The 70 GHz/1 MW long-pulse ECRH system on the advanced stellarator W VII-AS

For plasma start-up and heating in the new Garching W VII-AS stellarator an ECRH microwave system is under construction which comprises five subsystems each designed to generate, transmit and radiate into the plasma 200 kW of millimeter-wave power with pulse length up to 3 s at a frequency of 70 GHz. Each subsystem consists of a microwave power source module, a transmission line and a quasi-optical wave launching antenna for plasma irradiation in the almost perfectly linearly polarized 'Gaussian-like' HE11 mode. The overall efficiency in the desired mode of a complete transmission line is expected to be approximately 87%. The microwave power and the mode composition at the gyrotron output and in the transmission line are measured by special calorimeters and wavenumber spectrometers, respectively. Design and realization of the system are described together with extensive low-power and first high-power measurements. (author)