Sawtooth, Neo-Classical Tearing Mode and Error Field Studies in JET

A range of ITER relevant plasma stability issues has been studied in the JET tokamak; these include the effects of fast particles on sawteeth, control of NTMs via sawtooth seeds, scaling of m=2, n=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 b-limit, suggesting the value of local current drive near q=1. The marginal-b (bN,marg) below which the m=3, n=2 NTM is unconditionally stable has been examined; it is found that bN,marg scales almost linearly with r* and typical H-mode (with q953) discharges are meta-stable to NTMs (ie a sufficiently large seed can always destabilise an NTM). The m=2, n=1 NTM 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 b-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.
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