Optical refractivity studies of plasma focus.

摘要

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.;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.;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 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.