Discharge Cathode Assembly Research Articles

Dormant Cathode Erosion in a Multiple-Cathode Gridded Ion Thruster

DOI: 10.2514/1.37031 A rectangular gridded ion thruster discharge chamber is investigated for operation with multiple discharge cathode assemblies. The multiple-cathode approach attempts to increase thruster throughput and lifetime by operating three discharge cathode assemblies sequentially, possibly providing a threefold increase in discharge chamber life. Previous multiple-cathode electric propulsion devices, such as the SPT-100, have shown dormantcathode erosion to be a life-limiting phenomenon. Similar results in a multiple-cathode discharge chamber may decrease the anticipated gain in discharge lifetime. To assess possible dormant-cathode sputtering erosion and to determine the operational configuration that minimizes this erosion, diagnostic cylinders are designed and used to measure plasma properties at the dormant-cathode locations. Each diagnostic cylinder appears similar to the active discharge cathode assembly, but is outfitted with Langmuir probes. Plasma properties are then used in a simple sputtering-erosion model to predict erosion of the dormant cathodes. Results indicate that the device should be operated at the 0 A electromagnet current configuration for minimum dormant-cathode erosion. For this optimum configuration,typicalnumberdensity,electrontemperature,andplasmapotentialvaluesare5:0 10 11 cm 3 ,5eV, and 27 V with respect to cathode common, respectively. The erosion model indicates that the dormant cathodes will suffer preoperation erosion, but the erosion rate is 26 times slower than the active discharge cathode assembly. Compared with a single-discharge-cathode-assembly thruster, the model predicts an increase in lifetime by a factor of 2.9 for a triple-discharge-cathode-assembly device.

Measurement of 30-Centimeter Ion Thruster Discharge Cathode Erosion

Relative erosion rates of the discharge cathode assembly of a 30-cm ion engine are measured using laser-induced fluorescence. Molybdenum and tungsten erosion products are interrogated downstream of the discharge cathode assembly during beam extraction. Erosion of the discharge cathode assembly is characterized for both keepered and unkeepered configurations. The erosion increases with both discharge current and voltage, and spatially resolved measurements agree with observed erosion patterns. Erosion rates are calculated using data from previous wear tests. Magnitudes and trends in the rates are correlated with both previous and subsequent wear tests. Laser-induced fluorescence is demonstrated to be a technique to measure relative internal erosion rates, and a path is identified for measuring absolute rates.

Erosion Processes of the Discharge Cathode Assembly of Ring-Cusp Gridded Ion Thrusters

An ion thruster discharge cathode assembly (DCA) erosion theory is presented based on near-DCA NSTAR plasma measurements and experimental results for propellant flow rate effects on ion number density. The plasma potential structures are utilized in an ion trajectory algorithm to determine the location and angle at the DCA of bombarding ions. These results suggest that the plasma potential structure causes a chamfering of the DCA orifice. Results from tests with an instrumented DCA show that increasing DC propellant flow rate causes a decrease in keeper orifice ion number density, most likely due to charge-exchange and elastic collisions. Combining these two results, the known wear-test and extended life test (ELT) DCA erosion profiles can be qualitatively explained. Specifically, the change in the wear profile from the DCA downstream face to the orifice for the ELT may be a result of the reduction in DCA propellant flow rate when the thruster operating point is changed from the TH 15 to TH 8.

Performance and Flatness of a Multiple-Cathode, Rectangular Ion Thruster Discharge Chamber

Results from the extended life test of the Deep Space One flight spare ion engine show that enlargement of the discharge cathode assembly orifice due to erosion of the orifice by ion bombardment limits the throughput of the Ion thruster, and therefore limits its operational lifetime to approximately three years. Future deep-space missions will require significantly longer operational lifetime, perhaps as long as 7-14 years. In an effort to increase lifetime, an ion thruster discharge chamber designed for operation with multiple discharge cathode assemblies was investigated. The multiple-cathode discharge chamber approach attempts to incase lifetime by operating three discharge cathode assemblies sequentially. Simulated ion thruster operation of the multiple-cathode discharge chamber with the active discharge cathode assembly located on centerline and off centerline for a variety of magnetic field configurations was accomplished. Results indicate that the configuration with permanent magnets and 0 A electromagnet current provided the best performance and flatness with optimum values of 194 ± 6 W/A at 0.89 ± 0.03 propellant efficiency and 0.55 ± 0.02, respectively. Finally, operation of the dormant cathodes with propellant flow is suggested to reduce preoperation erosion of those units.

