David P. Morris

Development of an Annular Helicon Source for Electric Propulsion Applications

Abstract : The performance of typical electrostatic propulsion systems, such as the Hall thruster, is limited in part by inefficiencies in the electron bombardment ionization process. These limitations become especially pronounced at the operating conditions required to achieve high thrust-to-power ratios. One approach for achieving significant increases in efficiency at such operating conditions is to replace the typically-employed DC ionization mechanism with a helicon source, which is widely regarded as an efficient method for creating a high-density, low-temperature plasma. Standard cylindrical helicons, however, are not amenable to straightforward integration with annular Hall thrusters. A rigorous mathematical treatment of helicon wave physics has been completed to establish the boundary conditions required to create an annular helicon source for both the m=0 and m=1 azimuthal modes. This analysis reveals no fundamental barriers to creation of an annular helicon source so long as the radial boundary conditions are set appropriately.

MEMS Gate Structures for Electric Propulsion Applications

Abstract : A MEMS gate prototype is under development to extract and accelerate charged particles for use with field emission cathodes and the nanoparticle field extraction thruster at the University of Michigan. Preliminary simulations suggest the desirability of a unity aspect ratio in the emission channel design to achieve electric field uniformity. Low emission threshold cubic boron nitride films have been grown, and gated testing with these films along with carbon nanotubes is in progress.

Developmental Progress of the Nanoparticle Field Extraction Thruster

The Nanoparticle Field Extraction Thruster (NanoFET) is an electrostatic accelerator technology currently under development at the University of Michigan to accelerate micro-/ nano-particles. The concept exists in two configurations: the liquid configuration stores and transports particulate propellant using microfluidics while the dry configuration eliminates the liquid feed system in favor of particle transport via piezoelectric actuators. Microgravity flight tests of the liquid-NanoFET concept indicate good agreement with theory regarding the threshold electric field for liquid surface instabilities. This threshold electric field was observed to increase in low Bond number systems as the channel diameter decreased and appeared to be governed by the largest characteristic channel orifice dimension. Particle liftoff and extraction from both liquidand air-filled reservoirs were also demonstrated in the microgravity environment. On the ground, preliminary experiments showed that particle liftoff electric fields could be reduced with the application of inertial accelerations from piezoelectrics to the charging electrode. Both the recent microgravity and ground test results for the NanoFET concept are presented.

Nanoparticle Field Extraction Thruster (nanoFET): Introduction to, Analysis of, and Experimental Results from the "No-liquid" Configuration

This paper introduces a nanoparticle field extraction thruster (nanoFET) concept that does not depend on the liquid delivery of micro and nano-particles for extraction and acceleration. The no-liquid approach potentially provides important advantages such as allowing the use of smaller particles for propellant, which may offer a greater specific impulse. The most likely developmental obstacles are the adhesion of the particles to the source electrode and the cohesion between the particles. Adhesion and cohesion models are presented along with proposed methods of overcoming each. A method of using the applied charging electric field to overcome the adhesion force is investigated, which predicts that it may be possible to remove particles with diameters down to hundreds or even tens of nanometers from a planar electrode with only the application of a high strength electric field. To investigate this particle removal model, eight test cases, involving 4 particle sizes and 2 electrode materials, are presented. A method of transporting the dry particle propellant through an ultra-fine sieve prior to the charging and accelerating stages is investigated as a method of overcoming the cohesion between the particles. A simple proof-of-concept experiment is presented which indicates that this method is capable of breaking the cohesion force under appropriate conditions, which helps to guide future research.

APPLICATION OF DUAL GRIDS TO COLD CATHODE/FIELD EFFECT ELECTRON EMISSION

This paper will present an analysis of dual grid field emitter technology, illustrating the substantial power savings that can be obtained from existing technologies by controlling the final velocity of emitted electrons. Additionally, using two grids instead of one allows the efficient use of a wider range of emission technologies. Higher field strength requirements do not necessitate higher power requirements as with single grid devices. Basic illustration of the concept including conservation of power analysis will be followed by examples of system integration. Application to ion thrusters, hall thrusters, electrodynamic tether propulsion, and ground experiments will be described.

