Requirements on localized current drive for the suppression of neoclassical tearing modes

A heuristic criterion for the full suppression of an NTM was formulated as η_NTM ≡ j_CD,max/j_BS ≥ 1.2 (Zohm et al 2005 J. Phys. Conf. Ser. 25 234), where j_CD,max is the maximum in the driven current density profile applied to stabilize the mode and j _BS is the local bootstrap current density. In this work we subject this criterion to a systematic theoretical analysis on the basis of the generalized Rutherford equation. Taking into account only the effect of j _CD inside the island, a new criterion for full suppression by a minimum applied total current is obtained in the form of a maximum allowed value for the width of the driven current, w_dep, combined with a required minimum for the total driven current in the form of w_depη _NTM, where both limits depend on the marginal and saturated island sizes. These requirements can be relaxed when additional effects are taken into account, such as a change in the stability parameter Δ′ from the current driven outside the island, power modulation, the accompanying heating inside the island or when the current drive is applied preemptively. When applied to ITER scenario 2, the requirement for full suppression of either the 3/2 or 2/1 NTM becomes w_dep ≲ 5 cm and w_depη _NTM ≳ 5 cm in agreement with (Sauter et al 2010 Plasma Phys. Control. Fusion 52 025002). Optimization of the ITER ECRH Upper Port Launcher design towards minimum required power for full NTM suppression requires an increase in the toroidal injection angle of the lower steering mirror of several degrees compared with its present design value, while for the upper steering mirror the present design value is close to the optimum.