ECRH antenna at 140 GHz on FTU Tokamak

The quasi-optical beam launching antenna for the Electron Cyclotron Resonan ce Heating (ECRH) experiment at 140 GHz, 1.6 MW to the plasma, on the Frasc ati Tokamak Upgrade (FTU) has been successfully exploited in experimental o perations. It provides four beams 400 kW each, with independent poloidal an d toroidal steering capability and a maximum power density of 60 kW/cm(2) a t the plasma edge. The beam radius in the plasma is approximate to 20 mm, a llowing a very high localisation of the absorbed power. The main characteri stics of the antenna are: Four movable launching mirrors under vacuum, far from the plasma edge (no m ovable parts near the plasma). Oblique toroidal injection capability at fixed angles obtained with reflect ions on two fixed stainless steel plates, gold plated, inserted at the side s of the port. Vacuum gate separating the main vacuum from the appendix containing the two matching mirrors and the barrier window (for safety and maintenance). All movements are transferred outside the vacuum chamber through bellows an d linear displacement. Actuators and encoders are in air. Full capability to adjust small movements of the machine during cool-down a nd warm-up, preserving the alignment. Pattern measurements at low power before installation were performed to cha racterise the effects of the real system on the ideal shape and the polaris ation of the launched beams, with particular attention to diffraction effec ts. Final control of the shape of the beams and the alignment of the mirror s in the system is performed after installation, under vacuum and with the tokamak at liquid nitrogen temperature, with the aid of a retractable probe and using a thermopile as the RF detector. (C) 2001 Elsevier Science B.V. All rights reserved.