MeV range bremsstrahlung measurements of the runaway electron distribution function in disruption mitigation experiments at the ASDEX Upgrade and JET tokamaks

The generation of runaway electrons (REs) is one of the major threats for the realization of a fusion power plant and their mitigation is the subject of an intense research effort worldwide. Techniques rely- ing on massive gas (MG) or shattered pellet injection (SPI) have been developed and are currently under test in major tokamaks. However, the extrapolation of the experimental results to ITER is a challenging task, due to the significantly different RE physics regimes that are expected at ITER. A detailed under- standing of the physics underlying the generation and dissipation of the REs is therefore mandatory. In recent years, progress in the development of gamma-ray spectroscopy (GRS) systems with MHz count- ing rate capabilities [1, 2] have made it possible to develop dedicated instruments for RE studies [3, 4, 5]. GRS measures the energy spectrum of the bremsstrahlung hard x-ray (HXR) emission of MeV range REs as they slow down in the plasma after a disruption. From the measured HXR spectrum, information on the RE energies is obtained using deconvolution, with a typical temporal resolution of a few ms. In this paper, we present the instruments that are available for this task both at the ASDEX Upgrade (AUG) tokamak and at the Joint European Torus (JET), with examples of results.