The detection of the non-absorbed millimeterwave power during EC heating and current drive

Fusion-oriented devices using powerful source of electron cyclotron (EC) waves are subjected to high levels of RF-stray radiation. This radiation is potentially harmful for any object made of RF-absorbing material exposed to it. Special material or/and suitable techniques to scrambling the radiation could be needed, depending on the effective expected level of such radiation. Therefore arises the need for a reliable method to foresee the distribution and the level of the RF-stray radiation. A multi-modal antenna, with sensitivity approximately constant for any incoming linear polarization, with great radiating surface and low directivity has been developed. Its purpose is collecting 140 GHz stray radiation from FTU tokamak and Wendelstein 7-AS stellarator plasmas during ECRH and current drive experiments. Theoretical predictions for stray radiation levels are obtained by two different methods: a multiple ray tracing code developed for FTU and a multi-cell code, originally written for W7-X. Both codes have been applied to FTU geometry, resulting in a good agreement with the experimental data from the multi-modal antenna. Descriptions of the antenna design and of the results from both codes are presented. (C) 2001 Elsevier Science B.V. All rights reserved.