Sawtooth, neo-classical tearing mode and error field studies in JET

Abstract

A range of ITER relevant plasma stability issues have been studied in the JET tokamak; these include the effects of fast particles on sawteeth, control of NTMs via sawteeth seeds, scaling of (2,1) NTM thresholds and the effect of error fields on plasma rotation and vice-versa. The sawtooth studies have examined both the effects of ICRF and NBI fast particles, and models used for ITER predictions have been validated. Large sawteeth, due to fast particle stabilisation, have been shown to trigger m=3, n=2 NTMs at low beta. Conversely the use of ICCD to produce small short period sawteeth has been shown to raise the NTM beta-limit, suggesting the value of local current drive near q=1. The m=2, n=1 NTM, which when destabilised results in severe degradation of plasma performance, has been studied in a joint scaling experiment with the DIII-D tokamak. These experiments using closely matched non-dimensional parameters (plasma shape, R/a, q, gyro-radius and collisionality) on DIII-D and JET show similar NTM beta-limits and consistent scalings. The issue of error field thresholds and their variation with plasma rotation has also been studied, helping to clarify scalings to ITER.