Herriott cell interferometry for millimeter-scale plasma measurements

Abstract

A Herriott cell consists of two concave mirrors positioned on opposite sides of plasma so as to create multiple laser paths through the plasma. Added to a traditional interferometry diagnostic, the Herriott cell multiplies the effective path length through the medium and thereby increases instrument resolution. Previous work used a planar geometry to validate the use of Herriott cells in interferometer applications where the numerous mirror reflections will significantly degrade the phase front quality. The current work extends the Herriott cell capability to a point configuration. In this geometry the multipass beams converge near a single point within the plasma, useful for performing density measurements in very small scale length plasmas. Ray tracing analysis is used to illustrate example measurement geometries attainable with the instrument. It is shown that the configuration results in two convergence points for the laser paths, which somewhat complicates the interpretation of the experimental data. In addition, the multitude of passes through the high density, small scale length passes increases the likelihood for deleterious refractive effects. The diagnostic capability is demonstrated with measurements of the electron and neutral densities in the plasma exhaust of a Micro Pulsed Plasma Thruster. The measurements are validated with similar, lower resolution measurements, obtained using a single-pass interferometer.