Dynamics of the ionizing and magnetic fronts in the radialcompression stage of a DPF current sheath

Further results regarding the sudden drop of dense plasma focus (DPF) neutron yield at low pressure are presented. Experimental measurements of temporal evolution of magnetic piston MP (interface plasma-magnetic field) and current sheath ionizing front IF (interface plasma-neutral gas) are performed in the radial compression stage of DPF discharges. At the optimum deuterium pressure for neutron yield (1.5 mb), MP and IF have similar r(t) functions, so that the thickness of the sheath remains practically constant (~0.2 cm) in the whole stage. At the lower pressure limit for neutron yield (1 mb) the sheath begins the radial compression stage with a smaller thickness (~0.1 cm) and in the last 35 ns the IF moves forward (relatively to the MP) with a velocity of 7.5E+7 cm/s and the thickness in the layer reaches a value of ~0.5 cm. This sudden acceleration of the IF is interpreted as the main cause of the lower pressure limit in the neutron yield