Optical refractivity studies of plasma focus

Abstract

A study of a Plasma Focus device by optical diagnostic techniquesbased on plasma refractivity is reported. The device produces at theaxis of symmetry of its coaxial electrodes a transient dense plasma fila-ment, of density >10 19 electrons cm -3 and electron temperature ~2.5keV, by a focusing mechanism.

A review of earlier experimental and theoretical studies of the Plasma Focus permits a detailed description of the four principal phases of the discharge.

The principles of showdowgraphy, schlieren photography and inter-ferometry and their application to plasma diagnostics are discussed in detail. It is shown that in the present study the plasma refractivity is mainly due to free electrons.

The Plasma Focus used in the study is described and the experimental details of using the diagnostic techniques are discussed. A shutter has been developed to produce a pulse of duration one nanosecond from a Q- switched ruby laser and is used to obtain good temporal resolution of the transient plasma.

The refractivity studies lead to a detailed understanding of the PIasma Focus. Interferometry provides the time dependence of the plasmaelectron-density profile, permitting calculation of the time-dependent plasma line-density and the focusing efficiency. Multiple-expo sure shadowgrams lead to values of the velocity and acceleration of the plasma boundary, the dense-filament lifetime and the growth rates of plasma instabilities. An ion temperature is deduced from the damping of acceleration-driven instabilities.

The significance of the results from these original studies is discussed. They are compared with the results from other measurements on the Plasma Focus and with the results of a numerical fluid code. The limitations of the optical techniques used are considered and improvements are suggested. Future studies of the Plasma Focus based on these improved techniques are proposed.