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Multiple-Kilowatt-Class Graphite Heater for Large Hollow Cathode Ignition

Abstract

A graphite heater design capable of operating in the multiple-kilowatt power range with an estimated life of over 40 years of continuous operation has been developed to address the power and operational life requirements of large lanthanum hexaboride hollow cathodes. Larger hollow cathodes are capable of increased discharge currents, but require higher heater powers to ignite. Legacy heater designs suffer from material interactions and increased failure rates at high temperatures and are currently not viable for high-power operation; a graphite heater can provide high power and long operational life simultaneously. Two models, a simplified circuit model and a finite-element model, have been developed to predict the operating temperature and resistance of the graphite heater design. The sublimation-limited operational life of the heater was estimated using a Hertz-Knudsen evaporation model and an alternate vacuum sublimation model proposed by Thieberger. A prototype heater has been fabricated and tested using a large-diameter lanthanum hexaboride hollow cathode to demonstrate the feasibility of the graphite heater design. The prototype has repeatedly achieved cathode ignition and has been tested at up to 4.5 kW of heater power. The temperatures of the cathode and heater element were measured during heating prior to ignition and used to calculate the heater resistance, which was found to agree fairly well with the predicted values. The exposed portion of the heater element and the cathode tube tip reached temperatures of just over 1300 and 1200◦C, respectively. For a peak surface operating temperature of 1500◦C, both heater sublimation life models predict a 1% loss life of approximately 400 kh