A refractive index gradient (RING) diagnostic for transient discharges or expansions of vapor or plasmas

The Refractive Index Gradient (RING) diagnostic described in this paper uses a fast, silicon, photodiode quadrant detector with a differential amplifier to temporally detect the refraction of a CW laser by transient discharges or expansions of vapor, gas, or plasma. The method is a local, one-dimensional, time-resolved, quantitative, species-discriminating (i.e., atoms or electrons) Schlieren technique. The diagnostic is easy to field, sensitive (minimum deflection angles detectable ∼ 0.3 μrad), and fast (risetime = 11 ± 1 ns). Circuit design, performance, and diagnostic theory will be discussed. The RING diagnostic has been applied to measurements on several ion sources under development at Sandia National Laboratory for use on intense applied-B ion diodes in the light ion-beam fusion program. Sources studied include different types of flashboards, a thermal evaporation lithium ion source, and a laser-produced ion source. Measurements have been made both in labs and directly on large pulsed power accelerators. In this paper to illustrate the utility of this technique, examples of measurements on LEVIS (Laser Evaporation Ion Source), a laser-produced, active, lithium ion source are given. Measured properties include vapor/plasma production thresholds, expansion velocities, and time-resolved gradient and density spatial profiles. Comparisons of the RING results with measurement using a Faraday cup and a double-floating Langmuir probe are presented