Herriott Cell augmentation of a quadrature heterodyne interferometer

A quadrature heterodyne interferometer is augmented with a Herriott Cell multipass reflector to increase instrument resolution, enable a separation of the phase shift due to neutral density from room vibrations, and optimize the diagnostic for small scale length measurements or for planar measurements. Analysis is used to illustrate retro-reflective planar measurement geometries attainable with the instrument and to show that phase front degradation and loss of scene beam intensity 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, and that 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. (C) 2003 American Institute of Physics.