A non-intrusive diagnostic that measures the relative population and temperature of species.
Figure 1. Early emission spectroscopy setup diagram (from ref. 1).
When excited atoms or ions relax, they emit light at a discrete frequencies corresponding to the difference in energy between the excited and relaxed electronic states.
This "natural emission" can be used to determine the relative populations of species and the temperature of the species assuming equilibrium.
At PEPL, the spectra are collected using either fiber optics or a series of lenses.
A scanning monochromator with a photo-multiplier tube (PMT) is used to measure the intensity of the emission over a range of frequencies or of a single frequency as a function of time or location.
Applications at PEPL have included plume diagnostics of arcjets and Hall thrusters and include internal plasma diagnostics of an ion engine and of a Hall thruster.
Figure 2. Early emission spectroscopy setup in PEPL.
Selected Relevant Publications
Foster, J., An Investigation of the Influence of a Transverse Magnetic Field on the Formation of Large Anode Fall Voltages in Low-Pressure Arcs,"
Ph.D. Dissertation, University of Michigan, 1996.
T. B. Smith, G. J. Williams, and A. D. Gallimore, "Spectroscopic Characterization of FMT-2 Discharge Ionization Processes,"
(presentation), 27th IEEE International Conference on Plasma Science (ICOPS), New Orleans, Louisiana, June 2000.
Williams, G. J., Haas, J. M., Smith, T. B., Beal, B. E., and Gallimore, A. D., "Instabilities and State Populations of the Discharge Plasmas in the FMT-2 Ion Engine and the P5 Hall Thruster,"
Proceedings of 26th International Electric Propulsion Conference," Kitakyushu, Japan, Oct. 17 – 21, 1999.