Matthew T. Domonkos

Performance Evaluation of the NEXT Ion Engine

The performance test results of three NEXT ion engines are presented. These ion engines exhibited peak specific impulse and thrust efficiency ranges of 4060 4090 s and 0.68 0.69, respectively, at the full power point of the NEXT throttle table. The performance of the ion engines satisfied all project requirements. Beam flatness parameters were significantly improved over the NSTAR ion engine, which is expected to improve accelerator grid service life. The results of engine inlet pressure and temperature measurements are also presented. Maximum main plenum, cathode, and neutralizer pressures were 12,000 Pa, 3110 Pa, and 8540 Pa, respectively, at the full power point of the NEXT throttle table. Main plenum and cathode inlet pressures required about 6 hours to increase to steady-state, while the neutralizer required only about 0.5 hour. Steady-state engine operating temperature ranges throughout the power throttling range examined were 179 303 C for the discharge chamber magnet rings and 132 213 C for the ion optics mounting ring.

Next: NASA's Evolutionary Xenon Thruster development status

NASA’s Glenn Research Center (GRC) is leading the development of NASA’s Evolutionary Xenon Thruster (NEXT) system. The central feature of the NEXT system is an electric propulsion thruster that inherits the knowledge gained through the NSTAR thruster that successfully propelled the Deep Space 1 to asteroid Braille and comet Borrelly, while significantly increasing the thruster power level and making improvements in performance parameters associated with NSTAR. The thruster under development has a 40 cm beam diameter, twice the effective area of the Deep-Space 1 thruster. It incorporates mechanical features and operating conditions to maximize the design heritage established by the flight NSTAR 30 cm thruster, while incorporating new technology where warranted to extend the power and throughput capability. To date three engineering-model thrusters have been manufactured at NASA GRC, and performance, wear, vibration, and integration testing of these thrusters is underway.

Laser-induced fluorescence characterization of ions emitted from hollow cathodes

Laser-induced fluorescence (LIF) was used to measure the mean and variance of the velocity distribution of xenon ions emitted from two hollow cathode assemblies operating at low power. High-energy ions detected in plume-mode and spot-mode operation are consistent with the potential-hill model of high-energy ion production. The distributions of velocities were modeled, yielding temperatures on the order of a few electronvolts in plume mode and 1 eV in spot mode. The LIF of neutral xenon atoms in the plumes indicated thermal temperatures significantly less than the temperatures associated with the ion velocity distributions.

Ion propulsion development activities at the NASA Glenn Research Center

The NASA Glenn Research Center (GRC) ion propulsion program addresses the need for high specific impulse ion propulsion systems and technology across a broad range of mission applications and power levels. Development areas include high-throughput NSTAR derivative engine and power processing technology, lightweight high-efficiency sub-kilowatt ion propulsion, micro-ion propulsion concepts, engine and component technologies for highpower (30 kW class) ion engines, and fundamentals. NASA GRC is also involved in two highly focussed activities: development of 5/10-kW class next-generation ion propulsion system technology, and development of high-specific impulse (> 10,000 seconds) ion propulsion technology applicable to deep-space and interstellar-precursor missions.

Low-Current Hollow Cathode Evaluation

An experimental investigation of the operating characteristics of 3.2 mm diameter orificed hollow cathodes was conducted to examine low-current and low flow rate operation. Cathode power was minimized with low orifice aspect ratio and the use of an enclosed keeper. Cathode flow rate requirements were proportional to orifice diameter and the inverse of the orifice length. Cathode temperature profiles were obtained using an imaging radiometer, and conduction was found to be the dominant heat transfer mechanism from the cathode tube. Orifice plate temperatures were found to be weakly dependent upon the flow rate and strongly dependent upon the current. Internal cathode pressures were measured to range from 39.2 to more than 66.5 kPa for spot mode emission. A model developed to describe the mass flow achieved good agreement with the experimental cathode pressures for heavy particle temperatures between 2900 and 6100 oC. Plasma properties measured within the insert region exclude the electron partial pressure from being responsible for the elevated cathode pressures. As such, it was concluded that the average heavy particle temperature in the orifice would have to be on the order of several thousand degrees C to account for the high internal pressure. Plasma parameters were also measured in the cathode-tokeeper gap and downstream of the keeper. The structure of the data enabled analysis of the current conduction processes in spot and plume modes. * Graduate Student Research Assistant, Student Member AIAA † Associate Professor, Associate Fellow AIAA ‡ Graduate Student Research Assistant, Student Member AIAA § Research Scientist, Member AIAA Copyright

