First Measurement of X-rays Generated by Runaway Electrons in Tokamaks Using a TimePix3 Device with 1 mm thick Silicon Sensor

An application study of modern pixel semiconductor detectors for characterization of runaway electron events in tokamaks is presented. Characterization techniques utilizing both spectroscopic measurements and monitoring of the intensity of secondary X-rays produced by the runaway electrons were used. Energy spectra of X-rays and time evolutions of their intensity on two tokamaks (Golem and Compass) were measured under different conditions and compared with results of standard runaway diagnostics. The energy spectra measured on both tokamaks have similar exponential shapes but with a significant variation in numbers of events per shot. The time evolutions of the X-ray intensity during several discharges on the tokamak Golem were measured using both the Timepix3 device and scintillation detectors (NaI:Tl and YAP:Ce). On a microsecond time scales, the signal time evolution measured by the TimePix3 device shows patterns in a form of unexpected or periodic-like increases of the intensity. We have also observed significant differences in number of events of the detected X-rays generated by the runaway electrons flying frontward and backward with respect to a limiter of the tokamak Golem. This fact declares that the runaway electrons have relativistic velocities. The experiments on the tokamak Compass provide similar results. Measurements in the immediate vicinity of tokamak Compass were impossible to perform because of a rapid change of the tokamak magnetic field. Measurements performed in the distance of at least 0.5 m from a diagnostic port of the tokamak Compass gave millions of correctly measured events per shot and an unknown number of events affected by pileups. The correctly measured events were used for construction of energy spectra and the time evolutions of the X-ray intensity.