Control of NTMs by ECCD on ASDEX Upgrade in view of ITER application

Abstract

The predictions of the generalised Rutherford equation for the stabilisation of neoclassical tearing modes (NTMs) are reviewed. They suggest that the stabilisation efficiency can be maximised by maximising the current density within the island, favouring narrow deposition over maximum total current. Also, for ITER, where it is expected that the minimum island size before stabilisation will be small with respect to the deposition width, a loss of efficiency for continuous injection is predicted, but can be recovered by phased injection with respect to the island's O-point. The paper compares in detail these predictions with dedicated experiments on ASDEX Upgrade and finds good qualitative agreement with the generalised Rutherford equation. For quantitative agreement, the experimental database is not yet firm enough. The conclusion for ITER is that JECCD should be optimised and that modulation capability of the gyrotrons should be foreseen to ensure optimum stabilisation efficiency in the small island regime.