The development of high power ignitrons with peak current ratings of up to 1000 kA and simultaneous charge transfer rates of 250–500 C is currently under way in a joint effort between Texas Tech University (TTU), Lawrence Livermore National Laboratory (LLNL) and industry. Research at TTU is concentrated on plasma diagnostics, novel anode designs, electrode placements and trigger schemes.1,2,3 Electrical measurements as well as optical plasma studies such as high speed framing photography, Mach-Zehnder interferometry and spectroscopy have been performed. The cooperative efforts have lead to the development of a new commercial tube (Richardson Electronics NL-9000). This paper describes plasma diagnostics performed on a demountable ignitron (DIG) which provides optical access to the discharge plasma through four viewports, two of which are on opposite sides on a common optical axis. The latter pair of viewports was used to perform plasma density studies using a Mach-Zehnder interferometer. Time resolved recordings of the interference patterns, either visually or electronically, during changes in plasma behavior (such as current conduction or plasma heating) yield time resolved information about the particle density. This technique was applied to the demountable ignitron during high current discharges. The light source was a 2W CW argon laser which was pulsed using a ferroelectric liquid crystal light valve. The resulting fringe patterns were recorded with a mechanical high speed camera. In the paper all experimental details, results and a theoretical evaluation are given. In addition, high speed framing photography was used to study the influence on electrode design and placement on the discharge plasma.