Multi-kilowatt-class heaters for large hollow cathodes
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.