Neo-Classical Tearing Mode Control through sawtooth Destabilisation in JET
The m=3, n=2 Neoclassical Tearing Mode (NTM) destabilisation requires the pre-existence of a magnetic seed island which is often generated by the plasma perturbations associated with sawteeth. A new strategy was recently developed at the Joint European Torus (JET) to control the NTM seed island by modifying the sawtooth activity. Ion Cyclotron Current Drive (ICCD), with waves tuned to the 2nd harmonic hydrogen resonance, has been used to destabilise the sawteeth and thus to increase the plasma pressure at which an NTM is triggered. As the 2nd harmonic hydrogen damping was most effective placing the resonance RIC on the Low Field Side (LFS) of the plasma, this paper will emphasize the LFS case analysis. In these experiments a scan in the RIC position with respect to the sawtooth inversion radius, Rinv, has been performed. The results are compared to the scaling, obtained for JET discharges with NBI heating only, of the normalised plasma pressure at which the NTM is triggered bNonset (= bt[%]a[m]B0[T]/Ip[MA]) with the normalised poloïdal ion Larmor radius. Two groups of discharges have been identified. When RIC is in a narrow range outside Rinv sawtooth destabilisation occurs, and bNonset is increased to a value significantly above the scaling law prediction and close to the ideal b-limit, bN~4. On the other hand, when RIC is well outside Rinv, or well inside Rinv, sawtooth destabilisation is lost and bNonset is close to the scaling law values. The sensitivity to the resonance position is observed in particular during the NBI power ramp; as the plasma pressure is increased, diamagnetic effects can lead to a drift of RIC significant enough to lose sawtooth destabilisation. Increases in the bNonset values are expected with a programmed change in the magnetic field in order to compensate resonance shifts. In order to develop this strategy, ways to maintain ICCD in presence of ELMs and ways to optimize the ICCD efficiency will also be presented.