Temporal evolution of neoclassical tearing modes and its effect on confinement reduction in ASDEX Upgrade

It is shown that (3,2) neoclassical tearing modes (NTMs) in ASDEX Upgrade can, under similar external control parameters, occur at significantly different values of beta(N), with the NTMs occurring at higher beta(N) sometimes having a less detrimental impact on the energy confinement. This is found to be due to a coupling between the (3,2) NTM and a (4,3) mode, which can either occur continuously and clamp the (3,2) amplitude for the time of the existence of the (4,3) or occur in bursts on a timescale much shorter than the usual resistive timescale, where each burst leads to a rapid reduction of the (3,2) NTM called an amplitude drop. The amplitude drops may be due to stochastization in the presence of two islands of different helicity and sufficient amplitude, which can act as an anomalous increase in resistivity. These observations point towards the possible existence of a regime where the energy confinement deterioration due to NTMs is acceptable for reactor scale experiments.