Simulating hypersonic atmospheric conditions in a laboratory setting using a 15-cm-diameter helicon source

Summary form only given. While a spacecraft is reentering the atmosphere or a hypersonic vehicle is in flight, low-frequency electromagnetic radiation cannot penetrate the plasma layer that forms around the high speed vehicle. This interferes with real-time telemetry from hypersonic vehicles and interrupts spacecraft communications during atmospheric reentry. Hypersonic atmospheric plasmas are difficult to simulate in a laboratory setting because they are high density (~109 - 1011 cm-3 depending on altitude) and low temperature (~2 - 5 eV). A 6-cm-diameter helicon source capable of creating plasma with these requirements has been designed, fabricated and tested at the University of Michigan Plasmadynamics and Electric Propulsion Laboratory (PEPL). We present Langmuir probe, retarding potential analyzer and residual gas analyzer data from helicon source operation with argon, nitrogen and air.

DEMONSTRATION OF FIELD EMISSION CATHODE OPERATION IN A PLASMA ENVIRONMENT

Field emission cathodes are being considered for use on low-power electrostatic and electromagnetic propulsion systems as well as on electrodynamic tether systems. In these systems, field emission cathodes prevent negative spacecraft charging by emitting a lowpower electron beam that is accommodated by the ambient plasma. Prototype cathodes were tested at the University of Michigan as part of a Student Space Systems Fabrication Laboratory satellite project. This paper presents experimental emission performance results of molybdenum and carbon nanotube field emission cathodes operating in a krypton plasma of density ne=1x10 11 m -3 and temperature Te = 0.5 – 1.0 eV.

Electron emission for electric propulsion: reducing power by mitigating space charge limits

Electron emission is a requirement for most forms of space electric propulsion, as well as many other space applications. For many such missions minimizing system power is critical to the viability of the mission. The generation of electrons requires an amount of power that varies depending on the source (cold cathode and thermionic sources are considered here, not hollow cathode or other sources requiring consumables) but regardless of the source, the electrons must leave as a beam that has sufficient power (velocity) to escape the spacecraft. The space charge limit refers to the maximum current that can cross a gap (the sheath between the spacecraft and the surrounding plasma for example) at a given velocity, and thus is a practical lower bound to the beam power required for a given current. For many systems, especially those using low-power electron sources, this means that power must be added to the electron beam, usually with biased grids. The minimization of this electron beam power requirement is the topic of this paper. Analytic solutions for the space charge limit in simple geometries have been developed up to three dimensions, and these demonstrate some basic performance tradeoffs. Less practical for the analytical approach are more complicated geometries and the addition of spacing and timing factors. These complications, however, can allow significant improvement in the space charge limit. Therefore, using particle-in-cell computer simulations (with the XOOPIC code developed at Berkley), a number of techniques for mitigating the space charge limit have been investigated, ranging from atypical geometries to spatial and chronological phasing of emission. Some of the advantages and disadvantages of some techniques, and the implications for electron emission system design, will be presented here.

FEA cathodes (FEACs) for space based applications

Summary form only given. In controlled high vacuum environments, field emitter array have shown substantial capability, but have failed in harsher environments more typical of space applications. We argue that a combination of localized arc suppression coupled with robust, low work function coatings such as zirconium, carbide can provide the needed raggedness to withstand energetic ions, oxygen fluxes, and adsorbates typical of a spacecraft environment. In this work, we present a survey of the requirements and capabilities of FEACs based on an analytical model of FEA performance; a summary of the model and its implementation shall be briefly discussed. From it we ascertain the relative advantages of FEACs as a function of materials, geometry, and space charge limitations, for EDTs, EP systems, and spacecraft charge control. We identify what is required of FEACs in terms of emission performance, lifetime, operational configurations and conditions (operation in a 1-10 microTorr pressure environment current densities of less than 0.1 Amps/cm/sup 2/, and gate voltages between 50-100 Volts). We discuss recent progress in modeling (device and space charge effects).

Development of a MEMS-based gate to enhance cold-cathode electron field emission for space applications

A gated structure of arrays of micron-sized holes has been developed at the University of Michigan. The structure can be positioned atop any uniformly structured planar emitting surface and biased to effect electron emission. The structure is designed to be compatible with a variety of emission surface technologies such as thin films (e.g., boron nitride), carbon nanotubes, and self-assembled nanostructures.