Measurement of Runaway Electron Plateau Final Loss Energy Deposition into Wall of DIII-D

Intentional runaway electron (RE) plateau-wall strikes with different initial impurity levels are used to study the effect of background plasma relativistic electron Z (as well as plasma resistivity for slow electrons) on RE-wall loss dynamics. RE wall loss time is found to be close to the avalanche time ($m_e$C/eE_{||}$)1n\lambda\sqrt{3(Z+5)/\pi}$, consistent with REs being lost by a series of MHD reconnection events, with timescale limited by current profile filling via avalanche. Local kinetic energy deposition is estimated with both hard x-ray emission and with infra-red imaging. At higher plasma impurity levels Z$\sim10$, energy deposition appears to be consistent with power balance estimates, as long as collisional dissipation during the final loss event is included. At low impurity levels Z$\sim1$, however, local energy deposition appears around 10× less than expected, indicating that the energy dissipation at low Z is still poorly understood.