Pulsed electromagnetic gas acceleration Semiannual report, 1 Jul. - 31 Dec. 1970
Potential and current distributions throughout MPD arc indicate that anode fall voltage varies inversely with local anode current density
Jahn, R. G.↗
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Anode phenomena in high-current discharges
Pinch and parallel-plate discharges, and analysis of anode region of quasi-steady MPD arc
Oberth, R. C.↗
Anode phenomena in high-current accelerators
Quasi-steady MPD accelerators current conduction and power loss mechanism investigation through local anode fall voltage and current density measurements at different arc current levels
Jahn, R. G.↗
Electrothermal oscillations and the quasilinear theory of electron enthalpy fluctuations in magnetohydrodynamic generators and magnetoplasmadynamic arc thrusters
Flucturations in electron density and temperature coupled through OHM's Law are studied for MHD power generator and MPD arc thruster applications. The dispersion relation based on linear theory is derived, and the two limiting cases of infinite ionization rate and frozen flow are examined. The nonlinear effects of the frozen flow case are then studied in the quasilinear limit. Equations are derived for the amplitude of the fluctuation and its effect upon Ohm's Law and the electron temperature equation. Conditions under which a steady state can exist in the presence of the fluctuation are examined, and effective transport properties are determined.
Smith, J. M.↗
Satellite auxiliary-propulsion selection techniques. Addendum: A survey of auxiliary electric propulsion systems
A review of electric thrusters for satellite auxiliary propulsion was conducted at JPL during the past year. Comparisons of the various thrusters for attitude propulsion and east-west and north-south stationkeeping were made based upon performance, mass, power, and demonstrated life. Reliability and cost are also discussed. The method of electrical acceleration of propellant served to divide the thruster systems into two groups: electrostatic and electromagnetic. Ion and colloid thrusters fall within the electrostatically accelerated group while MPD and pulsed plasma thrusters comprise the electromagnetically accelerated group. The survey was confined to research in the United States with accent on flight and flight prototype systems.
Holcomb, L. B.↗
Pulsed electromagnetic gas acceleration
Photographs of the exhaust plume of a pulsed MPD discharge through selected narrow band spectral filters reveal a species structure related to the location of the argon mass injection ports. This species structure provides the key to interpretation of time-resolved interferometric velocity measurements in the exhaust. The resulting exhaust velocity increases monotonically from 8500 m/sec at a position 5 cm downstream of the anode face to 16,500 m/sec 40 cm downstream. The latter value is approximately twice the Alfven critical speed for argon. The growth of the axial electric field near the downstream face of the anode indicates that the discharge operates in a starved mode. Data from biased double probes imply an electron temperature of 0.8 eV in the exhaust plume.
Jahn, R. G.↗
Quasi-steady plasma accelerators
Results are reported from a continued study of MPD thrusters operating in the quasi-steady mode and using electrode vapor as the propellant. Testing methods were refined, and performance data is now available for a fairly wide range of thruster geometries. Performance was found to be strongly dependent on thruster geometry, with some designs showing approximately twice the efficiency obtained during the preceding contract period. The radial clearance between electrodes, the attachment point of the arc on the cathode, and the electrical resistance of the electrodes appear to be important design variables.
Ducati, A. C.↗
Population inversion calculations using near resonant charge exchange as a pumping mechanism
Near resonance charge exchange between ions of a large ionization potential gas such as helium or neon and vapors of metals such as zinc, cadmium, selenium, or tellurium has produced laser action in the metal ion gas. The possibility of obtaining population inversions in near resonant charge exchange systems (Xe-Ca, Xe-Mg, Xe-Sr, Xe-Ba, Ar-Mg, N-Ca) was investigated. The analysis is an initial value problem that utilizes rate equations for the densities of relevant levels of the laser gas (Ca, Ba, Mg, or Sr) and an electron energy equation. Electron excitation rates are calculated using the Bohr-Thomson approximation for the cross section. Approximations to experimental values of the electron ionization cross section and the ion-atom charge exchange cross section are used. Preliminary results have been obtained for the Ca-Xe system and show that it is possible to obtain gains greater than 10 to the 14th power/m with inversion times up to 8x10 to the minus 7th power second. A possible charge exchange laser system using a MPD arc plasma accelerator is also described.
