Characterization of Startup Runaway Electrons in DIII-D Plasmas*

Startup runaway electrons (REs) reaching an energy of 25 MeV are produced in low density ohmic plasmas in the DIII-D tokamak. The generation and evolution of the observed startup REs are experimentally studied to support the ITPA analysis task which aims to understand the formation of startup REs and extrapolate to future devices. The evolution of RE quantity, maximum energy, and growth rate is determined using the Gamma Ray Imager measuring hard-x-ray (HXR) bremsstrahlung radiation with the key findings as follows: (1) the REs are observed from the early plasma current I_P ramp up phase and reach an energy of about 25 MeV in the current flattop; (2) the RE growth rate depends nonlinearly on the pre-fill gas pressure and consequently electron density during the I_P ramp, with 10% reduction in the line-averaged electron density resulting in approximately 4 times higher HXR signal at the end of the I_P ramp; (3) the RE quantity and energy continue to increase during the I_P flattop if not mitigated; (4) discharges with electron-cyclotron-assisted startup exhibit a reduced RE growth rate, but this does not always prevent the generation of sufficient RE seed as similar levels of the HXR signal can be observed in the discharges with and without assisted startup. These are among the first quantitative results obtained on tokamaks. They will be used for comparison with results from other devices, validation of startup models, and development of safe ITER discharge startup.