Traditional measurement tools, such as Langmuir probes and retarding potential analyzers, used in electric propulsion are intrusive. By probing the plasma with a physical object, they inevitably change the properties of the very plasma they are studying. They also run into serious thermal problems in the high-energy-density core of a thruster plume: i.e., they melt. Or vaporize, further contaminating the plasma.

Laser-induced fluorescence is a non-intrusive optical method for measuring the properties of gases and plasmas. The beam from a tunable laser is passed through the fluid, exciting an electronic or vibrational transition of one of the fluid species. More on this as time permits.