Alpha Channeling with High-field Launch of Lower Hybrid Waves

TL;DR

This paper investigates the potential for wave amplification of lower hybrid waves launched from the high-field side in tokamaks, which could enhance current drive efficiency in future fusion reactors.

Contribution

It quantitatively analyzes the radial gradients needed for wave amplification via high-field launch, expanding understanding of alpha channeling in fusion plasmas.

Findings

Wave amplification is achievable with specific radial gradients.

High-field side launch enhances wave amplification prospects.

Simulations demonstrate linear regime amplification possibilities.

Abstract

Although lower hybrid waves are effective at driving currents in present-day tokamaks, they are expected to interact strongly with high-energy particles in extrapolating to reactors. In the presence of a radial alpha particle birth gradient, this interaction can take the form of wave amplification rather than damping. While it is known that this amplification more easily occurs when launching from the tokamak high-field side, the extent of this amplification has not been made quantitative. Here, by tracing rays launched from the high- field-side of a tokamak, the required radial gradients to achieve amplification are calculated for a temperature and density regime consistent with a hot-ion-mode fusion reactor. These simulations, while valid only in the linear regime of wave amplification, nonetheless illustrate the possibilities for wave amplification using high-field launch of the lower hybrid wave.