Development and Implementation of Laser Schlieren Imaging and Interferometry to Diagnose Structure and Electron Density in a Dense Plasma Focus

The MJOLNIR (MegaJOuLe Neutron Imaging Radiography) dense plasma focus (DPF) at LLNL is a prototype source for performing neutron radiography of dynamic events. MJOLNIR's driver can store up to 2 MJ stored energy and produce currents up to 4.5 MA (so far commissioned to >3 MA). The DPF consists of two coaxial electrodes, which generate a plasma sheath by ionizing deuterium gas. The sheath implodes on the axis in a z-pinch geometry. When the pinch breaks apart, it produces a beam of ions that impacts the “target”, a region of the sheath assembled on axis past the pinch (n_e ~le19/cm
³
). The “beam-target” interaction produces a neutron burst on the order of tens of nanoseconds, used for radiography. We present our design of a laser interferometry diagnostic to measure the electron density of the pinch and target regions. It allows us to infer the ion density in the target region, study dynamics during implosion to optimize beam generation in the pinch region, and compare with hybrid fluid-kinetic particle-in-cell stimulations to benchmark models. The interferometer builds on an existing Schlieren imaging system, showing density gradients in the plasma and bounding the density. We compare the dynamics, instabilities, and structure of undoped deuterium, and argon- and neon-doped deuterium shots, using laser Schlieren imaging, visible self-emission imaging, neutron time-of-flight detectors, and EM probes.