unclear, in an environment of alpha-particle heating, whether the LHCD effect could work near the plasma center or whether the current near the plasma center would have to be driven by some other means.…”

Coupling of alpha channeling to parallel wavenumber upshift in lower hybrid current drive

“…Lower hybrid (LH) waves are predicted to be effective in driving substantial plasma current in tokamaks 1 , an effect that has enjoyed extensive demonstration in tokamak experiments 2 . Yet there remains a concern that, in extrapolating to a fusion reactor, high-energy α particles born in the plasma core could strongly damp the LH wave, thus significantly reducing current drive efficiency [3][4][5] . Fortunately, by coupling diffusion in energy to diffusion in space (known as alpha channeling), a favorable population inversion may appear along the diffusion path, causing the α particles to amplify rather than damp the wave 6 .…”