Rotation driven by fast ions in tokamaks

Abstract

Collective fast ion effects on flows in tokamaks are investigated analytically and numerically. A general analysis of noncollisional electrodynamic momentum transfer from fast ions to bulk plasma is presented, with polarization effects and dissipation in the bulk plasma taken into account. The analysis is illustrated using idealized simulations of fast ion orbits and radial electric fields in the Mega-Ampère Spherical Tokamak (MAST) [A. Sykes, R. J. Akers, L. C. Appel et al., Nucl. Fusion 41, 1423 (2001)], the Joint European Torus (JET) [P. H. Rebut et al., Nucl. Fusion 25, 1011 (1985)], and ITER [R. Aymar, P. Barabaschi, and Y. Shimomura, Plasma Phys. Controlled Fusion 44, 519 (2002)]. In the MAST simulation, prompt losses of beam ions injected counter to the plasma current drive up a radial electric field that saturates at a level such that beam ions subsequently injected are confined electrostatically. Although the actual radial electric fields in counterinjected MAST discharges are lower than this, the scenario explored in the simulation would be approached in MAST plasmas with sufficiently low collisionality. The JET simulation, although unrealistic, shows that a similar process could be driven by losses of fusion α-particles from a burning plasma. Test-particle simulations of α-particles in ITER suggest that performance-limiting instabilities such as neoclassical tearing modes and resistive wall modes could be affected significantly by flows associated with radial fast particle currents.