B-dot Probe Commissioning and First Results in the MegaJoule Neutron Imaging Radiography DPF

The MegaJoule Neutron Imaging Radiography (MJOLNIR) dense plasma focus (DPF) — capable of discharging 1.3 MJ of stored energy at peak currents up to 3.8 MA — was commissioned to investigate the viability of a dense plasma focus (DPF) as a source for dynamic radiography applications. Parasitic "restrike" currents in the DPF head divert current from the pinch and lower neutron yields. To resolve this and the time-dependent current dynamics of the plasma sheath, MJOLNIR was instrumented with two dual-coil B-dot probes and two baluns to suppress common-mode noise. To avoid direct plasma contact during sheath run-down, each probe is mounted behind and just outside the cathode rods. In this region, the azimuthal field decays much faster than 1/r, making precise radial placement and angular alignment essential for reliable measurements. Mechanical adaptations (vibrational-isolating gaps and Macor housings) ensured probe survival through repeated shots. In the inaugural campaign, raw dB/dt waveforms were acquired, delay-calibrated, and time-integrated to yield local current traces. Cross-diagnostic synchronization with our head-Rogowski coil and three optical light-gate monitors confirmed sheath arrival times and run-down velocities at two distinct z-locations. These first measurements demonstrate the probes' sensitivity to both primary return currents and secondary "restrike" effects in MJOLNIR. Ongoing work will investigate probe sensitivity and compare against simulation-derived synthetic signals to map current flows, quantify parasitic return paths, and optimize pinch formation for enhanced neutron yield.