A Resonant Heterodyne Interferometry Diagnostic for Measuring State Densities of Atoms and Ions

Abstract

Summary form only given. The direct measurement of ion or neutral atom densities in a plasma is usually obtained via emission diagnostics which need to be absolutely calibrated and suffer from a lack of spatial resolution. A resonant, heterodyne interferometer that, in general, can be used to quantitatively measure state densities of ions and atoms, has been developed. The system has been specifically designed to measure neutral hydrogen by interrogating the atomic transitions of Halpha. However, the technique may be applied to any transition in an atom or ion that can be probed with a tunable laser. As currently configured, our diagnostic system may be valuable for studying divertor operation or particle recycling in a tokamak, or any other system where obtaining quantitative information on neutral hydrogen is important. The interferometer operates with a He-Ne laser and a tunable diode laser (TDL) propagating co-linearly. The tunable diode laser has a linewidth <10-6 nm and a tuning range of 6 nm. This configuration serves the purpose of separating resonant and non-resonant phase effects in the interferometer signals. Phase shifts induced by plasma electrons and background vibrations are common to both the measured phase shifts and a simple subtraction leaves only the resonant portion for analysis. This feature provides robust noise immunity and high sensitivity. The measured quantities are integrated along the laser beam line-of-sight with a transverse resolution of less than a millimeter. Because state densities, as well as line profiles, can be determined directly from phase-shift measurements, no intensity calibration is required. We have tested the diagnostic using a plasma opening switch, filled with a plasma from an inverse pinch plasma source, demonstrating the basic feasibility and operation of the resonant heterodyne interferometer. Using hydrogen as a working gas, the properties of the fill plasma and the subsequent dynamics of the opening switch operation were studied. Using argon and helium as working gases, wall contamination (in the form of desorbed hydrogen), was investigated. Measurements of the n=2 state density of neutral hydrogen have been made with a resolution of 2times1011 cm-2, line integrated density