Department of Mechanical Engineering-Engineering Mechanics

Projects

Current projects in the Isp Lab:

 

Field Emission Cathodes

Field-emission cathodes have recently received much interest for use as zero-flow “cold” electron emitters for sub-100-W Electric Propulsion (EP) thrusters.  Field emission cathodes rely on Fowler Nordheim emission from very sharp (10-nm-radius) electrode tips.  The lifetime of microfabricated field emitters is, so far, incompatible with EP applications.  Because of the fragility of the nanometer-sized tips, the structures are susceptible to damage that blunts the tip and destroys the device functionality.

This research investigates the discharge characteristics of a field-emission cathode for use in EP that has the ability to be re-generated when the emitter tip becomes damaged.  Emitter tip re-generation is possible by using Taylor cone formation from an operating liquid-metal-ion-source (LMIS) in an effort to solidify, or quench, the ion emitting cone to preserve the sharp tip.  The tip can then be used for electron emission.

Bismuth Hall Thrusters

Hall thrusters operating at power levels greater than 30 kW would have significant benefits for space missions of interest to the Air Force. Although scaling state-of-the-art Hall thrusters to larger powers is straightforward from a physical standpoint, development of such devices has been impeded by financial considerations. Currently, Hall thrusters operating at the power levels of interest require multi-million dollar ground-testing facilities and consume tens of thousands of dollars of xenon propellant in a single day of testing.

This project is developing critical technology for high-power Bismuth Hall thrusters potentially having superior performance over state-of-the-art devices at a fraction of the cost of existing technology. Bismuth is significantly cheaper than Xenon and since it is solid at room temperature, does not require the pumping capacity needed for Xenon powered thrusters. Also when compared with xenon, bismuth has a higher atomic weight and lower ionization potentials, implying superior physical efficiency in the plasma discharge. (Hall thruster operating on bismuth propellant is shown to the left.)

The work centers on an MTU-patented concept for supplying metal vapor propellant to a Hall thruster using a temperature-controlled segmented anode. The metal propellant is maintained in a liquid state within the primary anode using waste heat dissipation within the thruster. The propellant supply rate is controlled through power-sharing shim electrodes, which intercept a fraction of the plasma discharge current, thereby regulating the liquid metal temperature within the primary anode and hence the evaporation rate.

This work is also supported by industry partners Aerojet and Aerophysics, Inc.

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Alternate Metal Propellant Thrusters

In addition to bismuth, the Isp Lab is investigating alternate metal propellant options for Hall thrusters Two candidates, Mg and Zn, show early promise for certain mission applications. Magnesium has an atomic weight of 24.3 amu and ionization potential of 7.6 eV, while zinc has an atomic weight of 65.4 amu and an ionization potential of 9.2 eV. Given these properties, magnesium would be ideal for missions requiring a specific impulse of approximately 4000 s whereas zinc suits missions near 2400 s. Both magnesium and













zinc are readily available and are both far less expensive than traditional propellants such as krypton or xenon. As an added benefit, lunar and Martian studies have shown that magnesium could be harvested in-situ, allowing for the possibility of re-fueling an exhausted propellant supply. Initial experiments using both metals yielded some early success as shown in the images of the operating zinc thruster (top right), magnesium thruster (bottom left), and magnesium cathode (bottom right) which were first operated in February of 2009. Development of Mg and Zn Hall thrusters continues in the Isp Lab.

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Bismuth Cathode

Using bismuth in place of gases such as xenon for Hall thruster propellant could potentially offer both physical and economical gains. As research continues to develop Hall thrusters that are fueled with bismuth, it will become advantageous to maintain one propellant supply rather than multiple supplies for the anode and cathode. The recent development of a bismuth Hall thruster at Michigan Tech, operated using a xenon LaB6 cathode, provided a motive to explore the feasibility of developing an entire bismuth system.

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Refractory Powder Metallurgy

powder_met

Refractory metals have various thermal and material properties that are desirable for use in plasma thrusters. The Isp Lab has the ability to sinter refractory materials in a high vacuum or inert environment to create parts with controlled densities while retaining open porosities. This process has been used to make large molybdenum discs with ~2 micron pores at 60 percent density which are used in bismuth Hall thrusters.

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Hall Thruster Thermal Modeling

Control and Management of condensable metal propellants in Hall thrusters requires detailed knowledge of the Hall thruster temperature field and heat transfer during operation. Computational codes such as MuSES, IDEAS, and TRASYS are used to predict temperature fields. Numerical results are calibrated against test data. The Isp lab also maintains a Mikroscan 5104 IR thermal imaging camera for experimentally examining the thermal properties of condensable metal laboratory cathodes and Hall thrusters in operation.

 

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Electron Dynamics in Hall Thruster Fields

Hall thrusters employ a magnetic field applied transverse to an accelerating electric field to emulate virtual accelerating electrodes. Historically, the cross-field electron mobility in HEA's has been shown to be 1,000 times greater than predicted by classical plasma theory, yet an explanation for this discrepancy is absent.

