Time History and Spectra of the Light Emitted by Multimegavolt Water Switches

We study the physics of water switching at 4 to 5 MV as part of Sandia\´s Z-Refurbishment program. We present time histories of the light emitted by two sets of water switches and correlate the light emission to the accelerator power pulse. Framing camera images demonstrate how the initial "bushy" streamers collapse to narrow current channels at late time. Spectroscopy of the water switches indicates that the early-time streamers are already hot-dense blackbodies with temperatures between 7000 K and 9000 K. Late time hydrogen emission lines allow estimates of electron densities at that time ranging from 0.2 to 2 times 10¹⁸/cm³.