Measurement of stray millimeter-wave radiation from a 70-GHz ECH/ECCD system in Heliotron J

Abstract Stray millimeter-wave radiation from a 70-GHz electron cyclotron heating and current drive (ECH/ECCD) system has been measured in the Heliotron J helical device. Two rotatable diode detectors located at the outboard side ports are used to pick up the stray radiation: one is installed at the ECH launcher port and the other is installed at a toroidal angle of 135 deg far from the ECH launcher port. Both detectors are rotated to measure the polarization of the stray radiation. The results show that at the toroidal position far from the ECH launcher, the polarization is not fully randomized before plasma breakdown, whereas the polarization is uniform after a quasi-stationary plasma is generated. The polarization near the ECH launcher is not uniform even in a quasi-stationary plasma. Plasma experiments scanning the electron density indicate that the EC power absorption estimated from the stray radiation at the toroidal position far from the ECH launcher is correlated with the single-pass absorption rate calculated by the TRAVIS ray-tracing code. These results indicate that the diagnostic using a simple diode detector can be used as a real-time monitor of EC power absorption only when the plasma with finite density is produced and the detector is placed far from the ECH launcher port.