Momentum Conservation in Current Drive and Alpha-Channeling-Mediated Rotation Drive

TL;DR

This paper develops a self-consistent quasilinear theory for electrostatic waves in fusion plasmas, demonstrating how alpha channeling can drive plasma rotation while conserving momentum, resolving previous theoretical inconsistencies.

Contribution

It introduces a linear-quasilinear theory that captures alpha channeling and rotation drive, emphasizing the importance of nonresonant particle response for momentum conservation.

Findings

Waves can drive currents without momentum injection in collisionless plasmas.

Steady-state waves with nonuniform structure enable rotation drive, unlike growing plane waves.

The theory respects local and global momentum conservation laws.

Abstract

Alpha channeling uses waves to extract hot ash from a fusion plasma, while transferring energy from the ash to the wave. Intriguingly, it has been proposed that the extraction of this charged ash could create a radial electric field, efficiently driving ExB rotation. However, existing theories ignore the response of the nonresonant particles, which play a critical role in enforcing momentum conservation in quasilinear theory. Because cross-field charge transport and momentum conservation are fundamentally linked, this non-consistency throws the whole effect into question. Here, we review recent developments that have largely resolved this question of rotation drive by alpha channeling. We build a simple, general, self-consistent quasilinear theory for electrostatic waves, applicable to classic examples such as the bump-on-tail instability. As an immediate consequence, we show how waves can drive currents in the absence of momentum injection even in a collisionless plasma. To apply this theory to the problem of ash extraction and rotation drive, we develop the first linear theory able to capture the alpha channeling process. The resulting momentum-conserving linear-quasilinear theory reveals a fundamental difference between the reaction of nonresonant particles to plane waves that grow in time, versus steady-state waves that have nonuniform spatial structure, allowing rotation drive in the latter case while precluding it in the former. This difference can be understood through two conservation laws, which demonstrate the local and global momentum conservation of the theory. Finally, we show how the oscillation-center theories often obscure the time-dependent nonresonant recoil, but ultimately lead to similar results.