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  • 1
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    Current-driven plasma acceleration versus current-driven energy dissipation. I - Wave stability theory (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The dominant unstable electrostatic wave modes of an electromagnetically accelerated plasma are investigated. The study is the first part of a three-phase program aimed at characterizing the current-driven turbulent dissipation degrading the efficiency of Lorentz force plasma accelerators such as the MPD thruster. The analysis uses a kinetic theory that includes magnetic and thermal effects as well as those of an electron current transverse to the magnetic field and collisions, thus combining all the features of previous models. Analytical and numerical solutions allow a detailed description of threshold criteria, finite growth behavior, destabilization mechanisms and maximized-growth characteristics of the dominant unstable modes. The lower hybrid current-driven instability is implicated as dominant and was found to preserve its character in the collisional plasma regime.
    Keywords: PLASMA PHYSICS
    Type: AIAA PAPER 90-2610
    Format: text
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  • 2
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    Recombination lasing in a magnetoplasmadynamic arcjet (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The plasmadynamic recombination laser concept is verified experimentally in a high-power quasisteady MPD arcjet operating at 4 kA and 12 g/sec of argon. Measurements of the spatial variation of electron temperature, electron density, and population densities in the arc exhaust flow confirm that inverted populations of the 4p to 4s Ar II transitions are established by collisional-radiative recombination of the Ar III ion. Using an optical cavity aligned transversely to the flow, recombination lasing of seven such transitions, 5145, 4880, 4764, 4727, 4658, 4579, and 4545 A, is observed spectrophotographically and photoelectrically over the entire 1-msec discharge.
    Keywords: PLASMA PHYSICS
    Type: Journal of Applied Physics; 51; Jan. 198
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  • 3
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    Pulsed electromagnetic gas acceleration (1975)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Terminal voltage measurements with long cathodes in a high power, quasi-steady MPD discharge show that the critical current for the onset of voltage fluctuations, which was previously shown to be a function of cathode area, approaches an asymptote for cathodes of very large surface area. Floating potential measurements and photographs of the discharge luminosity indicate that the fluctuations are confined to the vicinity of the cathode and hence reflect a cathode emission process rather than a fundamental limit on MPD performance. Photoelectric measurements of particular argon neutral and ion transitions show that the higher electronic states are populated more heavily than would be calculated on the basis of Saha-Boltzmann equilibrium at the local electron temperature and number density. Preliminary optical depth measurements show that for a current of 4 kA and an argon mass flow of 12 g/sec, a population inversion exists between the upper and lower states of the 4880 A argon ion transition.
    Keywords: PLASMA PHYSICS
    Type: NASA-CR-142904 , SAR-634X
    Format: application/pdf
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  • 4
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    Pulsed electromagnetic gas acceleration (1974)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Detailed measurements of the axial velocity profile and electromagnetic structure of a high power, quasi-steady MPD discharge are used to formulate a gasdynamic model of the acceleration process. Conceptually dividing the accelerated plasma into an inner flow and an outer flow, it is found that more than two-thirds of the total power in the plasma is deposited in the inner flow, accelerating it to an exhaust velocity of 12.5 km/sec. The outer flow, which is accelerated to a velocity of only 6.2 km/sec, appears to provide a current conduction path between the inner flow and the anode. Related cathode studies have shown that the critical current for the onset of terminal voltage fluctuations, which was recently shown to be a function of the cathode area, appears to reach an asymptote for cathodes of very large surface area. Detailed floating potential measurements show that the fluctuations are confined to the vicinity of the cathode and hence reflect a cathode emission process rather than a fundamental limit on MPD performance.
    Keywords: PLASMA PHYSICS
    Type: NASA-CR-140996 , SAR-634W
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  • 5
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    Physical processes in hollow cathodes (1976)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Detailed measurements of magnetic field and floating potential inside large hollow cathodes in a quasi-steady MPD arc facility reveal a cavity plasma that is concentrated near the orifice for most operating conditions. However, for certain combinations of current and mass flow, the peak surface current density penetrates into the cavity up to one diameter upstream of the orifice. These measured distributions are consistent with a conduction model which suggests that maximum current penetration, and the more efficient cathode emission which accompanies it, occurs when the dominant mean free path for electron energy exchange is comparable to the cavity diameter.
