MHD modes in regular and reversed shear scenarios and possibilities for their control through current drive

Abstract

The onset conditions for neoclassical tearing modes are shown to be over a wide range of experiments in good agreement with predictions of the polarization current model. In the presence of sufficiently large seed islands, this model extrapolates unfavourably to reactor-grade devices operating with peaked current profiles. The expected decrease of the seed islands produced by sawteeth at the (3, 2) and (2, 1) resonant surfaces may counteract this trend, but extrapolation of this effect has large uncertainties. Active control of this mode on ASDEX Upgrade and COMPASS-D through modulated or unmodulated current drive at the resonant surface has been demonstrated to stabilize this mode, in the electron cyclotron current drive (ECCD) case with a small fraction of driven current (2% of Ip). In advanced scenarios, magnetohydrodynamic (MHD) modes can contribute to the stationarity of the current profile. Fishbones can clamp the current profile development near the q = 1 or q = 2 surface, without significantly deteriorating energy confinement, whereas double-tearing modes, acting in a similar form, lead to substantial confinement losses. First experiments on ASDEX Upgrade with application of central electron cyclotron resonance (ECR)-heating and current drive to discharges with an internal transport barrier show a substantial effect on MHD stability, affecting the passage of the q-profile through qmin = 2, and degrading or prolonging the core reversed shear phase, depending on the current drive direction.