Kristina K. Jameson

LaB6 Hollow Cathodes for Ion and Hall Thrusters

Deep space missions and satellite station-keeping applications continue to demand higher power ion thrusters and Hall thrusters capable of providing high thrust and longer life. Depending on the thruster size, the hollow cathodes may be required to produce discharge currents in the 10-100 A range with lifetimes in excess of 10 years. A lanthanum hexaboride (LaB 6 ) hollow cathode has been developed for space applications to increase the current capability from the cathode and ease the handling and gas purity requirements. This cathode uses a LaB 6 insert in an all-graphite hollow cathode structure with an integral graphite keeper. Three different sizes of the LaB 6 cathode have been successfully operated at discharge currents of up to 100 A to date. Although the LaB 6 cathode insert operates at a higher temperature than the conventional BaO dispenser cathode, LaB 6 offers the capability of long life and orders of magnitude less sensitivity to propellant impurities and air exposure than conventional dispenser cathodes.

Hollow cathode theory and experiment. I. Plasma characterization using fast miniature scanning probes

A detailed study of the spatial variation of plasma density, temperature, and potential in hollow cathodes using miniature fast scanning probes has been undertaken in order to better understand the cathode operation and to provide benchmark data for the modeling of the cathode performance and life described in a companion paper. Profiles are obtained throughout the discharge and in the very high-density orifice region by pneumatically driven Langmuir probes, which are inserted directly into the hollow cathode orifice from either the upstream insert region inside the hollow cathode or from the downstream anode-plasma region. A fast transverse-scanning probe is also used to provide radial profiles of the cathode plume as a function of position from the cathode exit. The probes are extremely small to avoid perturbing the plasma; the ceramic tube insulator is 0.05cm in diameter with a probe tip area of 0.002cm2. A series of current-voltage characteristics are obtained by applying a rapid sawtooth voltage wave...

Efficacy of Electron Mobility Models in Hybrid-PIC Hall Thruster Simulations

The cross-field electron mobility in Hall thrusters is known to be enhanced by wall collisionality and turbulent plasma fluctuations. Although progress has been made in understanding the plasma-wall interaction and instabilities responsible for the anomalous transport, a predictive model based on the underlying physics of these processes has yet to emerge. Hybrid-PIC simulations of the Hall thruster have typically depended on semi-empirical models of the mobility to provide sufficient electron current to match experimental results. These models are capable of qualitatively predicting the plasma response over a wide range of operating conditions, but have limited quantitative capabilities unless they are calibrated with experimental data. The efficacy of several electron mobility models in reproducing the plasma response of a 6 kW laboratory Hall thruster are assessed. With respect to a two-region mobility model that is frequently reported in the literature, a three-region model for the mobility is shown to significantly improve the agreement with experimentally measured profiles of the plasma potential and electron temperature.

Wear Mechanisms in Electron Sources for Ion Propulsion, 2: Discharge Hollow Cathode

The wear of the keeper electrode in discharge hollow cathodes is a major impediment to the implementation of ion propulsion onboard long-duration space science missions. The development of a predictive theoretical model for hollow cathode keeper life has long been sought, but its realization has been hindered by the complexities associated with the physics of the partially ionized gas and the associated erosion mechanisms in these devices. Thus, although several wear mechanisms have been hypothesized, a quantitative explanation of life test erosion profiles has remained incomplete. A two-dimensional model of the partially ionized gas in a discharge cathode has been developed and applied to understand the mechanisms that drove the erosion of the keeper in two long-duration life tests of a 30-cm ion thruster. An extensive set of comparisons between predictions by the numerical simulations and measurements of the plasma properties and of the erosion patterns is presented. It is found that the near-plume plasma oscillations, predicted by theory and observed by experiment, effectively enhance the resistivity of the plasma as well as the energy of ions striking the keeper.

Hollow cathode and keeper-region plasma measurements

The successful performance of the NSTAR ion thruster in Deep Space 1 mission, coupled with the recently completed 30,032 hour life test of the flight spare thruster, has accelerated with the implementation of electric propulsion in NASA missions.

Potential fluctuations and energetic ion production in hollow cathode discharges

Ions with energies significantly in excess of the applied discharge voltage have been reported for many years in hollow cathode discharges. Models of dc potential hills downstream of the cathode and instabilities in postulated double layers in the cathode orifice have been proposed to explain this, but have not been substantiated. Measurements of the dc and rf plasma density and potential profiles near the exit of hollow cathodes by miniature fast-scanning probes suggests that turbulent ion acoustic fluctuations and ionization instabilities in the cathode plume significantly increase the energy of the ions that flow from this region. Increases in the discharge current and/or decreases in the cathode gas flow enhance the amplitude of the fluctuations and increase the number and energy of the energetic ions, which increases the erosion rate of the cathode electrodes. The transition from the quiescent “spot mode” to the noisy “plume mode” characteristic of these discharges is found to be a gradual transition...

