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

TL;DR

This paper explores how alpha-channeling in lower hybrid current drive can be coupled with the parallel wavenumber upshift, revealing conditions for wave amplification and implications for tokamak operation.

Contribution

It identifies the coupling between alpha-channeling and wavenumber upshift, and analyzes how wave launch location influences wave amplification in tokamaks.

Findings

Wave amplification is coupled with the parallel wavenumber upshift.

Launching waves from the high-field side favors alpha-channeling.

Symmetries in the interaction constrain amplification conditions.

Abstract

Although lower hybrid waves have been shown to be effective in driving plasma current in present-day tokamaks, they are predicted to strongly interact with the energetic alpha particles born from fusion reactions in eventual tokamak reactors. However, in the presence of the expected steep alpha particle birth gradient, this interaction can produce wave amplification rather than wave damping. Here, we identify the flexibilities and constraints in achieving this amplification effect through a consideration of symmetries in the channeling interaction, in the wave propagation, and in the tokamak field configuration. Interestingly, for standard LH current drive that supports the poloidal magnetic field, we find that wave amplification through alpha channeling is fundamentally coupled to the poorly understood parallel wavenumber upshift. In so doing, we show that wave launch from the tokamak high-field side is favorable both for alpha-channeling and for achieving the parallel wavenumber upshift.