Controlling dense plasma focus neutron source size and pulse length for radiography

A dense plasma focus (DPF) is a pulsed-power-driven coaxial plasma railgun that terminates in a Z-pinch. The MegaJOuLe Neutron Imaging for Radiography (MJOLNIR) DPF produces bursts of deuterium fusion neutrons within a spot size and pulse length dependent on the spatio-temporal extent of the pinching plasma column. We investigated the effect of machine parameters on the plasma column and when able, the neutron spot size and pulse width and report theoretical scalings and experimental evidence of these effects. The plasma column dynamics are measured with a fast-framing camera, the neutron pulse width using neutron time-of-flight detectors, and the neutron spot size using a rolled edge penumbral imaging neutron camera. The plasma column radius at stagnation was observed to be smaller with a lower gas-fill pressure, higher current, smaller anode radius, and with the presence of dopants. The neutron spot size was also observed to be smaller with higher current, smaller anode radius, earlier in time, and with the presence of dopants. The neutron pulse width was observed to be narrower in time at lower pressure and smaller anode radius, and with the presence of dopants.

The observations are consistent with a 1D shock theory, the onset time for instabilities in a plasma column, and the effect of high Z materials on plasmas.



This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 at LLNL.