Nonresonant suppression of the sawtooth instability by passing ions in tokamaks

A new interpretation is presented for the well-known observation of sawteeth stabilization, during ion cyclotron resonant heating, in the Joint European Tokamak [Campbell et al., Phys. Rev. Lett. 60, 2148 (1988)] and the Tokamak Fusion Test Reactor [Phillips et al., Phys. Fluids B 4, 2155 (1992)]. It is shown that the radial fluxes of superthermal and thermal ions exchange a finite amount of power with the m=1 mode (associated with the sawtooth instability) which is suppressed. The dominant contribution to this effect is provided by the passing ions, which experience in the presence of the mode a much faster (than the trapped ions) radial diffusion. This is a nonresonant electrostatic interaction between the passing ions and the finite parallel electric field (along the perturbed magnetic field line), which takes place at any mode frequency. For the requirements of the present theory, the concepts of the finite parallel electric field, and the diffusion of passing ions, in the presence of a magnetic mode are also developed. The present work also provides the interpretation for the suppression of the sawtooth instability by passing ions, during tangential neutral beam injection, in the Japan Atomic Energy Research Institute Tokamak Upgrade (JT-60U) [Kramer et al., Nucl. Fusion 40, 1383 (2000)].