Plans for a new ECRH system at ASDEX upgrade

Construction of a new ECRH system for ASDEX Upgrade with a power of 4 MW and a pulse duration of 10 sec with linearly polarized Gaussian output has started last year. Four gyrotrons with single stage depressed collector will generate the power. A particular feature will be the possibility to operate them at different frequencies. The first gyrotron can work at 105 GHz and at 140 GHz, making use of the resonances of the diamond vacuum window at these frequencies. A second step-tuneable gyrotron is designed to work at several frequencies within the same interval. A diamond output window mounted at the Brewster angle allows broadband transmission. Two gyrotrons will be fed from one thyristor controlled power supply 70 kV / 80 A. However, each gyrotron will have its own series tetrode modulator with an output of 60 kV / 40 A.The body will be driven from a switching power supply 45 kV / 0.3 A with 5 kHz modulation capability. The whole system is planned to be operated remotely. The transmission line, similar to the one which is used now in the first ASDEX Upgrade ECRH system, will have a quasioptical section next to the gyrotrons and a 70 m long corrugated waveguide line with 87 mm i.d. ... mehrat normal air pressure. The corrugation profile was optimised for the broadband transmission. The quasioptical section contains phase correcting mirrors, a pair of broadband polarisation mirrors and a switchable mirror to direct the beam into a dummy load or a calorimeter load. The dummy load is located central in the system with a rotatable mirror which can be directed towards each one of the 4 gyrotrons. At the torus side of the transmission line we will install short pulse loads (˜ 0.1 sec) which allows testing of the transmission line every day before the experiments start. For the torus window we will also use a diamond window resonant at the two frequencies. However, for the step-tuneable transmission line we think of using a tuneable double disk diamond window, instead of a Brewster angle window, to allow transmission of an arbitrary polarisation without the need for polarising mirrors inside the torus. The launchers will allow to scan the whole poloidal crossection at toroidal angles from -25° to +25° at moderate speed between the pulse. Over a limited range the poloidal scan is planned to be fast (10° in 100 msec) with the intention of a feedback controlled power deposition. The launching mirror will be made of graphite with a conductive copper coating.