1 EXW / P 7-25 Extension of the ECRH Operational Space with O 2-and X 3-Heating Schemes to control Tungsten Accumulation in ASDEX Upgrade

ASDEX Upgrade is operated since several years with tungsten coated plasma facing components. H-mode operation with good confinement has been demonstrat ed. Nevertheless purely NBI-heated H-modes with red uc gas puff, moderate heating power or/and increased trian gularity tend to accumulate tungsten, followed by a radiative collapse. Under these conditions, central electron heating with ECRH (X2) changes the impurity transpo rt in the plasma center, reducing the central tungsten concen tration and, in many cases, stabilizing the plasma. In order to extend the applicability of central ECRH to a wider range of magnetic field and plasma current additio nal ECRH schemes with reduced single pass absorption have be en implemented: X3 heating allows to reduce the mag netic field by 30 %, such that the first H-modes with an ITER-l ike value of the safety factor q95 = 3 could be run in the tungsten-coated device. O2 heating increases the cu toff density by a factor of 2 allowing to address h igher currents and triangularities. In case of central X3 heating, the X2 resonance lies close to the pedestal top at the high-field side of the plasma, serving as a beam dump. For O2, holo graphic mirrors have been developed which guarantee a second pass through the plasma center. The beam position o n these reflectors is controlled by fast thermocoup les. Strayradiation protection has been implemented using sni ffer-probes.