Very-Near-Field Plume Investigation of the Anode Layer Thruster

The plasma properties of the very-near-e eld (10‐50 mm) plume of the D55 anode layer thruster (TAL) were measured. The D55 is the 1.35-kW TAL counterpart to the SPT-100 and was made by the Central Scientie c Research Institute of Machine Building of Kaliningrad, Russia. The thruster was tested in the 6 m diameter £ 9 m longvacuumchamberattheUniversityofMichigan’ sPlasmadynamicsandElectricPropulsionLaboratory,andthe diagnosticprobes werepositioned using a three-axis translation tablesystem. Water-cooled Hall probes, a Faraday probe, emissive probes, and langmuir probes were used to examine the near-e eld plasma properties. Water-cooled Hall probes were employed to exploretheeffectof the closed-drift current on theradial magnetic e eld. The change in the magnetic e eld during thruster operation was found to be less than 5% over the region examined, which indicated that the Hall current was limited to several tens of amperes. Evidence also indicated that the closed-drift current extended between 5 and 10 mm downstream of the anode. Ion current density proe les showed that the annular beam focuses within 40 mm of the thruster exit plane. Plasma potential measurements indicated that ion acceleration occurred primarily within 10 mm of the anode. The highest electron temperature measured in this investigation occurred immediately downstream of the anode, and the temperature decreased with axial distance from the thruster. The low-energy electrons were cone ned to the high-density core of the plasma beam.

Low Power Pulsed MPD Thruster System Analysis and Applications

Roger M. MyersSverdrup Technology, Inc.Lewis Research Center GroupBrook Park, OhioMatthew DomonkosUniversity of New MexicoAlbuquerque, New MexicoandJames H. GillandSverdrup Technology, Inc.Lewis Research Center GroupBrook Park, OhioPrepared for the1993 Joint Propulsion Conferencecosponsored by the AIAA, SAE, ASME, and ASEEMonterey, California, June 28-July 1, 1993

Very near-field plume investigation of the D55

The plasma properties of the very near-field (10 to 50 mm) plume of the D55 anode layer thruster (TAL) were measured as part of an effort lead by NumerEx of Albuquerque, NM to model the processes within TALs. The D55 is the 1.35 kW TAL counterpart to the SPT-100 and was made by TsNUMASH of Kaliningrad, Russia. The thruster was tested in the 6 m diameter by 9 m long vacuum chamber at (lie Plasmadynamics and Electric Propulsion Laboratory (PEPL), and the diagnostic probes were positioned using a three axis translation table system. A Faraday probe, water-cooled Hall probes, emissive probes, and Langmuir probes were used to examine the near-field plasma properties. Water-cooled Hall probes were employed to explore the effect of the closed drift current on the radial magnetic field. The change in the magnetic field due to the Hall current was found to be less than five percent over the region examined. Ion current density profiles showed that the annular beam focuses within 40 mm of the thruster exit plane. Similarly, the electron temperature and number density radial profiles showed peaks near the discharge chamber at 10 mm axially, and the peaks moved toward the axis within 40 mm. The peak electron temperature decreased with axial distance, while the number density remained approximately constant over the very near-field region. Nomenclature

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.

A Particle and Energy Balance Model of the Orificed Hollow Cathode

A particle and energy balance model of orificed hollow cathodes was developed to assist in cathode design. The model presented here is an ensemble of original work by the author and previous work by others. The processes in the orifice region are considered to be one of the primary drivers in determining cathode performance, since the current density was greatest in this volume (up to 1.6 x 10(exp 8) A/m2). The orifice model contains comparatively few free parameters, and its results are used to bound the free parameters for the insert model. Next, the insert region model is presented. The sensitivity of the results to the free parameters is assessed, and variation of the free parameters in the orifice dominates the calculated power consumption and plasma properties. The model predictions are compared to data from a low-current orificed hollow cathode. The predicted power consumption exceeds the experimental results. Estimates of the plasma properties in the insert region overlap Langmuir probe data, and the predicted orifice plasma suggests the presence of one or more double layers. Finally, the model is used to examine the operation of higher current cathodes.