Imaging Runaway Electrons in Slide-Away and Killer Pellet Discharges in DIII-D

Runaway electrons (REs) produced by acceleration of slide-away electrons in very low density ohmic discharges, and by rapid shutdown induced by argon pellets, have been studied by imaging synchrotron emission (SE) from 700-1000 nm, providing new data on the equilibrium and formation physics of RE beams. Trace levels of quiescent RE current (QRE) are produced in ohmic discharges with $n_e=4\times10^{18}/m^3$. The synchrotron emission forms 1 or 2 crescents near the q=1.5 and 2 surfaces, which survive fast transients due to low density locked modes. 2mm argon pellets with velocity $\sim$185 m/s produce REs when the pellet is strongly ablated upon reaching the core, forming $\sim$0.5 s long plateaus of several hundred kA when the RE seeds are formed inside $\rho\sim$0.35. Discharges in which the pellet survives, passing completely through the plasma to hit the centerpost, do not form enough RE seeds to provide an imageable synchrotron emission in the 700-1000 nm range.