Speaker
Description
We present a systematic quantification of the energy balance during controlled runaway electron (RE) beam terminations in the TCV tokamak.
Disruptions in tokamaks can generate relativistic REs capable of carrying a large fraction of the plasma current at multi-MeV energies. A sudden loss of confinement of the REs can deposit extreme energy densities onto plasma-facing components (PFCs), leading to surface melting or vaporization. Understanding how RE energy is redistributed during beam termination is therefore central to disruption mitigation and protection of PFCs. In TCV, after RE beam formation, compressing the plasma against the central column triggers a collapse in a reproducible way, both in time and in space, enabling unprecedented diagnostic access to the termination phase. The reproducibility of the collapse also permits systematic variation of beam and termination parameters, enabling controlled, repeatable tests of dissipation mechanisms. The graphite inner wall is used as a calorimeter via embedded thermocouples, measuring the energy deposition, while infrared thermography captures the spatial heating distribution. Bolometry, magnetic reconstructions, and induced-current simulations quantify radiative emission and power coupled to machine structures. Monte Carlo simulations with GEANT4 complement the experimental analysis by linking the surface heating profiles and the RE energy spectrum.
The combination of these diagnostics and analysis demonstrate that the PFCs become a powerful diagnostic for beam energy and deposition physics. Calorimetry enables direct measurements of the total RE beam energy and the combination with surface temperature measurements allow to estimate RE penetration depth of multi-MeV electrons into graphite. The absence of melting, surface ablation and damage result in more accurate measurement and observation of discharge dependent heat patterns. Results show consistent energy accounting across all channels, with representative discharges yielding approximately 10–20 kJ radiated, 14–15 kJ conducted to the inner wall, and about 4 kJ inductively coupled to surrounding structures. Combining these with reconstructed magnetic energy (≈10–20 kJ) indicates total RE beam energies of 10–30 kJ. A key finding is the clear toroidal symmetry of heat deposition, demonstrated by simultaneous infrared and calorimetric measurements at multiple sectors. Deposition depth in the order of few mm are demonstrated, consistent with simulations. This approach provides a new, experimentally grounded framework for studying discharge termination in the presence of REs by directly connecting the plasma dynamics to the PFC response. The methodology and results are broadly applicable to assessing disruption scenarios in all modern tokamaks, where quantifying, and mitigating, RE–surface interaction is critical for safe operation.