Progress in the new ECRH system for ASDEX Upgrade

Summary form only given. The new ECRH system which is currently under construction at ASDEX Upgrade is aimed for a total power of 4 MW, generated by 4 gyrotrons, and a pulse length of 10 sec. The system should allow for central heating at different neoclassical tearing modes (NTM) through the possibility of current very localized feedback controlled power deposition with a narrow beamwidth by using a fast steerable launcher. The transmission line components are especially designed to cope for the large bandwidth of gyrotron to be installed can work at 105 GHz and at 140 GHz. The operating modes are TE/sub 17.6/ and TE/sub 22.8/. Here we make use of the 3/spl lambda//2 and 4/spl lambda//2 resonances of the single disk synthetic diamond vacuum window at these frequencies. A second gyrotron is step-tunable, i.e. it can work at several frequencies within the same frequency range. For each frequency the gyrotron oscillates in a different cavity mode. Short pulse test gyrotrons at FZK and IAP have demonstrated efficient generation of power in excess of 1 MW at all these frequencies. The frequency can be changed between two ASDEX Upgrade pulses and requires a resetting of the cryomagnetic field at the cavity, the gun magnetic field and the collector magnetic field. There is one complicating feature with such a gyrotron: the output beam leaves the gyrotron window at slightly different angles and positions so that mirror adjustment in the matching optics unit is necessary. In order to limit the number of required phase correcting mirror sets we choose 4 frequencies as our main operating modes. The torus window required for the step-tunable gyrotron should be in principle broadband, e.g. a Brewster window. To avoid constraints in the polarization and the quasi-optical design, a tunable-disk window with a remote controlled adjustment of the distance between the disks is used. For the two-frequency gyrotrons a single-disk window is sufficient.