OPTICAL PROBING OF FIBER Z-PINCH PLASMAS

An experimental study of optical probing of a dense z-pinch plasma usi ng the MAGPIE (mega-ampere generator for plasma implosion experiments) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)] is reported. The generator was operated with a peak current of 1.1 MA rising in 150 ns (10%-90%). The loads were 33 mu m diam carbon fibers . Faraday rotation was used to investigate the distribution of the cur rent flowing in the plasma. A measurable Faraday rotation angle was ob served only in a time window from 50 to 60 ns after the current start, due to the fact that this effect depends on a combination of the magn etic-field strength and electron number density. A new type of self-re ferencing cyclic radial shear interferometer was used to evaluate the plasma density profiles which are necessary for the reconstruction of the current distribution. It was calculated that similar to 110 kA was flowing in the plasma at 52 ns after the current start. Shadowgraphy was used to study the dynamics of the plasma and to investigate the fo rmation of instabilities. Plasma instabilities were observed at very e arly times (similar to 5 ns). These instabilities appeared to be not e ntirely axi-symmetric implying the existence of m = 1 and maybe higher modes as well as m = 0. The perturbations increased with time and evo lved into density islands (isolated plasma fragments) distributed alon g the axis at late times (similar to 70 ns). (C) 1998 American Institu te of Physics.