Evolution and Mitigation of Runaway Electrons Emerging during Tokamak Plasma Startup

Startup runaway electrons (REs) reaching an energy of over 30 MeV have been observed in DIII-D 1.5s after discharge initiation with first-time measurements of the time-dependent startup RE quantity and energy spectra. Without mitigation, the maximum observed RE energy continues to rise during the plasma current, Ip, ramp and flattop, eventually reaching the upper limit of the RE diagnostic in some cases. Surprisingly, the growth rate of maximum RE energy is lower during the Ip ramp than during the Ip flattop, which is encouraging for mitigation applied during the ramp. Sensitive measurements of hard-x-ray bremsstrahlung were obtained from the Gamma Ray Imager diagnostic, resolving individual photons with energies from 0.5 - 30 MeV at over a 1 MHz count rate. REs with energy on the order of 0.5 MeV are detected immediately after the burn-through phase over a wide range of startup parameters in DIII-D, including an 80-fold variation in pre-fill pressure, from 0.29 to 24 mPa. This is consistent with a recent hypothesis that startup REs may emerge, in varying quantities, in all tokamak plasmas [P.C. de Vries, Nucl. Fusion 63 (2023) 086016]. The evolution of RE quantity and energy depend nonlinearly on pre-fill pressure and density during the Ip ramp. Electron cyclotron heating (ECH) during plasma initiation and the Ip ramp suppresses the growth of startup RE quantity and energy, likely due to increased plasma temperature and density. Additionally, ECH during the Ip flattop induces continuous RE loss across the measured energy range, demonstrating a potential method of startup RE mitigation. These results have already motivated a new modeling and validation effort for the development of safe startup scenarios for ITER and other future tokamaks, and they move the U.S. to the experimental forefront of this international effort.