Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA.
Rev Sci Instrum. 2023 Mar 1;94(3):033506. doi: 10.1063/5.0135272.
Hollow cathodes used in electric propulsion typically have an external heater to raise the thermionic electron emitter to emission temperatures. Heaterless hollow cathodes that are heated by a Paschen discharge have been historically limited to low discharge currents (<5 A) due to arcing and inefficient heating. A new heaterless technology was previously developed for cathodes up to 50 A, utilizing a refractory metal tube to extend the gas feed line partway into the thermionic insert region. A high voltage (>700 V) Paschen discharge is ignited between the keeper and the tube, which quickly transitions to a lower voltage (<80 V) thermionic discharge from the inner tube surface and heats the thermionic insert by radiation. This "tube-radiator" configuration eliminates arcing and inhibits the long-path-length discharge between the keeper and gas feed tube upstream of the cathode insert that caused inefficient heating in prior designs. This paper describes extending this technology developed for a 50 A cathode to one that is capable of 300 A. The larger cathode uses a 5-mm diameter tantalum tube-radiator and a 6-A, 5-min ignition sequence. Ignition was challenging because the high heating power required (≥300 W) is difficult to maintain with the low voltage (<20 V) keeper discharge that exists prior to igniting the thruster discharge. To achieve self-heating from the lower voltage keeper discharge, the keeper current is raised to 10 A once the LaB insert starts emitting. This work shows that the novel tube-radiator heater is scalable to large cathodes capable of tens of thousands of ignitions.
电推进中使用的空心阴极通常具有外部加热器，以将热电子发射器加热到发射温度。由于电弧和加热效率低，历史上一直限制无加热器空心阴极的放电电流低于 5A（<5A）。以前已经开发出一种新的无加热器技术，可用于高达 50A 的阴极，利用难熔金属管将气体进料线部分延伸到热电子插入区域。在保持器和管之间点燃高压（>700V）的帕邢放电，该放电迅速过渡到低电压（<80V）的热电子放电，来自内管表面，并通过辐射加热热电子插入物。这种“管辐射器”配置消除了电弧，并抑制了保持器和阴极插入物上游的气体进料管之间的长路径长度放电，这在先前的设计中导致加热效率低下。本文描述了将为 50A 阴极开发的技术扩展到能够达到 300A 的阴极。较大的阴极使用 5mm 直径的钽管辐射器和 6A、5min 的点火序列。点火具有挑战性，因为需要高加热功率（≥300W），而在点燃推进器放电之前存在的低电压（<20V）保持器放电很难维持。为了从低电压保持器放电实现自加热，一旦 LaB 插入物开始发射，将保持器电流提高到 10A。这项工作表明，新型管辐射器加热器可扩展到能够承受数万次点火的大型阴极。
