The role of kinetic instabilities in post-disruption runaway electron beam formation after argon injection in DIII-D

Kinetic instabilities in the MHz range driven by relativistic runaway electrons (REs) have been observed for the first time during the current quench of a tokamak disruption and are well-correlated with intermittent losses of REs that can suppress sustained RE beam formation. When the instabilities exceed a threshold power RE beam formation is no longer observed, enabling a first understanding of the critical argon (Ar) quantity required to form a RE beam. Improvements to the DIII-D gamma ray imaging system to measure high gamma fluxes enabled the first diagnosis of the post-disruption RE seed energy distribution function and its dependence on pre-disruption parameters. The instabilities are observed when RE energy (E_RE) exceeds 2.5 MeV, the number of modes grows linearly with maximum E_RE, and their frequencies lie in the range 0.1–3 MHz, below the ion cyclotron frequency. Possible plasma waves excitable by REs in this region are proposed. Increasing the quantity of injected Ar results in a strong dissipation of REs, a reduced number of high-energy REs, and correspondingly a smaller amplitude of kinetic instabilities that then enables RE beam formation. Increasing the pre-disruption plasma current increases the available flux and is found to raise the fraction of high-energy RE seeds and further destabilizes the kinetic instabilities. Thus, a higher Ar quantity was necessary to form the RE beam at high pre-disruption plasma current. No such kinetic instabilities are observed after the injection of Ar pellets, resulting in reliable production of RE beams. This work opens new directions to understand RE beam formation and suppression as well as to develop validated models to design the disruption mitigation system of future tokamaks such as ITER.