A comparison of spectroscopic and Thomson scattering measurements of electron densities and temperatures in a transient expanding plasma flow

Abstract

The expansion flow of helium plasma in the end loss region of a theta pinch has been studied. The accuracy of several spectroscopic techniques for determining plasma properties in the transient, inhomogeneous plasma has been evaluated. Plasma at a location of 2.5 cm outside the end of a 25- cm long theta-pinch coil is characterized by Ne ∼1016 cm−3, Te ∼6 eV. Radial electron density profiles were obtained by: (1) Abel inversion of plasma continuum emission from chords across the plasma column to produce relative radial electron density profiles, with normalization by a new technique, whereby a theoretical He II 4686-Å spectral profile is calculated and compared to that observed experimentally being emitted across the diameter of the plasma column; and (2) direct prediction of an absolute electron density profile by Abel inversion of He II 4686-Å spectral profiles obtained by viewing chords across the plasma column. Electron temperatures were obtained from spectroscopic measurements across the diameter of the plasma flow by: (1) measurement of the ratio of the He II 4686-Å line intensity to the underlying continuum intensity; and (2) measurement of the absolute intensity of the He II 4686-Å line. The accuracy of these spectroscopic techniques in a pulsed discharge, with plasma undergoing flow expansion, will be indicated by comparison of the results with those from Thomson scattering.