Demonstration of Safe Termination of Megaampere Relativistic Electron Beams in Tokamaks

Runaway electron current reconstitution after a nonaxisymmetric magnetohydrodynamic flush

Benign termination of mega-ampere (MA) level runaway current has been convincingly demonstrated in recent JET and DIII-D experiments, establishing it as a leading candidate for runaway mitigation on ITER. This comes in the form of a runaway flush by parallel streaming loss along stochastic magnetic field lines formed by global magnetohydrodynamic instabilities, which are found to correlate with a low-Z injection that purges the high-Z impurities from a post-thermal-quench plasma. Here, we show the competing physics that govern the postflush reconstitution of the runaway current in an ITER-like reactor where significantly higher current is expected. The trapped "runaways" are found to dominate the seeding for runaway reconstitution, and the incomplete purge of high-Z impurities helps drain the seed but produces a more efficient avalanche, two of which compete to produce a 2-3 MA step in current drop before runaway reconstitution of the plasma current.

Energy distribution of lost high-energy runaway electrons based on their bremsstrahlung emission in the EAST tokamak

We report in this paper the study towards revealing the energy distribution of lost high-energy runaway electrons based on their bremsstrahlung emission. The high-energy hard x-rays are originated from the bremsstrahlung emission of lost runaway electrons in the experimental advanced superconducting tokamak (EAST) tokamak, and the energy spectra are measured using a gamma spectrometer. The energy distribution of the runaway electrons is reconstructed from this hard x-ray energy spectrum using a deconvolution algorithm. The results indicate that the energy distribution of the lost high-energy runaway electrons can be obtained with the deconvolution approach. In the specific case in this paper, the runaway electron energy was peaked around 8 MeV, covering from 6 MeV to 14 MeV.

Hot-Tail Runaway Seed Landscape during the Thermal Quench in Tokamaks

Runaway electron populations seeded from the hot tail generated by the rapid cooling in plasma-terminating disruptions are a serious concern for next-step tokamak devices such as ITER. Here, we present a comprehensive treatment of the thermal quench, including the superthermal electron dynamics, heat and particle transport, atomic physics, and radial losses due to magnetic perturbations: processes that are strongly linked and essential for the evaluation of the runaway seed in disruptions mitigated by material injection. We identify limits on the injected impurity density and magnetic perturbation level for which the runaway seed current is acceptable without excessive thermal energy being lost to the wall via particle impact. The consistent modeling of generation and losses shows that runaway beams tend to form near the edge of the plasma, where they could be deconfined via external perturbations.