Effect of preionization on the dynamics of current sheath in a small plasma focus device

In the dense plasma focus (PF) device, the dynamics of the current sheath is an important parameter for good focusing or pinching. In this paper, we present the effect of preionization on current sheath dynamics, its pinching evidence and details of its filamentary structure by measuring the axial run-down velocity, confinement time and focus amplitude in a miniature PF device using a Rogowskicoil and high-voltage (HV) probe. For this purpose, the discharge is produced in deuterium gas with deposited energies lying in the range of 3.3 kJ. The Rogowski coil and HV probe signals reveal the formation of an axisymmetric parabolic current sheath propagating down the coaxial tube, and longer confinement time of it during the collapse phase and increase in focus amplitude with preionization source. The axial run-down velocity of the current sheath is increased from 2.1 to 3.2 cm/mu s. The focus duration is from 45 to 72 ns and the focus amplitude is from 1.8, which increases to 2.9 with preionization at the optimum pressure. The effect of the working pressure on the current sheath dynamics and its spatial evolution are also studied.