Hollow Cathode and Keeper-region Plasma Measurements Using Ultra-fast Miniature Scanning Probes

In order to support the development of comprehensive performance and life models for future deep space missions that will utilize ion thrusters, we have undertaken a study of the plasma structure in hollow cathodes using an new pneumatic scanning probe diagnostic. This device is designed to insert a miniature probe directly into the hollow cathode orifice from either the upstream insert region in the interior of the hollow cathode, or from the downstream keeper-plasma region at the exit of the hollow cathode, to provide complete axial profiles of the discharge plasma parameters. Previous attempts to diagnose this region with probes was Limited by the melting of small probes in the intense discharge near the orifice, or caused significant perturbation of the plasma by probes large enough to survive. Our new probe is extremely compact, and when configured as a single Langmuir probe, the ceramic tube insulator is only 0.5mm in diameter and the current collecting conductor has a total area of 0.002 cm2. A series of current-voltage characteristics are obtained by applying a rapid sawtooth voltage waveform to the probe as it is scanned by the pneumatic actuator into and out of the plasma region, The bellow-sealed pneumatic drive scans the probe 4 cm in the cathode insert region and 10 cm in the anode/keeper plasmas region at average speeds of about 1 mm/msec, and the residence time at the end of the insertion stroke in the densest part of the plasma near the orifice is measured to be only 10 msec. Since the voltage sweep time is fast compared to the motion of the probe, axial profiles of the plasma density, temperature and potential with reasonable spatial resolution are obtained. Measurements of the internal cathode pressures and the axial plasma-parameter profiles for a hollow cathode operating at discharge currents of up to 35 A in xenon will be presented.

Evidence of nonclassical plasma transport in hollow cathodes for electric propulsion

Measurements, simplified analyses, and two-dimensional numerical simulations with a fluid plasma model show that classical resistivity cannot account for the elevated electron temperatures and steep plasma potential gradients measured in a 25–27.5A electric propulsion hollow cathode. The cathode consisted of a 1.5cm hollow tube with an ∼0.28cm diameter orifice and was operated with 5.5SCCM (SCCM denotes cubic centimeter per minute at STP) of xenon flow using two different anode geometries: a segmented cone and a circular flat plate. The numerical simulations show that classical resistivity yields as much as four times colder electron temperatures compared to the measured values in the orifice and near-plume regions of the cathode. Classical transport and Ohm’s law also predict exceedingly high electron-ion relative drift speeds compared to the electron thermal speed (>4). It is found that the addition of anomalous resistivity based on existing growth rate formulas for electron-ion streaming instabilities ...

Hall Thruster Cathode Flow Impact on Coupling Voltage and Cathode Life

The cathode coupling voltage in Hall thrusters, which is the voltage difference between the cathode and the thruster beam plasma potential, is considered an indicator of the ease with which electrons flow from cathode to anode.Historically, the coupling voltage has beenminimizedby increasing the amount of propellant injected through the hollow cathode due to early observations that thismaximizes the discharge (or anode) efficiency.However, recent experiments described here show that the total thruster efficiency is independent of the cathode flow over the range from 5 to 10%of the propellant injected into the thruster body through the anode. For this reason, cathode flow rates can be reduced closer to the classic plume mode limit characteristic of the hollow cathode design without impacting the total thruster efficiency. Such reductions in cathode flow rate can significantly extend the cathode life, especially for higher-power Hall thrusters with larger discharge currents, where the normal Hall thruster cathode flow split will significantly exceed the optimum level for cathode operation and life.

Plasma processes inside dispenser hollow cathodes

A two-dimensional fluid model of the plasma and neutral gas inside dispenser orificed hollow cathodes has been developed to quantify plasma processes that ultimately determine the life of the porous emitters inserted in these devices. The model self-consistently accounts for electron emission from the insert as well as for electron and ion flux losses from the plasma. Two cathodes, which are distinctively different in size and operating conditions, have been simulated numerically. It is found that the larger cathode, with outer tube diameter of 1.5cm and orifice diameter of 0.3cm, establishes an effective emission zone that spans approximately the full length of the emitter when operated at a discharge current of 25A and a flow rate of 5.5sccm. The net heating of the emitter is caused by ions that are produced by ionization of the neutral gas inside the tube and are then accelerated by the sheath along the emitter. The smaller cathode, with an outer diameter of 0.635cm and an orifice diameter of 0.1cm, do...