Experiments with reduced single pass absorption at ASDEX Upgrade – instrumentation and applications

Reflecting gratings have been installed in the vacuum vessel of ASDEX Upgrade for all beamlines of the electron cyclotron resonance heating system. Potentially unabsorbed millimetre wave power after the first pass through the plasma is redirected towards the plasma centre. This increases the efficiency of heating schemes with reduced single pass absorption like O-2 or X-3. In order to monitor beam position and power, thermocouples were installed into the gratings. A numerical model was developed to evaluate the beam intensity during short pulses from the thermocouple measurement in a non-stationary environment. An experiment was carried out, where only the X-3 resonance is present in the plasma, and the millimetre wave beam shine-through was measured successfully as a function of the central plasma electron temperature. This allows to deduce the X-3 absorption experimentally. Scanning the launching angles, it seems possible to measure the 2D beam cross section after the first pass through the plasma.