Runaway electron diagnostics for the COMPASS tokamak using EC emission

An electron cyclotron emission (ECE) diagnostic of suprathermal electrons was utilised for runaway electron (RE) experiments purposes in the COMPASS tokamak. Our vertical ECE (V-ECE) system consists of a 16-channel heterodyne radiometer and an E-band horn antenna with a 76.5-88 GHz frequency range front-end. Simulations used for the design of the diagnostic showed a possibility of detecting the emission of low-energy (50-140 keV) runaway electrons. We realized measurements with both extraordinary (X-) and ordinary (O-) mode linear polarizations. The amplitudes of the X-mode and O-mode signals are similar, which can be explained by depolarised reflected radiation. V-ECE measurements in low-density flattop discharges and in discharges with massive gas injections of high- Z elements show correlations with other RE diagnostics. Our results are in the agreement with the principles of the primary runaway generation mechanisms.