Strategies for modifying alpha driven TAE thresholds through q profile and ion temperature control

Abstract

Two methods are examined for influencing the stability thresholds of alpha driven toroidal Alfven eigenmode (TAE) instabilities in the deuterium-tritium (DT) operation of the Tokamak Fusion Test Reactor (TFTR). These are: (a) increasing the TAE drive through raising the central q(0) value (in order to centre the peak alpha pressure gradient on the lower m gap locations) and (b) decreasing the TAE damping rate on the background ions through cooling perturbations such as helium puffs or injected pellets. A gyrofluid model of the TAE is used that is inherently non-perturbative and includes continuum damping, ion/electron/neutral beam Landau damping, ion FLR effects and a generalized resistivity (finite E1 effects). Specific TFTR DT cases are analysed using a combination of measured and inferred profiles. The resulting predicted thresholds are generally consistent with an observed lack of strong TAE activity. However, it is found that special techniques, especially increasing q(0), can significantly lower the alpha driven TAE threshold into ranges that should be experimentally accessible