Heaterless 300 A lanthanum hexaboride hollow cathode.

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 LaB6 insert starts emitting. This work shows that the novel tube-radiator heater is scalable to large cathodes capable of tens of thousands of ignitions.

Development of a 50-A heaterless hollow cathode for electric thrusters.

Hollow cathodes in electric thrusters normally use an external heater to raise the thermionic electron emitter to emission temperatures. These heaters are a potential single-point failure in the thruster and add a separate power supply to the power processing unit. Heaterless hollow cathodes are attractive for their compact size and potential higher reliability but have only been reliably demonstrated to date in small hollow cathodes capable of discharge currents below around 5 A. A new heaterless LaB6 hollow cathode has been developed that is capable of discharge currents from 5 to 50 A. The cathode configuration extends the gas feed tube at cathode potential part way into the emitting insert region of the cathode. A high-voltage Paschen discharge is struck from the tube to the keeper that heats the tube tip, which then efficiently heats the insert by radiation. This configuration eliminates the arcing observed in prior large heaterless designs that coupled the high-voltage Paschen discharge to the orifice plate or the insert itself. Discharge current-voltage characteristics show that the presence of the tube does not significantly perturb the insert-region plasma. Startup uses a simple 3 min ignition procedure, and voltage traces of the keeper discharge reveal that much of the present tube-radiator's 100-to-150 W heating power comes from an intermediate thermionic discharge sustained by the tube during the transition between the Paschen discharge and LaB6 insert thermionic regime. This novel heating mechanism enables an unprecedented class of higher-current heaterless hollow cathodes for the next generation of high-power electric propulsion systems.

Compact scanning retarding potential analyzer.

A miniature scanning electrostatic energy analyzer has been developed for measurements of the ion energy distribution in low-pressure plasma discharges. The retarding potential analyzer utilizes four grids to reject the incident electron flux and discriminate the ion energy. It features a compact size of 5-mm diameter and 2.5-mm thickness and uses a simple configuration to mount the grids and provide voltage isolation using high temperature ceramic cement that avoids complicated grid alignment or MEMS fabrication techniques. Mounting on a fast-scanning two-axis probe drive eliminates overheating of the ceramics and subsequent current leakage, which permits insertion into challenging locations and provides profiles of the ion energy in the plasma discharge. Guidelines for the analyzer design for high density operation are discussed, and the performance of the analyzer is shown for measurements in a hollow cathode plume in xenon plasmas.

Propagation of ion acoustic wave energy in the plume of a high-current LaB_{6} hollow cathode.

A frequency-averaged quasilinear model is derived and experimentally validated for the evolution of ion acoustic turbulence (IAT) along the centerline of a 100-A class, LaB_{6} hollow cathode. Probe-based diagnostics and a laser induced fluorescence system are employed to measure the properties of both the turbulence and the background plasma parameters as they vary spatially in the cathode plume. It is shown that for the three discharge currents investigated, 100 A, 130 A, and 160 A, the spatial growth of the total energy density of the IAT in the near field of the cathode plume is exponential and agrees quantitatively with the predicted growth rates from the quasilinear formulation. However, in the downstream region of the cathode plume, the growth of IAT energy saturates at a level that is commensurate with the Sagdeev limit. The experimental validation of the quasilinear model for IAT growth and its limitations are discussed in the context of numerical efforts to describe self-consistently the plasma processes in the hollow cathode plume.

Ion acoustic turbulence in a 100-A LaB6 hollow cathode.

The temporal fluctuations in the near plume of a 100-A LaB(6) hollow cathode are experimentally investigated. A probe array is employed to measure the amplitude and dispersion of axial modes in the plume, and these properties are examined parametrically as a function of cathode operating conditions. The onset of ion acoustic turbulence is observed at high current and is characterized by a power spectrum that exhibits a cutoff at low frequency and an inverse dependence on frequency at high values. The amplitude of the turbulence is found to decrease with flow rate but to depend nonmonotonically on discharge current. Estimates of the anomalous collision frequency based on experimental measurements indicate that the ion acoustic turbulence collision frequency can exceed the classical rate at high discharge current densities by nearly two orders of magnitude.

A high power ion thruster for deep space missions.

The Nuclear Electric Xenon Ion System ion thruster was developed for potential outer planet robotic missions using nuclear electric propulsion (NEP). This engine was designed to operate at power levels ranging from 13 to 28 kW at specific impulses of 6000-8500 s and for burn times of up to 10 years. State-of-the-art performance and life assessment tools were used to design the thruster, which featured 57-cm-diameter carbon-carbon composite grids operating at voltages of 3.5-6.5 kV. Preliminary validation of the thruster performance was accomplished with a laboratory model thruster, while in parallel, a flight-like development model (DM) thruster was completed and two DM thrusters fabricated. The first thruster completed full performance testing and a 2000-h wear test. The second successfully completed vibration tests at the full protoflight levels defined for this NEP program and then passed performance validation testing. The thruster design, performance, and the experimental validation of the design tools are discussed in this paper.

Compact lanthanum hexaboride hollow cathode.

A compact lanthanum hexaboride hollow cathode has been developed for space applications where size and mass are important and research and industrial applications where access for implementation might be limited. The cathode design features a refractory metal cathode tube that is easily manufactured, mechanically captured orifice and end plates to eliminate expensive e-beam welding, graphite sleeves to provide a diffusion boundary to protect the LaB6 insert from chemical reactions with the refractory metal tube, and several heater designs to provide long life. The compact LaB(6) hollow cathode assembly including emitter, support tube, heater, and keeper electrode is less than 2 cm in diameter and has been fabricated in lengths of 6-15 cm for different applications. The cathode has been operated continuously at discharge currents of 5-60 A in xenon. Slightly larger diameter versions of this design have operated at up to 100 A of discharge current.