Direct observations of runaway electrons during disruptions in the JET tokamak

Runaways generated during disruptions in tokamaks may have damaging consequences in large machines because of the high power generated by their localized deposition on the vessel walls. In an investigation of the runaways generated in disruptions in JET, detailed time and space resolved X ray images of the runaway beam in flight have been obtained for the first time and these allow a detailed diagnosis and analysis of the production and movement of the runaways. These measurements are now possible because of the development, fur the JET DT campaign, of radiation protected soft X ray cameras. The measurements show that the runaways are generated at the vessel centre in a region with small minor radius and they then move to interact with a small area of the wall. The observed radiation power both in detectors installed in the torus and in the radiation protected cameras shows reasonable agreement with values calculated from the runaway current and energy. The current density profile and q profile of the runaway beam are also determined. After the start of the disruption there is a delay before the onset of runaway generation and this may offer control possibilities.