Investigation of the plasma behaviour in the dense plasma focus using laser light scattering techniques

Abstract

The frequency spectrum of density fluctuations in a Plasma Focus device has been investigated by Thomson scattering of laser light. A ruby laser scattering diagnostic has been developed to enable the co-operatively scattered ion feature, Si(k,w) to be measured simultaneously along different density fluctuation k vectors. Data derived from this apparatus is interpreted in terms of the time and space variations of local plasma parameters such as electron and ion temperatures, drift velocity, bulk velocity and level of turbulence. The total scattered intensity, S

(k) is shown to depend on the direction in whichthe fluctuations are sampled by the scattering geometry, with scattered intensities along a Je vector parallel to the current flow enhanced considerably above thermal levels. The data is found to be most satisfactorily interpreted in terms of a double radial shell model of the plasma, each shell with its own characteristic particle temperatures, density, drift and bulk velocity. At peak compression of the pinch onto the z-axis of symmetry for example, the core plasma has typically electron and ion temperatures Te = 200 eV, Ti = 250 eV respectively, while the outer sheath region exhibits a shot to shot variation in the temperature structure, with Te=2.3 keV, Ti=850 eV and Te = L.65 keV, T

= 1.4 keV respectively. An examination of the relative enhancementand broadening of S

(k,w) along the various k-vectors has been interpretedin terms of current driven turbulence. The various mechanisms for inducing turbulence have been considered and lower-hybrid drift, electron-cyclotron drift and ion acoustic instabilities are thought to play a role in the various phases of the plasma discharge. The development of the multiple Je scattering diagnostic has overcome the ambiguity in interpreting scattering results from an irreproduceable inhomogeneous plasma. This problem has restricted the interpretation of the results in previous scattering experiments on the Plasma Focus using single Je vector scattering arrangements.