Measuring MeV-range photons from runaway electrons and fast ion reactions in SPARC

The SPARC tokamak1 has the potential to generate runaway electrons (REs) during plasma start-up and disruptions, with energies that, if not mitigated, could rapidly reach the MeV range and possibly damage plasma facing components2. A passive mitigation system is planned for post-disruptions REs2, while the strategy for start-up REs is to ramp-down any discharge that presents signs of them. SPARC will install, in the torus hall, two hard X-ray (HXR) monitors with tangential fields of view which cover both negative and positive plasma current. They will detect photon energies in excess of 100 keV, emitted by REs during bremsstrahlung interactions with the plasma and the tokamak vessel. Their goal is to monitor start-up RE formation, together with a cadmium detector installed down a beamline, and to measure the evolution of post-disruption REs. Starting from OpenMC3 and FISAPCT4 simulations, the preliminary design for the HXR monitors is here outlined, with particular emphasis on the challenges posed by neutron-induced background, external magnetic fields, and high counting rates. SPARC will also generate gamma-rays in nuclear reactions between plasma ions, and an estimate of this source of gamma-rays in SPARC beryllium-less plasmas is presented.

1 A. J. Creely et al., Journal of Plasma Physics 86.5 (2020)

2 R. A. Tinguely et al., Nuclear Fusion 61 (2021) 124003

3 P. Romano et al., Annals of Nuclear Energy 82 (2015) 90-97

4 J-Ch. Sublet et al., Nuclear Data Sheets 139 (2017) 77-137