Experimental Determination of the Current Sheath Structure in a Plasma Focus

Summary form only given. The current density profile associated to the current sheath produced in a 5.6 muF Mather type plasma focus operated at 20 kV was studied by means of a magnetic probe placed in the interelectrodic space at different axial positions along the gun. From the probe signal, both, the arrival time and the temporal width of the current sheath, were measured during the run down phase. The discharge initiation time, as well as the pinching time were determined using a Rogowski coil. A slightly increasing velocity for the traveling sheath along the gun, reaching Mach numbers of about 200 for nitrogen and 100 for hydrogen, were inferred from the data. Ultrafast images of the visible radiation emitted by the plasma obtained with an image converter (10 ns exposure time) show that the luminosity as well as the shape of the visible radiation emitted by the sheath depends on the filling gas type. It was also observed that the temporal width of the current distribution diminishes as it travels along the gun. The measurements indicate that the current distribution is spatially compressed during its evolution. A theoretical model considering the perturbing effect that the probe´s encasing body causes on the measurement itself, as well as the time-dependent nature of the problem due to the temporal variation of total discharge current, is presented. Using this model, the magnetic signals are used to infer the current density profile in the current sheath