SUMMARYA 30 cm diameterdivergent magneticfieldion thruster, developed for mercuryoperation at 2.7 kN, was modifiedand operatedwith xenonpropellant at a powerlevelof lO kH for 567 hr to evaluatethrusterperformance and lifetime. The majordifferences betweenthis thrusterand its baselineconfiguration were elimination of the threemercuryvaporizers, use of a maln discharge cathode witha largerorifice,reduction In discharge bafflediameter, and use of an Ion accelerating systemwith largeraccelerator grid holes. At a xenonion beamcurrentof 5 A the engineproduceda thrustof 0.33 N at a specific Impulseof 4220 sec and an efficiency of 6B percent. Therewas no measurable _r screengrid erosion. However,grid thickness measurement uncertainties, com-,,', binedwlthestimates of the effectsof reactiveresidualfacilitybackground gasesgavea minimumscreengrid lifetime of 7000 hr. Discharge cathodeorificeerosionrateswere measuredwith threedifferent cathodeswith different initialorificediameters.As the initialorificediameter was Increased from thatof the baselinevalue,the erosionrate decreased to near zero_ Threepotential problemswere identified duringthe wear test. Flrst,the upstream sideof the discharge baffleerodedat an unacceptable rate,nearly two ordersof magnitude greaterthanthat of the baselinemercurythruster. Second,two of the main cathodetubesexperienced oxidation, deformation, and failureafter227 and 243 hr of thruster operation.Thesefailuresare believed to be relatedto the cathodestartingtechnique.Third,the accelerator gridimpingement currentwas more than an orderof magnitude higherthan thatof the baseline mercurythruster, implyinga greatererosionrate. The impingement currentincreaseis primarily due to a greaterchargeexchangeIon currentresulting from the higherbeam current; operation at a higherneutral atom lossrate and; to a lesserextent,the higherfacilitypressure.. The chargeexchangeion erosionwas not quantified in this test. Therewere no measurable changesin the accelerator grid thickness or the accelerator grid ho]ediameters.
INTRODUCTIONInertgas ion thrustersystemsare beingconsidered for primarypropulsion for near-Earth, cislunar, and interplanetary missions(refs.I to 3). To reducethe specific mass and complexity of ion propulsion systems,it is deslrableto increase the thrustproducedby each thruster. Thls can be accomplished by increasing the ion beam current(propellant flow rate)and/orthe beam voltage (propellant velocity).Increasing the beam voltageincreases the thrust and specificimpulse, but, It also reducesthe ratioof outputthrustto input power. Therefore, it is oftendesirable to increasethe ion beam curr,_nt as muchas possiblewith minimalincreases in the beam voltage. However,unless changesin otheroperating conditions can be made to offsetthe increased beam This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States.
