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Jahn, R. G.

Publications and source records attributed to Jahn, R. G..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Flow field characteristics and performance limitations of quasi-steady magnetoplasmadynamic accelerators

Measurements of terminal voltage, exhaust velocity, current and potential distributions in a quasi-steady MPD arc operating over a range of currents from 3 to 25 kA countervene previous indications of basic limitations on MPD performance. Terminal voltage fluctuations, previously used as an indicator of a unique limiting value of the parameter current squared over mass flow, are shown to be related to cathode surface area rather than to interelectrode plasma phenomena. With improved electrodes and insulators, satisfactory arc operation at specific impulses somewhat beyond previously proposed limits has been demonstrated, and the significant thrust producing mechanisms in such arcs have been identified.

Boyle, M. J.↗
NASA NTRS1975

Pulsed electromagnetic gas acceleration

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.

Jahn, R. G.↗
NASA NTRS1974

Pulsed electromagnetic gas acceleration

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.

Jahn, R. G.↗
NASA NTRS1974

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.↗
NASA NTRS1973

Effects of insulator ablation on the operation of a quasi-steady MPD arc

Multimegawatt operation of quasi-steady MPD arcjets can involve serious ablation of the insulator surfaces within the arc discharge chamber. Various degrees of insulator ablation manifest themselves by significantly perturbing the voltage-current characteristics and the exhaust velocity profiles. Voltage-current characteristics for two different insulator materials, Plexiglas and boron nitride, are interpreted in terms of an empirical Ohm's law. Use of the refractory insulator material eliminates the ablation-dominated nature of the terminal voltage, but the exhaust stream is still disturbed by insulator material. An Alfven critical velocity model can be applied to this influence of insulator ablation on exhaust velocity. Appropriate changes in the propellant injection geometry eliminate this influence and result in arcjet operation which is independent of insulator material. A particular combination of propellant injection geometries reduces the terminal voltage for a given current and mass flow while maintaining insulator-independent operation, thus implying an improvement in the overall efficiency of the device.

Boyle, M. J.↗
NASA NTRS1973

Anode power deposition in quasi-steady MPD arcs

The power deposited in the anode of a quasi-steady MPD accelerator has been measured directly by thermocouples attached to the inside surface of a shell anode which provide a local measurement of anode heat flux. The results over a range of arc currents from 5.5 to 44 kiloamperes and argon mass flows from 1 g/sec to 48 g/sec show that the fraction of the total input power deposited in the anode decreases drastically from 50% at an arc power of 200 kW to 10% at 20 MW, and that anode power is not uniformly deposited in the anode. A theoretical model of the anode heat transfer, including effects of anode work function, electron thermal energy, and anode sheath, can be brought into reasonable agreement with the measurements, provided the effective range of the conduction electrons from within the discharge plasma to the anode surface is properly acknowledged.

Saber, A. J.↗
NASA NTRS1973

The hollow cathode in the quasi-steady MPD discharge

A large hollow cathode has been operated in a quasi-steady MPD discharge over a range of current from 7 to 30 kA and argon mass flow from 0.04 to 6.0 g/sec. The 1.3-cm-i.d. cathode cavity attains steady emission characteristics in some tens of microseconds without the assistance of auxiliary heating, low work function inserts, or external keeper electrodes. Measured current and potential distributions within the cavity reveal that the current attaches in a zone 1 to 2 cm long with a surface current density greater than 1000 A/sq cm and a local axial electric field less than 10 V/cm. Electron densities within the cavity, estimated from spectroscopic records, are above 10 to the 17th power per cu cm, at least one order of magnitude greater than has been reported for either ion engine hollow cathodes or conventional solid cathodes in similar arc discharges.

Von Jaskowsky, W. F.↗
NASA NTRS1973

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.↗
NASA NTRS1973

Pulsed electromagnetic acceleration

Direct measurements of the power deposited in the anode of a multimegawatt MPD accelerator using thermocouples attached to a thin shell anode reveal a dramatic decrease in the fractional anode power from 50% at 200 KW input power to less than 10% at 20 MW power. The corresponding local power flux peak at a value of 10,000 W/sq cm at the lip of the anode exhaust orifice, a distribution traced to a corresponding peak in the local current density at the anode. A comparison of voltage-current characteristics and spectral photographs of the MPD discharge using quartz, boron nitride and plexiglas insulators with various mass injection configurations led to the identification of different voltage modes and regions of ablation free operation. The technique of piezoelectric impact pressure measurement in the MPD exhaust flow was refined to account for the effects due to probe yaw angle.

