Schlieren diagnostics of the plasma focus

The dynamics and the structure of the plasma layer in a 1.3 kJ plasma focus were investigated by means of the Schlieren technique. A mode-locked dye laser provides a repetitively pulsed illumination source, which is used for stroboscopic recordings of the plasma sheath. During the rundown phase the plasma layer attains a minimum thickness at a finite distance from the surface of the centre electrode. The observed broadening of the plasma sheath near the centre electrode surface indicates reduced temperatures due to plasma heat losses to the electrode. During the collapse phase the evolution of periodic density modulations of the plasma sheath was observed in the gradient region of the layer adjacent to the zone of the magnetic field. The corresponding wave vector is parallel to the direction of the current. From the Schlieren images the sheath parameters, including electron density, electron density gradient and the growth rates of hydrodynamic instabilities, were determined.