Detection method for direct studies of fast electrons via Cherenkov effect in the ISTTOK tokamak

Summary form only given. Measurements of the energetic particles (electrons and ions) are one of the main scientific tasks for various experimental devices generating high-temperature plasmas. The significance of such studies is proportional to the amount of information about different processes occurring inside plasma, in which such charged particles are involved. For measurements of fast electrons in tokamaks there were developed special Cherenkov-type probes of high spatial- and temporal-resolution. The applicability of the Cherenkov detectors for measurements of fast electrons in high-current plasma discharges has been studied theoretically and experimentally, with particular attention to their capabilities. In all cases the practical application of such detectors requires the selection of appropriate radiators and consideration of geometrical- and thermal-limitations. On the basis of our previous studies the Cherenkov-type detectors have been designed and adapted for the direct observation of fast (runaway) electrons in small-size tokamaks. During experiments with the ISTTOK facility discharges with low plasma currents (3-4 kA) were studied and runaway electrons of energies > 80 keV were successfully recorded. The electron measurements, as performed at different radial positions of the detector head, demonstrated a distinct increase in the radiation signal when the probe approached the bulk plasma. An analysis of the experimental results revealed good agreement between the obtained data and theory of runaway processes. This paper reports on the most important results of the experiment and its analysis.