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Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry
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Electron Cooling in a Magnetically Expanding Plasma.

J M Little1, E Y Choueiri1

  • 1Electric Propulsion and Plasma Dynamics Laboratory (EPPDyL), Princeton University, Princeton, New Jersey, 08544, USA.

Physical Review Letters
|December 8, 2016
PubMed
Summary
This summary is machine-generated.

Electron cooling in magnetic nozzles deviates from simple models. Experimental data reveals a polytropic expansion with exponent γe≈1.15, impacting plasma flow and ion acceleration efficiency.

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Area of Science:

  • Plasma Physics
  • Space Propulsion
  • Magnetic Nozzle Technology

Background:

  • Electron cooling is crucial for plasma flow and detachment in magnetic nozzles.
  • Current magnetic nozzle (MN) models often assume isothermal electron expansion (γe=1).
  • This assumption may not accurately represent real plasma behavior.

Purpose of the Study:

  • To experimentally investigate electron cooling in a magnetically expanding plasma.
  • To determine the polytropic exponent governing electron expansion in a magnetic nozzle.
  • To assess the implications for plasma flow and ion acceleration.

Main Methods:

  • Utilized a radio frequency plasma source and a magnetic nozzle (MN).
  • Conducted probe measurements of plasma density, potential, and electron temperature along the MN centerline.
  • Employed a quasi-1D fluid model incorporating classical electron thermal conduction.

Main Results:

  • Electron expansion in the MN followed a polytropic law with γe=1.15±0.03.
  • This experimentally determined value contradicts the commonly assumed isothermal expansion (γe=1).
  • The observed axial variations were consistent with a quasi-1D fluid model predicting γe≈1.19 in the weakly collisional limit.

Conclusions:

  • The polytropic nature of electron expansion (γe≈1.15) is critical for accurate magnetic nozzle modeling.
  • A new criterion based on the ratio of convected to conducted power predicts efficient ion acceleration.
  • Findings challenge existing assumptions and provide a more realistic understanding of plasma behavior in magnetic nozzles.