Investigating Stagnation Degradation Mechanisms in a Dense Plasma Focus Pinch*

The MJOLNIR dense plasma focus (DPF) at LLNL can deliver up to 4.4 MA of current to two coaxial electrodes, which generate a plasma sheath between them by ionizing deuterium gas. Driven by the JxB force, the sheath lifts off from the insulator and travels the length of the electrodes, converges on-axis, and forms a z-pinch geometry, producing neutrons in the stagnation and break-up phases. Experiments indicate that the sheath may not always sweep up all of the gas, leaving mass behind it; the large transient voltage induced during the stagnation and z-pinch breakup can turn this mass into an alternate current pathway, diverting current from reaching the z pinch. We have designed interferometry and Faraday rotation diagnostics to determine the electron density and the location of the current at stagnation. Faraday rotation, in combination with electron density, allows reconstruction of the azimuthal magnetic field, from which we can determine the plasma current. The current in the sheath region can be compared with the current measurements from a Rogowski coil at the base of the electrodes and B-dot probes mounted on the cathode rods along the pathway to the pinch region to determine if current is being lost between these locations during the discharge. Tracking the spatial current evolution will provide insight into the limitations that cause the neutron yield in MJ-class DPFs to stop increasing above a certain current threshold and help develop mitigation techniques.

LLNL-ABS-866149