Measurement of electron density gradients using a modified Jamin interferometer

Abstract

The use of a modified Jamin interferometer for the measurement of electron density gradients in a transient plasma has been demonstrated using a small Z pinch argon discharge. The interferometer was operated with 632.8 nm radiation from a He-Ne laser, and an interference pattern was produced using wedge shaped plates. With both beams of the interferometer passing along the length of the plasma and separated by 3.8 mm, differences in electron density were measured, and hence radial electron density gradients were derived. Electron density gradients as small as 5*1022 m-4 could be detected with good reproducibility. The Jamin interferometer used in this way was found to have several practical advantages over the Michelson and Mach-Zehnder interferometers, even for absolute electron density measurement. These advantages include simple alignment, fewer optical components, and a stable, easily viewed interference pattern even in the presence of mechanical vibration. The interferometer was also used to locate the position of the centre of the Z pinch discharge to within 0.2 mm.