The contact interaction of metal-like carbides with graphite at high temperatures in a vacuum

Heater materials and geometry are analyzed to address the requirements of large lanthanum hexaboride hollow cathodes. Larger hollow cathodes are capable of increased discharge currents but require higher heater powers to ignite. Existing cathode heaters suffer from material interactions and increased failure rates at high temperatures and are currently not viable for high-power operation; graphite heaters can provide order-of-magnitude increases in heater power and operational life simultaneously. Two models, a simplified circuit model and a finite-element model, were developed to predict the heater operating temperature and resistance. The operational life of the heater was estimated using a vacuum sublimation model. A graphite heater was fabricated and tested using a large-diameter lanthanum hexaboride hollow cathode to demonstrate feasibility as a high-power heater material. The prototype has repeatedly achieved cathode ignition and was tested at up to 4.5 kW of heater power. Heater voltage and current were measured prior to ignition and used to calculate the heater resistance. The measured resistance is within 8% of the predicted value for the circuit model and within 2.5% of the finite-element prediction. At cathode ignition temperatures, the vacuum sublimation model predicts an order-of-magnitude increase in operational life as compared to existing cathode heaters; for a peak surface operating temperature of 1500 °C, the vacuum sublimation model predicts a 1%-loss life of approximately 400 kh.

show abstract

“…I, but refractory carbides do not exhibit the same problems and do not react with graphite at high temperatures in vacuum. 35…”

Section: Limitationsmentioning

confidence: 99%

Multi-kilowatt-class heaters for large hollow cathodes

Wordingham

Taunay

Choueiri

2018

Review of Scientific Instruments

View full text Add to dashboard Cite

show abstract

“…Further iterations of the heater will use tungsten or titanium carbide leads in lieu of the high-temperature stainless steel; tungsten carbide has a melting point of 2,777 • C and titanium carbide melts at over 2,940 • C. 12 Though pure refractory metals are prone to failure as described in Section I, the refractory carbides do not exhibit the same problems and do not react with graphite at high temperature in vacuum. 30…”

Section: Planned Improvementsmentioning

confidence: 99%

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

Wordingham

Taunay

Choueiri³

2015

51st AIAA/SAE/ASEE Joint Propulsion Conference

View full text Add to dashboard Cite

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.

show abstract

The contact interaction of metal-like carbides with graphite at high temperatures in a vacuum

Cited by 2 publications

References 0 publications

Multi-kilowatt-class heaters for large hollow cathodes

Multi-kilowatt-class heaters for large hollow cathodes

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

Contact Info

Product

Resources

About