Herriott Cell Augmentation of a Quadrature Heterodyne Interferometer

摘要

A quadrature heterodyne interferometer is augmented with a Herriott Cell multi-pass reflector to increase instrument resolution and enable a separation of the phase shift due to neutral density from room vibrations. In addition, the use of the Herriott Cell enables variations in the multi-pass laser-beam geometry that optimizes the diagnostic for small scale length measurements or for planar measurements. Kay tracing analysis is used to illustrate retro-reflective planar measurement geometries attainable with the instrument. Analysis is performed to show that phase front degradation and loss of scene beam intensity, concomitant with the large number of reflections from the Herriott Cell mirrors, does not introduce a systematic measurement uncertainty. The diagnostic capability is demonstrated with measurements of the electron and neutral densities in the plasma exhaust from electric propulsion thrusters. Experimental data with up to 18 passes through plasma demonstrates that the instrument resolution to electron and neutral density increases almost linearly with number of passes. However, measurement uncertainty associated with room vibrations is shown to remain constant as the number of passes is increased. Therefore the Herriott Cell can be used to increase the signal of neutral density phase shifts relative to the noise of the phase shifts due to room vibrations. For the Pulsed Plasma Thruster plasma exhaust used to validate the instrument, the addition of the Herriott Cell reduces the density measurement uncertainty to equal or less than the uncertainty due to discharge irreproducibility. When used on plasma sources with higher reproducibility, the Herriott Cell interferometer should be an effective tool for high resolution electron and neutral density measurements.