Capability and Theory of the Planar Langmuir Probe

The Planar Langmuir Probe, PLP, in its simplest form is just a flat plate and perhaps a guard ring placed on the ram side of a Low Earth Orbit spacecraft such that it can intersect an uninterrupted flow of plasma. At small negative potential, it measures the ion ram current and thus the ion density with great accuracy, and when swept to positive potential, electrons are collected and the electron temperature may be extracted from the current-voltage characteristic. With rapid sampling, the turbulence spectrum of the plasma may also be sampled. AFRL has pioneered the development and application of the PLP for close to 2 decades. The PLP was used on numerous rocket flights in the 90's, was flown on the CHAMP satellite from 2002 to 2011, on the C/NOFS satellite from 2008 to present, and a new version is in development for possible future flights. This talk will review the design variations to date, data validation, lessons learned, and theory of operation. In particular we will present methods for extracting the electron temperature from the current-voltage characteristic, and address how the probes can measure higher moments of the electron distribution as suggested by the observations of Rother et.al. (Radio Sci, Vol 45, 2010).