Multiframing Mach–Zehnder interferometer for spatiotemporal electron density measurement in a field-reversed configuration plasma

Abstract

Interferometric measurement is used to establish the spatially and temporally resolved electron density distribution in high β plasma experiments. However, a series of interferograms are recorded separately on different shots. In order to avoid shot-to-shot data variation, we have developed a multiframing Mach–Zehnder interferometer system to produce multiframe interferograms on a single field-reversed configuration (FRC) plasma shot. In this interferometer system, a continuous wave He–Ne laser is used to illuminate the interferometer and the interferograms at various instances are recorded photographically by a rotating mirror framing camera at framing rates of 0.7–2.9 MHz. A simple electromechanically operated optical shutter with opening time of 300 μs was developed in our lab to gate the laser light to prevent both the considerable attenuation of laser light intensity and multiple exposure of the interferograms. The precise timing of the interferograms relative to FRC plasma discharge is determined with two optical fiber glasses pointed separately towards the first and the last microlenses in the framing camera to pick up their laser light pulse signals. Twenty-eight consecutive FRC plasma interferograms were recorded successfully on Kodak T-MAX 400 film in the first half discharge period of 10 μs with this system. From these interferograms, the spatiotemporal evolution of plasma density can be evaluated. © 1999, American Institute of Physics. All rights reserved.