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Plasma Diagnostics

High-speed Dual Langmuir Probe (HDLP)

A rapidly swept and actively compensated single Langmuir probe that measures electron temperature, electron energy distribution function, number density, floating potential, and plasma potential at rates as high as 10 MHz.


Above: Typical HDLP used with electric thrusters2.

In an effort to temporally resolve the electron density, electron temperature, and plasma potential for turbulent plasma discharges, a unique High-speed Dual Langmuir Probe (HDLP) diagnostic has been developed at PEPL.1-4 Comprised of a traditional single Langmuir probe of cylindrical geometry (exposed to the plasma) swept simultaneously with a nearby capacitance and noise compensating null probe (fully insulated from the plasma), microsecond-timescale sweep rates are achievable. Traditional thin-sheath Langmuir probe theory is applied for interpretation of the collected probe data, however specialized broadband high-voltage low-noise amplifiers, sensors, and DAQ are required to capture the long datasets required to capture a statistically significant record of measured plasma turbulence.

Langmuir probes were the first plasma diagnostic developed nearly a century ago by Irving Langmuir. Today they are still employed as one of the most useful and common means of measuring plasma density, electron temperature (or the Electron Energy Distribution Function, EEDF), plasma potential, or floating potential with excellent spatial resolution, and a simple theory of probe data interpretation. The temporal resolution of a rapidly swept Langmuir probe (and in HDLP configuration) has been examined in detail at PEPL and swept rates of nearly 10-MHz appears attainable for plasma environments typical to Hall Effect Thrusters (HETs) and Tokamak edge regions.2


Above: Photograph of a 100-kHz high-speed dual Langmuir probe positioned about 5 cm downstream of 600 Watt Hall thruster cluster with close-ups of probe (from ref. 4).

While only recently developed in 2008, the HDLP diagnostic has already revealed dramatic bursts of plasma downstream from modern Hall effect thrusters created by a natural ionization instability termed the Hall thruster breathing mode.3 The turbulent character of most plasmas, including those of electric propulsion devices, where transient features such as the Hall thruster breathing mode play an crucial role in the overall device efficiency and performance, make the HDLP a important diagnostic in the years to come.

 

Selected Relevant Publications

  1. Robert B. Lobbia and Alec D. Gallimore, "High-speed dual Langmuir probe," Rev. Sci. Instrum., Art. No. 073503 , Vol. 81, Issue 7, July 2010. (Published online by the American Institute of Physics, click here to view abstract.)
  2. Robert B. Lobbia and Alec D. Gallimore, "Temporal limits of a rapidly swept Langmuir probe," Physics of Plasmas, Art. No. 073502 , Vol. 17, Issue 7, July 2010. (Published online by the American Institute of Physics, click here to view abstract.)
  3. Lobbia, R. B., "A Time-resolved Investigation of the Hall Thruster Breathing Mode," Ph.D. Dissertation, University of Michigan, 2010.
  4. Lobbia, R. B. and Gallimore, A. D., "A Method of Measuring Transient Plume Properties," AIAA-2008-4650, 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Hartford, CT, July 20-23, 2008.
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