Avoidance of disruptions at high /3n in ASDEX Upgrade with off-axis ECRH

摘要

Experiments on disruption avoidance have been carried out in H-mode ASDEX Upgrade plasmas: the localized perpendicular injection of ECRH (1.5 MW ~ 0.2P_(tot)) onto the q =2 resonant surface has led to the delay and/or complete avoidance of disruptions in a high β_N scenario (I_p =1 MA, B_t =2.1 T, q_(95) ~ 3.6, with NBI ~7.5 MW). In these discharges (at low q_(95) and low density) neoclassical tearing modes (NTMs) are excited: the growth and locking of the m =2/1 mode leads to the disruption. The scheme of the experiment is successfully applied in the same way as in previous disruption avoidance experiments in FTU and ASDEX Upgrade. As soon as the disruption precursor signal (the locked mode detector and/or the loop voltage) reaches the preset threshold, the ECRH power is triggered by real-time control. A poloidal scan in deposition location (P_(dep)) has been carried out by setting the poloidal launching mirrors at different angles in each discharge. The results depend on p_(dep): complete disruption avoidance can be achieved when the power is injected close to or onto the 2/1 island. When ECRH is injected outside the island (either at radii inside or outside the q — 2 surface), the discharge is disrupted as in the reference case.