Progress with the new multi-frequency ECRH system for ASDEX upgrade

Summary form only given. A multi-frequency ECRH system is currently under construction at the ASDEX Upgrade tokamak experiment. The system employs depressed collector gyrotrons, step-tunable in the range 105-140 GHz, with a maximum output power of 1 MW and a pulse length of 10 s. One two-frequency GYCOM gyrotron is in routine operation at ASDEX Upgrade since 2006. A further extension of the system with 3 more gyrotrons is underway. These gyrotrons will be step-tunable and operate at two additional intermediate frequencies between 105 and 140 GHz. They will be equipped with broadband Brewster windows. Since the transmission of the power from the gyrotron to the tokamak is in normal air, a second broadband vacuum window is required at the tokamak vacuum barrier. These windows must be broadband also for elliptically polarized beams. Therefore tunable double disc CVD diamond windows will be installed at the torus. To condition and test the gyrotrons a new long-pulse dummy load, capable of 1 MW, 10 s and operation in normal air has been operated very reliably. One of the main applications of the new ECRH system will be the suppression of neoclassical tearing modes (NTM). For this reason fast-steerable mirrors have been installed. This capability will allow feedback control of the deposition on the time scale of NTM growth, providing the possibility to validate this scheme for ITER in ASDEX Upgrade. For NTM stabilization experiments a fast modulation capability of the gyrotrons is required and was tested. This is especially important for future experiments like ITER where the width of the driven EC current will be larger than the marginal island size of the NTM leading to a loss of current drive efficiency in the non-modulated case.