Trapped particle precession in advanced tokamaks

Abstract

Plasma equilibrium parameters affect the precessional drift velocities of trapped particles, which can significantly influence plasma stability (e.g. collisionless trapped electron driftwaves and fishbones). The calculation outlined here computes the bounce-averaged drift velocities of trapped particles for toroidally symmetric plasmas of arbitrary aspect ratio with elliptical and triangular shaping of the poloidal plasma cross section. Local equilibrium parameters are specified to describe the geometry and magnetic field structure on a given up down symmetric flux surface. Making use of an expansion in the neighbourhood of the flux surface the precessional drift velocities of trapped particles can be calculated. The calculation is then applied to flux surfaces with the properties characteristic of JET and also of the START experiment, where it has been postulated that favourable trapped particle drift effects may be responsible for improved confinement. Our calculation fails to confirm this idea, and indeed suggests that the drive for instability from toroidal drifts at START is similar to that in JET. The picture is complex with the competing effects of aspect ratio, plasma shaping, shear and pressure gradients. Desirable features needed in the equilibria (e.g. high pressure gradients and low shear) are discussed. Drifts are also calculated in extreme equilibria where mod B mod does not decrease monotonically around the flux surface to a minimum on the outboard mid-plane.