Wear Testing and Analysis of Ion Engine Discharge Cathode Keeper

Experimental and analytical investigations were conducted to predict the wear of the discharge cathode keeper orifice in the NASA Evolutionary Xenon Thruster. The ion current to the keeper was found to be highly dependent on the beam current, and the average keeper ion current density was nearly identical to that of the NASA Solar Electric Propulsion Technology and Applications Readiness (NSTAR) thruster for comparable beam current density. The ion current distribution was highly peaked toward the keeper orifice. The wear assessment predicted keeper orifice erosion to the same diameter as the cathode tube after processing 250 or 460 kg of xenon, depending on whether warm or cold ions, respectively, were assumed. Although the presented simple wear analysis does not predict failure, comparison with the NSTAR extended life test results suggests that the discharge cathode assembly will continue to operate beyond the qualification goal of processing 405 kg of propellant. Probabilistic wear analysis showed that the ion energy at the keeper surface and the sputter yield contributed most to the uncertainty in the wear assessment. It is recommended that fundamental experimental and modeling efforts focus on accurately describing the plasma potential, ion temperature, and the sputtering yield.

Controlling Electron Backstreaming Phenomena Through the Use of a Transverse Magnetic Field

DEEP-SPACE mission propulsion requirements can be satisfied by the use of high specific impulse systems such as ion thrusters. For such missions. however. the ion thruster will be required to provide thrust for long periods of time. To meet the long operation time and high-propellant throughput requirements, thruster lifetime must be increased. In general, potential ion thruster failure mechanisms associated with long-duration thrusting can be grouped into four areas: (1) ion optics failure; (2) discharge cathode failure; (3) neutralizer failure; and (4) electron backstreaming caused by accelerator grid aperture enlargement brought on by accelerator grid erosion. The work presented here focuses on electron backstreaming. which occurs when the potential at the center of an accelerator grid aperture is insufficient to prevent the backflow of electrons into the ion thruster. The likelihood of this occurring depends on ion source operation time. plasma density, and grid voltages, as accelerator grid apertures enlarge as a result of erosion. Electrons that enter the gap between the high-voltage screen and accelerator grids are accelerated to the energies approximately equal to the beam voltage. This energetic electron beam (typically higher than 1 kV) can damage not only the ion source discharge cathode assembly. but also any of the discharge surfaces upstream of the ion acceleration optics that the electrons happen to impact. Indeed. past backstreaming studies have shown that near the backstreaming limit, which corresponds to the absolute value of the accelerator grid voltage below which electrons can backflow into the thruster, there is a rather sharp rise in temperature at structures such as the cathode keeper electrode. In this respect operation at accelerator grid voltages near the backstreaming limit is avoided. Generally speaking, electron backstreaming is prevented by operating the accelerator grid at a sufficiently negative voltage to ensure a sufficiently negative aperture center potential. This approach can provide the necessary margin assuming an expected aperture enlargement. Operation at very negative accelerator grid voltages, however, enhances ion charge-exchange and direct impingement erosion of the accelerator grid. The focus of the work presented here is the mitigation of electron backstreaming by the use of a magnetic field. The presence of a magnetic field oriented perpendicular to the thruster axis can significantly decrease the magnitude of the backflowing electron current by significantly reducing the electron diffusion coefficient. Negative ion sources utilize this principle to reduce the fraction of electrons in the negative ion beam. The focus of these efforts has been on the attenuation of electron current diffusing from the discharge plasma into the negative ion extraction optics by placing the transverse magnetic field upstream of the extraction electrodes. In contrast. in the case of positive ion sources such as ion thrusters, the approach taken in the work presented here is to apply the transverse field downstream of the ion extraction system so as to prevent electrons from flowing back into the source. It was found in the work presented here that the magnetic field also reduces the absolute value of the electron backstreaming limit voltage. In this respect. the applied transverse magnetic field provides two mechanisms for electron backstreaming mitigation: (1) electron current attenuation and (2) backstreaming limit voltage shift. Such a shift to less negative voltages can lead to reduced accelerator grid erosion rates.