Tearing mode formation and radiative edge cooling prior to density limit disruptions in ASDEX Upgrade

Tearing mode formation is investigated for ohmic density limit discharges on the ASDEX Upgrade tokamak at medium and high safety factors (q(95) = 3.8, 4.9, 6.0). Low electron temperatures inside magnetic islands and the observation of localized C III impurity radiation suggest that a thermal instability, as proposed by Rebut and Hugon, destabilizes the (m,n) = (3,1) and (4,1) islands, which grow during the current profile contraction phase. In contrast, the (2,1) islands appear to be thermally stable. Minor disruptions lead to step-wise loss of confinement, first localized at the q = 2 surface, then after a time delay comparable to the resistive time-scale, at the q = 3 surface and after a second time delay, at the q = 4 surface. It is found that (3,1) islands, unlike (2,1) islands, are quenched by the high heat flux during minor disruptions.