ON THE WAY TO A MULTI-FREQUENCY ECRH SYSTEM FOR ASDEX UPGRADE
The ECRH system for ASDEX Upgrade is currently extended by 4 multi-frequency units of 1 MW, 10 s each. The first unit, a two-frequency system is operational since early 2007. The contribution reports on the commissioning of this unit and on two examples of application: at 140 GHz the system is crucial for H-mode operation with the fully W-coated plasma facing components now present in ASDEX Upgrade for moderate values of density and safety-factor, to suppress central accumulation of heavy impurities. At 105 GHz the EC-beam is used as probe for a collective Thomson scattering diagnostic, which uses the launcher of the second unit as receiver. The other 3 units shall additionally operate at intermediate frequencies, requiring alternative solutions for the windows. For the gyrotron window a Brewster-angle set-up is used successfully. At the torus a Fabry-Pérot-type double-disk window is used. It has been tested successfully with low power.
