Stagnation Column Radius and Neutron Spot Size in the MJOLNIR 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. The "beam-target" interaction produces a neutron burst lasting on the order of tens of nanoseconds, which we use for radiography. The smaller the neutronproducing region ('spot size') is, the higher resolution neutron images we can obtain. In order to determine the effect of stagnation column radius on spot size, a number of anode geometries have been fielded; differences include the size of the hollow, the length of the anode, and the angle of the taper at the tip. We compare the dynamics, instabilities, and radius of the stagnation column and spot size across differing anode geometries using laser interferometry, visible self-emission imaging, and neutron radiographs.