Chubb, D. L.↗
Pulsed electromagnetic gas accelleration
Direct measurement with thermocouples of the power deposited in the anode of a multi-megawatt magnetoplasmadynamic discharge has shown the fractional anode power to decrease from 50% at 200 kW to 10% at 20 MW. Using local measurements of current density, electric potential, and electron temperature, the traditional model for heat conduction to the anode is found to be inadequate. Other experiments in which the voltage-current characteristics and exhaust velocities of MPD arcs using Plexiglas and boron nitride chamber insulators and various mass injection configurations show that ablation can affect nominal accelerator operation in several distinct ways. The incorporation of a hollow cathode in a 7 kA plasma discharge has shown that a stable current attachment can be realized in the cavity without the aid of cathode heaters, keeper electrodes, or emissive coatings.
Jahn, R. G.↗
Acoustics of the piezo-electric pressure probe
Acoustical properties of a piezoelectric device are reported for measuring the pressure in the plasma flow from an MPD arc. A description and analysis of the acoustical behavior in a piezoelectric probe is presented for impedance matching and damping. The experimental results are presented in a set of oscillographic records.
Dutt, G. S.↗
Hollow cathode physics
Hollow cathode configurations with cavity diameters up to 2 cm have been operated in quasi-steady MPD discharges at tens of kA and argon massflows up to 16 g per sec. Internal probe measurements exhibit densities of current attachment of 1.5 kA per sq cm in a 1 cm wide zone at the end of the cavity coincident with low electric fields of less than 10 V per cm. This zone is further characterized by a sheath of bright AII and continuum radiance which is absent at low massflows. Electron densities exceed 10 to the 17th power per cu cm, indicating a conductivity of 10,000 mhos per m at the cavity exit for an electron temperature of only a few eV.
Von Jaskowsky, W. F.↗
Physical processes in hollow cathodes
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.
Krishnan, M.↗
Recombination lasing in a magnetoplasmadynamic arcjet
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.
Campbell, E. M.↗
Recombination lasing in a magnetoplasmadynamic arcjet
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 ArII transitions are established by collisional-radiative recombination of the ArIII ion. Using an optical cavity aligned transversely to the flow, recombination lasing of four such transitions, 5145, 4880, 4764, and 4727 A, is observed spectrophotographically and photoelectrically over the entire 1-msec discharge.
Campbell, E. M.↗
Anode power in quasisteady magnetoplasmadynamic accelerators
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.
Saber, A. J.↗
Electric propulsion technology
The advanced electric propulsion program is directed towards lowering the specific impulse and increasing the thrust per unit of ion thruster systems. In addition, electrothermal and electromagnetic propulsion technologies are being developed to attempt to fill the gap between the conventional ion thruster and chemical rocket systems. Most of these new concepts are exagenous and are represented by rail accelerators, ablative Teflon thrusters, MPD arcs, Free Radicals, etc. Endogenous systems such as metallic hydrogen offer great promise and are also being pursued.
Finke, R. C.↗
A study of the applicability/compatibility of inertial energy storage systems to future space missions
The applicability/compatibility of inertial energy storage systems like the homopolar generator (HPG) and the compensated pulsed alternator (CPA) to future space missions is explored. Areas of CPA and HPG design requiring development for space applications are identified. The manner in which acceptance parameters of the CPA and HPG scale with operating parameters of the machines are explored and the types of electrical loads which are compatible with the CPA and HPG are examined. Potential applications including the magnetoplasmadynamic (MPD) thruster, pulsed data transmission, laser ranging, welding and electromagnetic space launch are discussed.
Weldon, W. F.↗
Recombination lasing in a magnetoplasmadynamic arcjet
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.
Campbell, E. M.↗