The University of Michigan
Department of Aerospace Engineering
| Plasmadynamics & Electric Propulsion Laboratory |
Plasma Diagnostics

Langmuir Probe

Measures electron temperature, number density, floating potential and plasma potential.

Above: Typical double Langmuir probe used with electric thrusters.

A Langmuir probe is comprised of an exposed conductor (e.g., wire) immersed within a plasma. The theory of interpreting the data acquired (namely the current drawn from the plasma at a sequence of different bias voltages) from Langmuir probes is well established (see references below). Nobel Laureate Irving Langmuir pioneered the use of electrostatic probes to measure the electron temperature, number density, floating potential, and plasma potential in ionized gases (in the 1920's). Druyvesteyn noted that the second derivative of the probe current with respect to the bias voltage is proportional to the electron energy distribution function. The analysis by Laframboise enabled accurate evaluation of experimental data for cylindrical and spherical probes regardless of sheath size. PEPL makes extensive use of planar and cylindrical Langmuir (single, double, and triple) probes for evaluating plasma properties in the plumes of thrusters and in near electrode regions. The small size of typical Langmuir probes coupled with their relatively simple theory of operation make them an indispensable and widely used plasma diagnostic. While PEPL usually constructs custom probes sized to each experiment, commercially available systems do exist.

Above: Typical single Langmuir probe used with electric thrusters (from J. Linnell's Ph.D. Dissertation).

Selected Relevant Publications

  1. Langmuir, I., "Scattering of Electrons in Ionized Gases," Physical Review, Vol. 26, No. 5, Nov. 1925.
  2. Druyvesteyn, M. J., 1930 Z. Phys. 64, 781.
  3. Laframboise, J. G., Theory of Spherical and Cylindrical Langmuir Probes in a Collisionless, Maxwellian Plasma at Rest, UTIAS Report No. 100, June 1966.
  4. 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.
  5. Domonkos, M. T., Marrese, C. M., Haas, J. M., and Gallimore, A. D., "Very Near-Field Plume Investigation of the D55," AIAA-97-3062, 33rd Joint Propulsion Conference, Seattle, WA, July 6-9, 1997.
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