    Keywords: PLASMA PHYSICS
    Type: AIAA PAPER 76-984 , International Electric Propulsion Conference; Nov 14, 1976 - Nov 17, 1976; Key Biscayne, FL
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  • 6
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    Anode power in quasisteady magnetoplasmadynamic accelerators (1978)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Anode heat flux in a quasi-steady MPD accelerator has been measured directly and locally by thermocouples attached to the inside surface of a shell anode. These measurements show that over a range of arc current from 5.5 to 44 kA, and argon mass flow from 1 to 48 g/s, the fraction of the total arc power deposited in the anode decreases from 50% at 200 kW to 10% at 20 MW. A theoretical model of the anode heat transfer asserts that energy exchange between electrons and heavy particles in the plasma near the anode occurs over distances greater than the anode sheath thickness, and hence the usual anode fall voltage, electron temperature, and work function contributions to the anode heat flux are supplemented by a contribution from the interelectrode potential. Calculations of anode heat flux using the measured current density, plasma potential, and electron temperature in the plasma adjacent to the anode agree with the direct measurements and indicate that the decrease in anode power fraction at higher arc powers can be attributed to the smaller mean free paths in the interelectrode plasma.
    Keywords: PLASMA PHYSICS
    Type: AIAA Journal; 16; Apr. 197
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  • 7
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    Current driven instabilities of an electromagnetically accelerated plasma (1988)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Quantitative measurements of the plasma instability that strongly affects the efficiency and lifetime of electromagnetic plasma accelerators were obtained. Experimental results are presented for the dispersion relations (wave phase velocities), spatial growth rates, and stability boundaries of accelerators. The measured critical wave parameters are found to agree well with theoretical predictions. A kinetic theoretical model was used to study current driven instabilities which are intrinsic to the high-current-carrying magnetized plasma of a magnetoplasmadynamic accelerator. Under certain conditions, marginally stable waves travelling almost perpendicular to the magnetic field are found which travel at a velocity equal to that of the electron current.
    Keywords: PLASMA PHYSICS
    Type: International Electric Propulsion Conference; Oct 03, 1988 - Oct 06, 1988; Garmisch-Partenkirchen; Germany
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  • 8
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    Recombination lasing in a magnetoplasmadynamic arcjet (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The plasmadynamic recombination laser concept is verified experimentally in a high power quasi-steady MPD arcjet operating at 4 kA and 12 g/sec of argon. Measurements of the spatial variation of electron temperature, electron density, and population densities in the arc exhaust flow confirm that inverted populations of the 4p to 4s Ar II transitions are established by collisional-radiative recombination of the Ar III ion. Using an optical cavity aligned transversely to the flow, recombination lasing of four such transitions, 5145, 4880, 4764 and 4727 A, is observed spectro-photographically and photoelectrically over the entire 1-msec discharge.
    Keywords: PLASMA PHYSICS
    Type: AIAA PAPER 77-242 , Aerospace Sciences Meeting; Jan 24, 1977 - Jan 26, 1977; Los Angeles, CA
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  • 9
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    Pulsed electromagnetic gas acceleration (1975)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Terminal voltage measurements with various cathodes and anodes in a high power, quasi-steady magnetoplasmadynamic (MPD) are discussed. The magnitude of the current at the onset of voltage fluctuations is shown to be an increasing function of cathode area and a weaker decreasing function of anode area. Tests with a fluted cathode indicated that the fluctuations originate in the plasma adjacent to the cathode rather than at the cathode surface. Measurements of radiative output from an optical cavity aligned to examine the current-carrying portion of a two-dimensional, 56 kA magnetoplasmadynamic discharge reveal no lasing in that region, consistent with calculations of electron excitation and resonance radiation trapping. A voltage-swept double probe technique allows single-shot determination of electron temperature and electron number density in the recombining MPD exhaust flow. Current distributions within the cavity of MPD hollow cathodes for various static prefills with no injected mass flow are examined.
    Keywords: PLASMA PHYSICS
    Type: NASA-CR-145895 , SAR-6344Y
    Format: application/pdf
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  • 10
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    Laser processes in plasmadynamic flows (1975)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Radiation emitted from the exhaust flow of a multi-megawatt, quasi-steady magnetoplasmadynamic accelerator has been examined photoelectrically for argon mass flows from 6 to 32 g/sec and arc currents up to 25 kA. Vacuum ultraviolet radiation, indicative of resonance transitions, has been detected photoelectrically at various axial locations downstream of the arc chamber, consistent with a relaxation length calculated using a collisional-radiative recombination model. Photoelectric measurements of particular transitions in the neutral and first ionic state of argon show that populations of excited electronic states of AII are in excess of that which would be expected on the basis of Saha-Boltzmann equilibrium at the local electron temperature and number density. Optical depth measurements indicate that for a current of 4 kA and argon mass flow of 12 g/sec, a population inversion exists between the upper and lower states of the 0.4880 micron argon ion transition.
    Keywords: PLASMA PHYSICS
    Type: AIAA PAPER 75-852 , American Institute of Aeronautics and Astronautics, Fluid and Plasma Dynamics Conference; Jun 16, 1975 - Jun 18, 1975; Hartford, CT
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