Measuring Characteristic Differences between High- and Low-Performing Discharges on the MegaJoule Neutron Imaging Radiography (MJOLNIR) DPF

A dense plasma focus (DPF) is a relatively compact coaxial plasma gun which completes its discharge as a Z-pinch. These devices are designed to operate at a variety of scales to produce short (<100 ns) pulses of ions, X-rays, and/or neutrons. LLNL recently constructed and commissioned a new device, the MJOLNIR (MegaJOuLe Neutron Imaging Radiography) DPF, which is designed for radiography and high-yield operations. MJOLNIR is one of the first DPF devices to be designed along with numerical simulations to optimize ion acceleration and target formation. In particular, particle-in-cell (PIC) simulations of discharges with the Chicago code have provided significant insight into the various physical factors that influence neutron yield. Operations in the original pulsed power configuration rated at 1 MJ of stored energy achieved neutron yields of up to 3.8E11 neutrons/pulse at 2.5 MA peak current. The pulsed power system has now been upgraded to a rating of 2 MJ and commissioning is underway at the higher stored energy. MJOLNIR is equipped with a wide range of diagnostics, including nuclear activation detectors, neutron time-of-flight detectors, a fast-framing camera, optical light gates, and a time-gated neutron and x-ray imager. Optical light gates and the framing camera have enabled run-down and run-in velocity measurements of the plasma sheath. We find run-down speeds steadily increase during the process of electrode clean-up. We present trends in these velocities, as well as an assessment of sheath symmetry and asymmetry, for high- and low-performing discharges. Current traces show evidence that is consistent with low-performance shots being plagued by early-in-time current re-strikes. We describe key insights from modeling that have influenced electrode design and enabled full voltage operations. Comparisons between modeling predictions and measurements are presented. Prepared by LLNL under Contract DE-AC52-07NA27344.