Motivation and design of a two-color interferometer for DARHT Axis-II*

Unsullied propagation of intense, relativistic electron beams through plasmas is required for multi-pulse flash radiography. Beam-target interactions result in a vaporized plume of target material that propagates upstream from the target, affecting the beam focus and quality. Simulations of the plume and its interaction with subsequent pulses is presented here as motivation for the two-color interferometry (TCI) system being developed for the Dual Axis Radiographic Hydrodynamic Test Facility’s Axis-II, a multi-pulse linear accelerator at Los Alamos National Laboratory. Beam focus positions were calculated with the beam-induced ionization model in LAMDA as a function of plume ionization, density, and volume. Plasma plume evolution, simulated by LASNEX/MCNP calculations, shows plume densities and temperatures consistent with the existence of a partially-ionized plasma. This drives the need for a TCI system, the design and theory of which will be presented. TCI uses interferometers of different wavelengths and can separate the phase contributions of free electrons and neutral particles, which is needed for accurate measurements of partially-ionized plasmas.