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, NHa, NI). Electrode power losses of less than 20% at megawatt 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 4 x 10 ~2 Pa. Nomenclature e = electronic charge, 1.6 x 10 ~1 9 C g = gravity acceleration, 9.81 m/s 2 H 0 = unheated propellant enthalpy, J/kg A P -specific impulses, s J = arc current, A (J 2 /m) c = onset parameter, A 2 -s/kg k = Boltzman's constant, 1.38 x 10~2 3 J/K m = mass flow rate, kg/s P = input power, W P a = power loss to anode, W P e = input electric power, W P L = power to water cooling system, W T = thrust, N T e = electron temperature, K V a = anode voltage drop, V AK = velocity increment required for a mission, m/s v = average exhaust velocity, m/s v\ = thrust efficiency f/ th = (P -P L )/P, thermal efficiency (j > = work function of anode material, V

NASA's Hall thruster program has base research and focused development efforts in support of the Advanced Space Transportation Program, Space-Based Program and various other programs. The objective of the base research is to gain an improved understanding of the physical processes and engineering constraints of Hall thrusters to enable development of advanced Hall thruster designs. Specific technical questions that are current priorities of the base effort are: (1) How does thruster life vary with operating point? (2) How can thruster lifetime and wear rate be most efficiently evaluated? (3) What are the practical limitations for discharge voltage as it pertains to high specific impulse operation (high discharge voltage) and high thrust operation (low discharge voltage)? (4) What are the practical limits for extending Hall thrusters to very high input powers? and (5) What can be done during thruster design to reduce cost and integration concerns? The objective of the focused development effort is to develop a 50 kW-class Hall propulsion system, with a milestone of a 50 kW engineering model thruster/system by the end of program year 2006. Specific program year 2001 efforts, along with the corporate and academic participation, are described.

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s e d t h r u s t e r s i n d i c a t e t h e p o t e n t i a l o f h i g h MPD t h r u s t e r performance. Extended t e s t s o f p u l s e d and s t e a d y -s t a t e MPD t h r u s t e r s y i e l d t o t a l impulses A a t l e a s t two t o t h r e e o r d e r s o f magnitude below t h a t necessary f o r c a r g o v e h ic l e p r o p u l s i o n . Performance t e s t s and d i a g n o s t i c s f o r l i f e -l i m i t i n g mechanisms o f megawatt-class t h r u s t e r s w i l l r e q u i r e h i g h f i d e l i t y t e s t stands whichh a n d l e i n excess o f 10 kA and a vacuum f a c i l i t y whose o p e r a t i o n a l p r e s s u r e i s l e s s t h a n 3~1 0 -~ t o r r .

Three applied-field MPD thruster geometries were tested with argon propellant to establish the influence of electrode geomety on thruster performance.