Runaway effects at the plasma boundary in ISTTOK

Experimental results on minor and major disruption events in discharges with runaway electrons in the small tokamak ISTTOK and corresponding numerical analysis are presented. Using the experimental data from the soft X-ray diagnostic the image of the plasma column in the presence of runaway electron instability and disruptions was reconstructed. The most probable zones of the interaction of energetic electrons with the plasma surrounding surfaces were detected as spots of the soft X-ray emission on the top and bottom parts of the plasma column during the development of the disruptions. Numerical analysis of the runaway instability allowed determining the runaway electrons characteristics, such as energy (WeRA≤30 keV) and density (nRA∼(0.001-0.01)ne). On the basis of this analysis the level of electrostatic turbulence due to the instability was estimated