Triggering of neo-classical tearing modes by mode coupling

The formation of magnetic islands with mode numbers (m>1,n⩾1) apparently triggered by sawtooth crashes has for a long time been a concern in JET discharges with auxiliary heating as it affects the peak performance, in terms of stored energy or of neutron yield (in DT experiments). The phenomenological association of the sawtooth crash with the onset of resistive modes is particularly relevant in the context of neo-classical tearing modes (NTM) theory that requires the formation of a `seed' island above a critical value. Relying on observations and on an understanding of the role of the collapse phase of the (m = 1,n = 1) instability, we propose as physical mechanism of destabilization of the metastable NTM a forced reconnection process due to electro-dynamic mode coupling. For a definite mode-number congruence and frequency matching conditions a bifurcation pattern appears in the behaviour of the driven mode. Modelling of a JET discharge shows that for perfect frequency matching conditions the triggering of the (m = 3,n = 2) NTM could be due to either two-mode toroidal coupling or three-mode non-linear coupling. The observed mode frequencies in the cases examined, suggest non-linear coupling, with the (m = 3,n = 2) mode being driven by (m = 1,n = 1) and (m = 4,n = 3) modes, the (m = 1,n = 1) being provided either by the sawtooth precursor or by fishbones.