Direct observations of runaway electrons during disruptions in the JET tokamak

Abstract

Runaways generated during disruptions in tokamaks may havedamaging consequences in large machines because of the high powergenerated by their localized deposition on the vessel walls. In aninvestigation of the runaways generated in disruptions in JET,detailed time and space resolved X ray images of the runaway beam inflight have been obtained for the first time and these allow adetailed diagnosis and analysis of the production and movement of therunaways. These measurements are now possible because of thedevelopment, for the JET DT campaign, of radiation protected softX ray cameras. The measurements show that the runaways are generatedat the vessel centre in a region with small minor radius and they thenmove to interact with a small area of the wall. The observed radiationpower both in detectors installed in the torus and in the radiationprotected cameras shows reasonable agreement with values calculatedfrom the runaway current and energy. The current density profile andq profile of the runaway beam are also determined. After the startof the disruption there is a delay before the onset of runawaygeneration and this may offer control possibilities.