Speaker
Description
As tokamak devices prepare to enter the burning plasmas era, as for example with the SPARC [1] experiment, one of the few remaining challenges is represented by runaway electrons (RE). Due to the plasma parameters necessary to create a burning plasma, REs can potentially reach tens of MeV of energy and carry multi-MA of current. Models of the interaction of REs with SPARC first wall [2] show that the dissipation of RE power could produce severe damage to the device. Thus SPARC will be equipped with a set of diagnostics for start-up RE detection, which will inform the plasma control system to interrupt an experiment before a RE beam could gain excessive energy. Among these diagnostics, the hard X-ray (HXR) monitors will detect bremsstrahlung radiation in excess of 100 keV, emitted by REs interacting with the bulk of the plasma or more likely with the plasma facing components.
In this contribution, we present the latest design of the HXR diagnostics, which will be two cylindrical 1.5 inch diameter x 1.5 inch height LaBr3 inorganic scintillators, coupled with PMTs. The two monitors will be located in two tangential penetrations in the tokamak hall wall, with a wide field to observe co- and counter current RE beams. The characterization of a prototype unit has been conducted [3], returning an estimate for its energy resolution, total efficiency, and dynamic range. We then discuss the strategy for implementing a digital acquisition chain capable of performing simultaneous measurements in spectroscopic and current modes. Due to their position, the detectors will operate in an unprecedented harsh environment, with high neutron fluxes (expected to reach few $10^{10}$ n/cm2/s) and high residual magnetic fields (~100 G) from the poloidal field coils. A neutron attenuator made of high density polyethylene (HDPE) has been designed using neutronics simulations, such as OpenMC [4] and FISPACT [5], to mitigate both the prompt neutron-induced and the delayed activation-induced backgrounds on the detector. Finally, we conclude presenting our strategy to minimize and correct the effect of the external magnetic fields by a combination of magnetic shielding and an LED-based monitor for the detector gain.
1A. J. Creely et al., J. Plasma Phys. 86.5 (2020) doi: 10.1017/S0022377820001257
2A. Freyer et al., APS-DPP 2024, “Implementation of a runaway electron module in HEAT”
3E. Panontin et al. Rev. Sci. Instrum. 95 (2024); doi: 10.1063/5.0219549
4P. Romano et al., Ann. Nucl. Energy 82 (2015), doi: 10.1016/j.anucene.2014.07.048
5J-Ch. Sublet et al., Nuclear Data Sheets 139 (2017) 77-137 doi: 10.1016/j.nds.2017.01.002
