A summary of performance and lifetime characteristics of pulsed and steady-state magnetoplasmadynamic (MPD) thrusters is presented. The technical focus is on cargo vehicle propulsion for exploration-class missions to the Moon and Mars. Relatively high MPD thruster efficiencies of 0.43 and 0.69 have been reported at about 5000 s specific impulse using hydrogen and lithium, respectively. Efficiencies of 0.10 to 0.35 in the 1000 to 4500 s specific impulse range have been obtained with other propellants (e.g., Ar, NH3, N2). Thermal efficiency data in excess of 0.80 at MW power levels using pulsed thrusters indicate the potential of high MPD thruster performance. Extended tests of pulsed and steady-state MPD thrusters yield total impulses at least two to three orders of magnitude below that necessary for cargo vehicle propulsion. Performance tests and diagnostics for life-limiting mechanisms of megawatt-class thrusters will require high fidelity test stands which handle in excess of 10 kA and a vacuum facility whose operational pressure is less than 3 x 10 to the -4 torr.

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Performance and lifetime assessment of MPD arc thruster technology

A new quadruple Langmuir probe diagnostic technique is applied to the plume of a pulsed magnetoplasmadynamic (MPD) thruster with argon propellant. The probe permits simultaneous measurement of electron temperature Te, density ne, and ion flow velocity u in the exhaust stream. For distances of 3-25 cm from the thruster face, Te is 8-1.5 eV, ne is 4.8-1.2 x 10 m~, and u is 0.7-1.8 x 10 m/s. The measurements suggest that the plume is well-collimated, and that the ion temperature is significantly higher than the electron temperature, consistent with an ion-conduction model of the thruster.

Application of a quadruple probe technique to MPD thruster plume measurements

When adcdischarge isoperated between acentral cathode andaring-shaped anodeatcurrents upto6X 103A andpressures on theorder ofafewtoffs, thestable discharge regime isfoundtobelim- ited. Theanodefail voltage isdetermined experimentally fromcalori- metric andpotential measurements. Inawiderange ofoperating condi- tions itsvariation correlates wellwiththeparameter J2/mh whereJ= current, mh=massflowratethrough thedischarge. Withincreasing values ofJ2/m,h theanodefall increases continuously fromsmall nega- tive values, through zero, anduptomoderate positive values. Then, within asmall increment ofJ2/mh, itjumps tovalues ontheorder of theionization potential ofthegas.A theoretical two-dimensional description oftheflowfield indicates thattheself-magnetic forces, whichareproportional toJ2,pinch thegastowards thedischarge axis, thereby leading toparticle starvation inthevicinity oftheanode. Basedupontheresults ofthis study itisshownthat, asaconsequence ofthealtering boundary conditions, whicharecharacterized bythe parameter J2/m,# theanoderesponds byadopting different operating modes.Existing theories yield values fortheanodefall which arein goodagreement withexperimental data.

Effect ofSelf-Magnetic Forceson theAnode Mechanism ofa High Current Discharge

In order to clarify the acceleration mechanism of applied-field magneto-plasma-dynamic arcjet (MPDA) plasma, the spatial profiles of the flow field and electromagnetic field near the outlet of the MPDA were measured using magnetic probes and the spectroscopic method. The plasma current densities and Lorentz forces acting on the plasma were evaluated experimentally. It was found that the azimuthal rotation of the exhausted plasma in the applied magnetic field is determined by a balance among the E×B drift, the diamagnetic drift, and the centrifugal force drift. Three components of the Lorentz force, i.e., the radial, the azimuthal, and the axial, were measured experimentally for the first time. The radial component Fr was dominant among the three components and the axial one (Fz) was weakened by the deceleration force, which spontaneously appeared in the applied-field MPDA plasma due to a diamagnetic effect of the high-beta plasma. It was demonstrated that the deceleration force can be converted to an acce...

Characteristics of electromagnetically accelerated plasma flow in an externally applied magnetic field

The thrust and discharge voltage of a quasisteady MPD arcjet have been measured, with emphasis on the transitional characteristics from the propellant over-fed to starved condition and from aerodynamic to electromagnetic acceleration. From the anode and cathode potential measurements, the transition into the starved condition was found to be preceded slightly by the transition of acceleration process when the discharge current was increased. At the beginning of the electromagnetic acceleration, the thrust became unstable. This was attributable to radial current expansion toward the outer anode region. At higher discharge current, the flow pattern settled and the quasisteady state was restored.

Transitional Behavior of MPD Arcjet Operation

https://ntrs.nasa.gov/api/citations/19710012242/downloads/19710012242.pdf

A. C. Malliaris

Papers

Quasi-steady MPD propulsion at high power Final technical report