Applied Mathematics and Applied Physics

Plasma Physics Colloquium

K. A. Lynch
Physics Department, Dartmouth

"Measuring thermal plasmas in the laboratory and in space"

Measuring ionospheric plasma requires access to both thermal ions and thermal electrons.  This is difficult on spacecraft because spacecraft charging potentials are on the order of or greater than the energy of the ionospheric bulk plasma.  Ionospheric Debye lengths are comparable to detector geometry dimensions and simple biasing methods for probes can result in distorted measurements or nonmonotonic barrier sheaths.  We describe a new university laboratory thermal plasma facility and present initial laboratory investigations exploring the formation of plasma sheaths with ionospheric electron energies, densities, and Debye lengths.  Our goal is to identify difficulties in and solutions for obtaining both thermal electron and ion velocity distributions on one payload.   An experimental setup in which two conducting spheres, with area ratios in excess of 100, are biased with respect to one another and allowed to float with respect to the plasma potential simulates a floating payload and biased-probe system.  Measurements of the resulting sheath structures around the two spheres are examined, and non-monotonic electron sheaths obtained by embedding a positively biased electrode within the sheath of a more negative conductor are also explored.   An extension of this model to the parameters of a recent sounding rocket experiment helps explain the response of that detector.  These initial plasma ion and electron sheath investigations  explore an interesting low energy parameter regime that is under-studied in the laboratory, and provide useful information for space-borne instrumentation such as DC electric field probes and thermal plasma instruments.  We welcome suggestions for questions to study with this new facility.

Prof. Thomas Pedersen, host