Abstract
It is shown that (3,2) neoclassical tearing modes (NTMs) inASDEX Upgrade can, under similar external control parameters, occur atsignificantly different values of βN, with the NTMs occurring athigher βN sometimes having a less detrimental impact on theenergy 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 andclamp the (3,2) amplitude for the time of the existence of the (4,3) oroccur in bursts on a timescale much shorter than the usual resistivetimescale, where each burst leads to a rapid reduction of the (3,2) NTMcalled an amplitude drop. The amplitude drops may be due tostochastization in the presence of two islands of different helicity andsufficient amplitude, which can act as an anomalous increase inresistivity. These observations point towards the possible existence ofa regime where the energy confinement deterioration due to NTMs isacceptable for reactor scale experiments.
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