Jahn, R. G.↗
NASA NTRS1973

Quasi-steady plasma acceleration.

A coaxial plasma accelerator driven by protracted pulses of current in the range of 10,000 to 100,000 A and synchronized mass flows from 1.0 to 36 g/sec argon attains, after some tens of microseconds, a stable magnetoplasmadynamic acceleration mode. This 'quasi-steady' discharge form is characterized by constant terminal voltage and current, a diffuse, fixed current distribution within the discharge, and a steady plasma efflux at velocities of approximately 20 km/sec. Measured potential distributions reveal that the bulk of the arc voltage gradient, exclusive of the electrode falls, occurs within two diameters of the cathode, and is normal to it. The anode fall voltage varies inversely with local current density, implying substantially lower anode losses at higher power arc operation. Spectroscopic, potential, and velocity measurements indicate the existence of a characteristic mass flow rate for a given current, below which arc operation becomes erratic.

Jahn, R. G.↗
NASA NTRS1972

Measurement of mass, momentum and energy distributions in a quasi-steady MPD discharge.

Piezoelectric impact pressure measurements in the exhaust flow of a quasi-steady MPD accelerator are used to compute the profile of momentum flux 25.4 cm downstream of a 2.8 MW discharge. The integral of this profile is in good agreement with the sum of calculated electromagnetic and measured electrothermal thrust components. Velocity vector profiles obtained by a time-of-flight technique reveal a radial variation of velocity ranging from 31,000 m/sec on the centerline to 11,000 m/sec at a radius of 20 cm. When combined with the momentum flux profiles, these velocities yield information on the distribution of mass and energy in the plasma exhaust.

Clark, K. E.↗
NASA NTRS1972

Exhaust plume structure in a quasi-steady MPD arc.

Results of spectroscopic and photographic investigations revealing a complex azimuthal species structure in the exhaust plume of a quasi-steady argon MPD arc. The features of the plume structure are found to be dependent on the arc current and mass flow rate. Time-resolved spectroscopic velocity measurements, obtained by a scanning Fabry-Perot interferometer, indicate argon ion jet velocities of 16,500 m/sec at a current of 16 kA and a matched mass flow of 6 g/sec. It is found that nearly half this velocity is attained in an acceleration region well downstream of the region of significant electromagnetic interaction. Temperature measurements suggest that simple expansion processes in the argon jets cannot fully explain the observed acceleration. It is possible that a transfer of momentum from the ablated core flow to the argon jets may contribute to the high argon velocities.

Bruckner, A. P.↗
NASA NTRS1972

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.↗
NASA NTRS1971

Hollow cathode, quasi-steady MPD arc

A quasi-steady MPD accelerator has been operated with four different hollow cathodes over a power range from 5 kilowatts to 5 megawatts. The absolute level of the argon mass flow, as well as the fractional division of the flow between the cathode and the six standard chamber injectors, is varied over a range of 1 to 12 grams per second. For a fixed total current, it is observed that the voltage increases monotonically with mass flow rate, compared to the usual experience with solid cathodes where the voltage decreases with mass flow rate. For a fixed percentage of flow through the cathode, each hollow cathode configuration displays a minimum impedance at a particular value of the total mass flow. It is asserted that in order to keep the discharge inside the hollow cathode the magnetic pressure and gasdynamic pressure have to match inside the cavity.

Parmentier, N.↗
NASA NTRS1971

Pulsed electromagnetic gas acceleration

Experimental data were combined with one-dimensional conservation relations to yield information on the energy deposition ratio in a parallel-plate accelerator, where the downstream flow was confined to a constant area channel. Approximately 70% of the total input power was detected in the exhaust flow, of which only about 20% appeared as directed kinetic energy, thus implying that a downstream expansion to convert chamber enthalpy into kinetic energy must be an important aspect of conventional high power MPD arcs. Spectroscopic experiments on a quasi-steady MPD argon accelerator verified the presence of A(III) and the absence of A(I), and indicated an azimuthal structure in the jet related to the mass injection locations. Measurements of pressure in the arc chamber and impact pressure in the exhaust jet using a piezocrystal backed by a Plexiglas rod were in good agreement with the electromagnetic thrust model.

Jahn, R. G.↗