Entrainment of MHD modes in ASDEX Upgrade using rotating non-axisymmetric perturbation fields

Recent ASDEX Upgrade experiments investigate the entrain ment of a fast rotating (around 2 - 5 kHz) m=2, n=1 tearing mode to a slowly rotating perturbation field (5 - 1 0Hz range) applied by in - vessel coils. W ithout entrainment the mode would have locked to the wall and could have caus e d a dis ruption. The ultimate goal of these experiments is active disruption avoidance by externally applied magnetic perturbati ons. In this work we start investigating the physics basis of this approach and our initial res ults can be co ns idered encouraging . This experiment used a set of 16 active magnetic perturbation coils, called B - coils, dis tributed along the toroidal angle on the low field s ide in an upper and a lower row of 8 coils, almost symmetric with res pect to the midplane [1]. Recently, new AC power supplies became available with 4 individual controllers , which made thes e experiments pos s ible. An improved H - mode plasma at moderate elongation with ? ? between 2 and 3 was selected as the target configuration. An external n=1 perturbation, applied with the B - coils, with a relative phase between the upp er and lower rows that maximizes the plas ma res pons e ( according to what was obs erved in previous exper iments [2]) produces at first a global rota tion b raking [3] . As the plasma rotation is reduced below approximately half its initial value , not indicating a direct causality , a fast rotating m=2, n=1 tearing mode is triggered , which s lows down and locks to the e xternally applied s lowly rotating fiel d , i.e. the mode can be entrai ned . Later, with reduced ampl itude, the mode unlocks fro m the perturbation field and rotates again with its natural frequency. In the pres ence of the l oc ke d mode a s trong decrease of ? ? was observed , which recovers as soon as the mode starts rotating again . Analysis of the ECE signals confirm s that the slowly rotating, entrained mode maintains its tearing nature, s ince an is land s tructure with amplitude os cillating in phas e with the a pplied perturbation was detected. The plasma response is also characterized by analysis of a large set of magnetic probes and kinetic diagnostics. The paper als o attempts to interpret t heoretically the experiments by comparing the experimental plasma resp ons e with tha t inferred from stability code. Moreover cons idering s implified torque balance models the measured plasma flow decele ration is interpreted according to the prediction of the neoclassical viscous and resistive layer terms.