ECRH SYSTEMS FOR LINEARLY POLARIZED PLASMA IRRADIATION IN THE HE11 MODE AT 28 AND 70 GHZ
The transmission lines and launching systems for ECRH at 28 GHz (200 kW, 40 ms) and 70 GHz (200 kW, 100 ms) on the Garching W VII-A stellarator are described. Periodically rippled wall mode converters transform TEOn gyrotron output mode mixtures into a pure TEO1 wave, which is used for long distance transmission through overmoded waveguides. The mode purity is conserved by using optimized tapers and corrugated bends. A mode converter with constant diameter and periodically modulated curvature transfers the circular symmetric TEO1 mode into the almost linearly polarized TE11 mode with an efficiency of 96-97%. In a further step the Gaussian HE11 mode with nearly no cross polarization is produced in a circumferentially corrugated mode converter (efficiency 98.5%). The overall efficiency of the complete microwave system in the wanted mode is about 90% at 28 GHz and about 87% at 70 GHz. Mode content and reflected power in the different sections of the transmission lines are controlled by several mode selective directional couplers and stacked thermographic paper mode pattern analyzers. The ECR waves are launched in the O mode from the low field side of the torus. The unabsorbed fraction of the beam is focussed and reflected into the plasma in the X-mode by a curved corrugated polarization twist reflector mounted on the inner torus wall. The RF power distribution on the reflector is monitored by coupling holes. Full power operation (about 200 kW) of the 28 GHz ECRH system has been achieved by decoupling the gyrotron from the plasma load by means of helix mode filters and newly developed, highly efficient anisotropic wall mode filters. The 70 GHz components have been tested at low power levels.
Bibliographic Reference: 4TH INTERNATIONAL SYMPOSIUM ON HEATING IN TOROIDAL PLASMAS, ROMA (ITALY), MARCH-21-28, 1984 VOL. II, PP. 1461-1468 1478 P., VOL. I AND VOL. II, 1984 WRITE TO MONOTYPIA FRANCHI, ST. LUCIA - VIALE UMBRIA NO. 28, 06012 CITTA DI CASTELLO, PERUGIA (ITALY), PRI
Record Number: 1989124039100 / Last updated on: 1987-01-01
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