Herriott Cell Interferometer for Density Measurements in Small-Scale Length Plasmas.

摘要

The introduction of a Herriott cell into a standard quadrature heterodyne interferometer is evaluated and demonstrated to increase the resolution of the system. Measurements of electron and neutral density during and after the current pulse are sought for modeling purposes for spacecraft contamination from Pulsed Plasma Thrusters. Testing is performed on the UIUC PPT-4, a coaxial electrothermal Pulsed Plasma Thruster pulsing at 20 J. Analytical and experimental analysis is conducted to determine the integrity of the phase front and the effect of multiple passes on the density measurements taken. The phase front quality is found to be acceptable for interferometric purposes and density measurements are taken for 2, 6, 14, and 18 passes in the Herriott cell. The advantage of the cell is obvious at late times when the external room vibrations induce an apparent phase shift in the same direction as neutral particles. Due to the same dependence on wavelength, 2 laser frequencies cannot be used to separate neutral and vibration contributions. The Herriott cell allows a density resolution increase linear with the number of passes that does not increase the vibrational component. Uncertainties from both vibrational sources and shot-to-shot variations of the thruster itself are investigated and characterized for this system. Due to variations in room vibrations on a day to day basis, the cell was unable to characterize the neutral density of the thruster. However, for single tests, neutral density measurements were acquired. The Herriott cell with 18 passes introduced a 9-fold increase in resolution over the standard 2-pass interferometric setup. At 200 microseconds for single tests at 14 and 18 passes ((caret)20 shots averaged) neutral density at the exit plane was shown to be no more than 1*10(exp 16)/cu cm. Peak electron density (4 microseconds) was shown to be 5.0*10(exp 15) +/- 1.1*10(exp 15)/cu cm.