Colleen M. Marrese

AN OVERVIEW OF MEMS-BASED MICROPROPULSION DEVELOPMENTS AT JPL

Development of MEMS (Microelectromechanical Systems) micropropulsion at the Jet Propulsion Laboratory (JPL) is reviewed. This includes a vaporizing liquid micro-thruster for microspacecraft attitude control, a micro-ion emgine for microspacecraft primary propulsion or large spacecraft fine attitude control, as well as several valve studies, including a solenoid valve studied in collaboration with Moog Space Products Division, and a piezoelectric micro-valve.

Modeling of Stationary Plasma Thruster-100 Thruster Plumes and Implications for Satellite Design

A computational model of a stationary plasma thruster (SPT) has been developed using a quasineutral particle-in-cell/direct simulation Monte Carlo (PIC-DSMC) model. This model is based on theoretical work showing that the plume consists of a quasineutral plasma with collisionless electrons in which the magnetic field can be neglected. Details of the PIC DSMC method are presented as well as axisymmetric and three-dimensional results. Comparisons are made to new and previously reported experimental data. The model is shown to produce results similar to laboratory measurements of the ion current density and plume-induced sputter erosion rates. The model does not compare as well with retarding potential analyzer measurements of the ion energy distribution. The results confirm previous observations that measurements made in some ground facilities may substantially overpredict the amount of backflow current that will be experienced under operational conditions. A surface-sputtering model is used to predict the impact the plume has on solar array interconnects and to show the impact an SPT thruster could have on a communications satellite. The results show that the thruster should be canted with respect to the solar array, lowering its effective thrust and specific impulse.

Miniature Ion Thrusters for Precision Formation Flying

Miniaturized ion thrusters are attractive for precision formation flying and many other future space missions. Their potential for high-efficiency and low-noise operation provides the mission designer with a uniquely attractive option in the mN thrust range. Caltech and JPL have designed and tested a small, 3 cm diameter ion thruster that demonstrates propellant efficiency > 80% and Isp > 3100s using a cathode generated xenon discharge. These investigations have shown that desirable discharge confinement efficiency is possible with reasonably sized magnets, and consequently low overall thruster weight. Robust, long-life ion extraction grid geometries were designed using computational tools and have performed well during experimental testing. Several flight-worthy cathode designs are currently under investigation for the neutralizer and discharge cathodes. In particular, recent design efforts have focused on miniature hollow and direct emission cathodes. Direct emission cathodes using hexaboride emitters are being considered for their applicability to precision formation flying missions, fast cycling times, scalability to even smaller thruster sizes (<3cm), and ability to run with low-purity propellants. Early testing of a direct emission LaB6 cathode has shown that such a cathode can provide good overall performance if consistent cathode heater performance can be maintained. Future testing will focus on the design and validation of efficient, flight- worthy cathodes and demonstration of thruster lifetime.

JPL micro-thrust propulsion activities

Formation flying and microspacecraft constellation missions pose new propulsion requirements. Formationflying spacecraft, due to the tight positioning and pointing control requirements, may need thrust control within 1- 20 uN to an accuracy of 0.1 uN for LISA and ST-7, for example. Future missions may have extended thrust ranges into the sub - mN range. However, all do require high specific impulses (>500 sec) due to long required thruster firings.

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.

Experimental and computational investigation of the performance of a micro-ion thruster

A micro-ion thruster assembly with a characteristic diameter of 3-cm has been developed at JPL for testing and optimization of various system parameters.

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

The design of a cathode to operate in an oxygen-rich environment

The primary problem with Hall plasma accelerator operation on oxygen is poor cathode performance and short lifetime. The primary problem with micro Hall thrusters is the absence of a stable low power cathode. Cathodes traditionally used for both applications employ thermionic emitters which are not efficient and which are easily oxidized in an oxygen-rich environment. The field emitter cathode presented in this report has the potential of filling both vacancies since it does not require a high-power heater and can be scaled down with the size of the thruster. The advantages to using Hf and HfC as emitting materials are low work functions and high resistance to oxygen poisoning. Preliminary investigations proved that HfC emitters can operate in 7.6 mTorr oxygen pressure environments. The initial cathode design employs an electrostatic lens that also acts as an ion filter to prevent thruster ions from bombarding the field emitters while decelerating the electron beam and keeping it focused to ensure efficient performance. Electron trajectories through the cathode and ion filtering capabilities are presented in this report as predicted by the charged particle code, MAGIC.

Pulsed Plasma Thruster Plume Study: Symmetry and Impact on Spacecraft Surfaces

Twenty-four witness plates were positioned on perpendicular arrays near a breadboard Pulsed Plasma Thruster (PPT) to collect plume constituents for analysis. Over one million shots were fired during the experiment at 43 J using fluorocarbon polymer propellant. The asymmetry of the film deposition on the witness plates was investigated with mass and thickness measurements and correlated with off-axis thrust vector measurements. The composition of the films was determined. The transmittance and reflectance of the films were measured and the absorption coefficients were calculated in the wavelength range from 350 to 1200 mn. These data were applied to calculate the loss in signal intensity through the films, which will impact the visibility of spaceborne interferometer systems positioned by these thrusters.

Fabrication and emitter measurements for a nanoklystron: A novel THz micro-tube source

A novel monolithic micro-tube source for submillimeter-wave (300 GHz to 3 THz) power generation is being developed at JPL. The tube takes the form of a simple reflex klystron with dimensions in the micrometer range, fabricated monolithically in silicon using standard MEMS (micro-electro-mechanical systems) fabrication techniques. While different types of cathodes are being considered, the effort has been focused so far on developing highly ordered arrays of carbon nanotubes (CNTs) working as cold field emitters. In this paper we report the progress and some preliminary results on the fabrication of monolithic nanoklystron cavities and CNT-based field emitters.