Fully kinetic modeling and ion probe beam experiemnts in a dense plasma focus Z-pinch

Abstract

The Z-pinch phase of a dense plasma focus (DPF) emits multiple-MeV ions from a ~cm length interaction. The mechanisms through which these physically simple devices generate such high-energy beams in a relatively short distance are not fully understood. We are exploring the mechanisms behind these large accelerating gradients using fully kinetic simulations of a DPF Z-pinch and ion probe beam measurements. Our particle-in-cell simulations have successfully predicted ion beams and neutron yield from kJ-scale DPFs <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> , which past fluid simulations have not reproduced. To access the regime of MJ-scale devices within computational resources, we have developed a handoff simulation starting from a fluid calculation near the end of rundown and continuing fully kinetic through the pinch. To probe the accelerating fields in our tabletop experiment, we inject a 4 MeV deuteron beam along the z-axis. For the first time, we have directly measured the gradients in the DPF and the acceleration of an injected ion beam. We observe > 50 MV/m acceleration gradients during 800 J operation using a fast capacitive driver <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . In addition, we have now experimentally measured and observed in simulations for the first time, electric field oscillations near the lower hybrid frequency. This is suggestive that the lower hybrid drift instability, long speculated to be the cause of the anomalous plasma resistivity that produces large DPF gradients, is playing an important role. Direct comparisons between the experiment and simulations enhance our understanding of these plasmas and provide predictive design capability for accelerator and neutron source applications.