Experimental Analysis of Energy Exchange from Runaway Electrons to the Companion Plasma in Benign Termination Scenarios on TCV and AUG

Benign termination of runaway electron (RE) beams is a promising complement to disruption mitigation in tokamaks. Energy transfer from RE beams to a cold, partially ionized companion plasma plays a key role in the effectiveness of this strategy [1]. In this study, energy gain and dissipation processes during the stable plateau phase of RE experiments on TCV and AUG are analyzed using experimental diagnostics. Radiated power, ohmic power, hard X-ray emission, and conductive energy loss—estimated results from fluid-regime considerations—are examined across discharges with varying injected gas quantities, species, and resulting neutral pressures. The influence of gas species and density on dissipation channels is characterized. Additional focus is placed on interpreting the evolution of companion plasma profiles, using measurements from Far InfraRed interferometer and Thomson scattering. The inferred radiative, conductive and convective energy losses provide experimental input for validating kinetic models of RE dissipation [2] and for constraining background plasma parameters used in companion plasma modeling. The analysis supports validation of physics-based models and extrapolation of benign termination scenarios to ITER-relevant conditions