Yield scaling and operating conditions for highest observed neutron yield shots above 3.5 MA on MJOLNIR DPF

MJOLNIR DPF requires high yield neutron pulses to perform flash neutron radiography with more neutrons contributing to a better image. Commissioning MJOLNIR above 3.5 MA requires operational modifications to take advantage of the higher current to produce higher yields.

Interleaving: Shots above 3.5 MA change the machine in a way that precludes back-to-back shots, requiring a low current (2.7 MA) “recondition shot” to reset the machine. The yield on the 3.5 MA shots was independent of the recondition shot yield and the number of reconditioning shots performed. Although interleaved, the 3.5 MA high yield shots still exhibit suitable consistency in pinch time, yield, and neutron shape for time-gated flash neutron imaging.

Regimes with lower than simulated yields: Above a critical pressure, the yield performs lower than simulations, coincident with the onset of asymmetries observed in the sheath. At low pressure, the measured pinch voltage spike reaches a ceiling of 200 kV, which is suppressed below model predictions and coincident with lower than expected yields and scalings. Measurements of rundown velocity and breakdown time for different fill pressures will be compared to determine when the shots diverge from simulations.

The highest yield shots at intermediate pressures exhibited favorable scaling ~I^4 from 2.7 up to 3.8 MA peak current and 1.2x10^12 neutrons.