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Results for “plasma sheaths”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Experimental measurements of the plasma sheath around pinhole defects in a simulated high-voltage solar array

An emissive Langmuir probe was used to measure the potentials within the plasma sheath developed around a hole in a simulated solar array at voltages between 50 and 450 V. The hole sizes were larger than actual pinhole defects; the plasma density was in the 10,000 per cu cm range, which is considerably lower than the density of 1,000,000 per cu cm found at low-earth-orbit altitudes. Despite these inadequacies in the simulation, the experiments indicate that this type of probe is a useful diagnostic technique for investigating the plasma sheaths developing around pinhole defects.

Gabriel, S. B.↗
NASA NTRS1989

High-frequency instability of the sheath-plasma resonance

Coherent high frequency oscillations near the electron plasma frequency (omega approx. less than omega sub p) are generated by electrodes with positive dc bias immersed in a uniform Maxwellian afterglow plasma. The instability occurs at the sheath-plasma resonance and is driven by a negative RF sheath resistance associated with the electron inertia in the diode-like electron-rich sheath. With increasing dc bias, i.e., electron transit time, the instability exhibits a hard threshold, downward frequency pulling, line broadening and copious harmonics. The fundamental instability is a bounded oscillation due to wave evanescence, but the harmonics are radiated as electromagnetic waves from the electrodes acting like antennas. Wavelength and polarization measurements confirm the emission process. Electromagnetic waves are excited by electrodes of various geometries (planes, cylinders, spheres) which excludes other radiation mechanisms such as orbitrons or beam-plasma instabilities. The line broadening mechanism was identified as a frequency modulation via the electron transit time by dynamic ions. Ion oscillations at the sheath edge give rise to burst-like RF emissions. These laboratory observations of a new instability are important for antennas in space plasmas, generation of coherent beams with diodes, and plasma diagnostics.

Stenzel, R. L.↗
NASA NTRS1989

High-frequency instability of the sheath-plasma resonance

Coherent high-frequency oscillations near the electron plasma frequency are generated by electrodes with positive dc bias immersed in a uniform Maxwellian afterglow plasma. The instability occurs at the sheath-plasma resonance and is driven by a negative RF sheath resistance associated with the electron inertia in the diodelike electron-rich sheath. With increasing dc bias, i.e., electron transit time, the instability exhibits a hard threshold, downward frequency pulling, line broadening, and copious harmonics. The fundamental instability is a bounded oscillation caused by wave evanescence, but the harmonics are radiated as electromagnetic waves from the electrodes acting like antennas. Wavelength and polarization measurements confirm the emission process. Electromagnetic waves are excited by electrodes of various geometries (planes, cylinders, spheres), which excludes other radiation mechanisms such as orbitrons or beam-plasma instabilities. The line-broadening mechanism has been identified as a frequency modulation via the electron transit time by dynamic ions. Ion oscillations at the sheath edge give rise to burstlike RF emissions. These laboratory observations of a new instability are important for antennas in space plasmas, generation of coherent beams with diodes, and plasma diagnostics.

Stenzel, R. L.↗
NASA NTRS1993

Plasma sheath effects on ion collection by a pinhole

The data tabulations presented, which express the results of particle-track trajectories through a simplified plasma-sheath electric field, should prove useful in the assessment of spacecraft 'pinhole-collection' effects. Attention is given to a method accounting for plasma-sheath effects in the application of these results. In addition, scaling rules are given for the application of these calculations to specific cases. A comparison is made between the present model's results and those of recent ion current measurements. Good agreement is obtained.

Herr, Joel L.↗
NASA NTRS1993

Plasma sheath effects on ion collection by a pinhole

This work presents tables to assist in the evaluation of pinhole collection effects on spacecraft. These tables summarize results of a computer model which tracks particle trajectories through a simplified electric field in the plasma sheath. A technique is proposed to account for plasma sheath effects in the application of these results and scaling rules are proposed to apply the calculations to specific situations. This model is compared to ion current measurements obtained by another worker, and the agreement is very good.

Herr, Joel L.↗
DOE OSTI2023

Resolving the mystery of electron perpendicular temperature spike in the plasma sheath

A large family of plasmas has collisional mean-free-path much longer than the non-neutral sheath width, which scales with the plasma Debye length. The plasmas, particularly the electrons, assume strong temperature anisotropy in the sheath. The temperature in the sheath flow direction (Te∥) is lower and drops toward the wall as a result of the decompressional cooling by the accelerating sheath flow. The electron temperature in the transverse direction of the flow field (Te⊥) not only is higher but also spikes up in the sheath. This abnormal behavior of Te⊥ spike is found to be the result of a negative gradient of the parallel heat flux of transverse degrees of freedom (qes) in the sheath. The non-zero heat flux qes is induced by pitch-angle scattering of electrons via either their interaction with self-excited electromagnetic waves in a nearly collisionless plasma or Coulomb collision in a collisional plasma, or both in the intermediate regime of plasma collisionality.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRSDate not supplied

Effect of Plasma Sheaths on Earth Re-entry MHD Processes

The first study of the full coupling between the aerothermodynamics, the magnetohydrodynamics, and the plasma sheaths within earth entry flows is here per-formed. The problem tackled herein is representative of a force-generating MHD patch located between the plenum and the frustum of a capsule entering the earth’s atmosphere at Mach 34. The reactions are obtained from a modified 11-species Park chemical solver. The physical model fully couples the drift-diffusion model for the sheaths to the multispecies Navier-Stokes equations for the plasma flow. The Hall and ion slip effects are taken into consideration within the plasma flow and within the sheaths. The effect of the electrode material on the MHD process is studied. Using thoriated tungsten instead of graphite leads to a four-fold increase in the Lorentz forces and also leads to significantly reduced heat fluxes on the cathode. This is attributed to the much higher electrical conductivity of the thoriated tungsten sheath reducing by orders of magnitude the plasma electrical resistance near the surfaces.

Bernard Parent↗