Laser Interferometry Measurements of a High Density Propagating Current Sheet
A helium-neon laser Mach-Zehnder quadrature interferometer is used to measure the electron number density in a propagating current sheet. In some cases the density rise rate was too high to be resolved by the system. Otherwise, the interferometer is used to measure time-resolved density at a particular location in the accelerator. The location of the beam is then moved between shots and, because of the repeatability of the discharge, multiple shots can be pieced together to create a temporally and spatially resolved view of the evolution of the sheet and of a wake of plasma that is left behind the sheet. The measurements confirm an emerging picture of the evolution of these structures that is based on high-speed photographs and current density measurements. They also reveal that the wake originates out of a bifurcation of the current sheet and that it is of comparable density to the sheet.