Alfvén wave heating and current drive

Abstract

Progress in the theory of Alfvén wave (AW) heating, current drive, and plasma flow, which may be relevant for improved confinement scenarios in tokamaks, is discussed. The effect of poloidal mode coupling on the power deposition of AW eigenmodes is investigated. Resonant absorption of the coupled side bands causes a broad power deposition close to the plasma boundary which can surpass the power deposition of the main global AW at the plasma center. A small population of impurities, such as carbon, in a hydrogen plasma can strongly modify the dispersion of the global AW, the AW continuum and the wave dissipation profile. The ponderomotive forces produced by rf fields are expressed as a sum of a gradient (radial derivative) part and of a wave momentum transfer force found to be proportional to wave dissipation. Finally, the rf force is balanced by the friction (or neo-classical viscosity) force to calculate relevant current drive and plasma specie flows. Estimates of AW power requirements to drive substantial heating, current, and poloidal flow are made for plasmas in the Tokamak Chauffage Alfvén wave experiment in Brazil (TCABR) [L. Ruchko et al., Nucl. Fusion 30, 503 (1996)]. The first results of Alfvén wave heating and current drive experiments with a low level of wave dissipation in the TCABR are discussed.

Nonlinear ponderomotive force by low frequency waves and nonresonant current drive

“…Low-frequency waves, Alfvén waves or fast waves, were considered as an attractive mechanism of driving plasma current because of its potential high efficiency, no density limit, and the convenience of high power rf generating and launching. 1,2 However, electron trapping may dramatically reduce the current drive efficiency in the subthermal resonant regime, although possibly the momentum carried by trapped electrons will be retained and returned by collisions or the bootstrap current in the steady state. 1,3 Alternatively, the possibility of increasing the current drive efficiency by helicity injection has been proposed by Ohkawa.…”

Double-Gap Alfvén Eigenmodes: Revisiting Eigenmode Interaction with the Alfvén Continuum

“…Damping on the continuum may affect the stability of shear Alfvén eigenmodes in a tokamak reactor, which, in turn, affects the confinement of energetic fusion products, α-particles. Also the absorbtion by the continuum prevents externally excited low frequency Alfvén waves from being used for plasma heating and current drive as only the plasma periphery is affected [7,8]. By contrast DGAEs can potentially couple the plasma edge to the core with implications for fast ion transport, heating and current drive, and the use of external antennas as a diagnostic of the plasma core.…”

Current Drive in a Ponderomotive Potential with Sign Reversal

“…These possibilities include the use of the non-resonant ponderomotive forces in driving plasma current [7][8][9][10][11]. Litwin [12] suggests that there is cancellation that reduces the so-called alpha effect relied upon by others [9].…”