Different modes of plasma sheath motion in the radial compression and pinch phases in plasma focus

Abstract

Two distinct modes of plasma sheath (PS) motion, the stable mode and the instability mode, in the radial compression and pinch phases in a small Mather-type plasma focus device (20 kV, 18 kJ) have been observed using laser differential interferometry. The two modes occur randomly from shot to shot in consecutive discharges and their probabilities are much influenced by the anode end structures. The instability mode is characterized by the rapid radial movement of the sliding front of the PS along the anode (centre electrode) end surface. The two modes show very different sheath and pinch configurations according to their different ways of motion. They also have notable influences on the neutron yield, in that the instability mode always produces a lower one and the stable mode a higher one. The mean neutron yield for consecutive discharges is made low and largely deviated by the co-existence of the two modes. The instability mode is found to be caused by the anode end geometry with a small transition arc, whose nature is of the m = 0 MHD type but only with a half wavelength, occurring at the plasma - electrode layer. The instability can be either avoided by a hemispherical-ended anode with a larger transition arc or stabilized by the stagnation of the radial movement of the sliding front of the PS in the initial radial compression by the hollow edge and the stable mode formed. The magnetic field introduced in front of the PS by the anode end discharge that occurs from the very beginning of the main discharge is also found to act as a stabilization mechanism for the instability and to cause some of the stable mode.