In this project, the defining characteristics of the accelerating region in a Hall thruster are reproduced in the construction of an electron trapping apparatus. The trap will be used to confine a one-component-plasma to study the mobility of electrons across the magnetic field. The trap utilizes radial magnetic mirror forces and an induced circular ExB drift to confine the plasma. Mobility is measured in response to field strength, collision frequency, and time-varying electrostatic perturbations representative of traveling plasma waves.

Many scientific devices rely on trapping hot, electrically charged particles in electromagnetic "bottles" to prevent collisions between the particles and solid walls. While many excellent techniques exist to confine either positively charged or negatively charged particles in such bottles, trying to simultaneously trap charges of both signs has been problematic. Magnetic fields are typically used as virtual walls for trapping charged particles, since such particles have difficulty crossing magnetic field lines. Theoretical calculations show that magnetic fields should be very effective for simultaneously confining both positive and negative charges. However, when used in the laboratory the rate at which particles escape magnetic devices can be more than 1,000 times greater than predicted by theory.

The poor effective trapping has been a problem in many disciplines. Most visibly, the inability to effectively trap particles has been a frustration for scientists trying to design fusion energy devices that have the potential to eliminate our reliance on fossil fuels. This project will study the problem of confining charged particles in magnetic fields in an attempt to understand the most basic discrepancy between theory and experiment. The results of this study could be used to improve charged particle confinement with benefits for fusion energy, space propulsion, and materials science applications.

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Xenon Hall Thrusters

Plasma thrusters are famous for their fuel efficiency: when compared with a chemical rocket, a typical plasma thruster may only require 1/10th the amount of propellant to accomplish a given mission. However, because the electric power available on a spacecraft is limited to a few 10's of kW, plasma thrusters are limited in the amount of instantaneous thrust they can provide. The result is that, while a plasma thruster requires less fuel, it will take longer to accomplish a given mission than a high-thrust chemical system. For certain missions, it is more important to get the spacecraft to its destination in a short time than it is to conserve propellant. These types of missions require more thrust than is currently available using state-of-the-art Hall thrusters.

Work at the Isp Lab is focusing on increasing the amount of thrust available from a Hall thruster for a given input electric power. These so-called High Thrust-to-Power ratio devices would be an enabling technology for many missions, civilian as well as military, requiring rapid re-positioning of space assets. Researchers are examining the various thrust and power loss mechanisms of current Hall thrusters with the intention of identifying means of improvement. A subtle, but critical, related area of research involves understanding the coupling voltage between the thermionic hollow-cathode/neutralizer and the thruster plasma.

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Annular Field Reversed Configuration Experiments

The ISP Lab is developing an experiment to study the formation and translation dynamics of annular field reversed configuration (AFRC) plasmas. AFRCs are plasma toroids, formed between two concentric coils as shown by the schematic on the right. A changing magnetic field produced by the coils induces an azimuthal diamagnetic current (yellow arrow) in the plasma, forming a closed toroidal magnetic field (orange arrows) to confine the plasma. The toroid (or plasmoid) can then be ejected from the coils as a self-contained unit. This propulsive capability turns an AFRC into a pulsed inductive plasmoid accelerator, candidates for pulsed high power electric propulsion.

Numerous formation studies on AFRCs have been conducted, yet the translation of an AFRC has not been previously reported. The primary goal of the ISP Lab's research is to translate the AFRC plasmoid and measure its velocity, momentum, and acceleration efficiency. A secondary goal of this research is to compare AFRC devices to similar technologies, using an electromagnetic launcher model developed by the ISP Lab. This model will also be used to aid experimental design and to develop scaling laws for AFRC devices.

The ISP Lab's AFRC experiment is a next-generation device of University of Michigan/U.S. Air Force Research Laboratory's AFRC formation experiment. Other co-axial geometry studies (the Coaxial Slow Source) have been completed by the University of Washington.

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Coulomb Propulsion

Spacecraft clusters, or formations, are an emerging trend in space mission design. Proposed concepts utilize swarms of small micro- or nanosatellites acting in collaboration. For many applications, the vehicles within the swarm must maintain accurate position with respect to others in the group. To achieve relative position control, some type of formation-keeping propulsion is required on each vehicle. While the propulsive forces necessary are small (10's to 100's of microNewtons), precise application and control of these forces is difficult in practice. Also, continuous application of formation-keeping forces would quickly exhaust the propellant supply of traditional micro-rocket engines. In 2001, the Isp Lab was the first to propose the use of inter-vehicle Coulomb forces for use in spacecraft formation control. This concept relies on electrically charging spacecraft by expelling charged beams. The vehicles then experience electrostatic attraction or repulsion from the other vehicles in the swarm. The entire formation exhibits coupled, collective motion that can be exploited to maintain relative position. Since Coulomb propulsion requires no fuel, the concept can essentially maintain a formation for an indefinite period of time. The power required to affect Coulomb propulsion has been shown to be on the order of milli-Watts up to a few Watts. The required on-board mass of the system has been shown to be very small. Research continues on charging mechanisms, charge sensing/serving techniques, spacecraft/plasma interaction, and distributed control.

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Carbon Nanotubes for Sputter Erosion Resistance

Electric propulsion devices make deep space missions feasible because of their superior efficiency over traditional chemical rockets. However, sputter erosion of critical surfaces in these thrusters is a life-limiting factor. To extend the lifetime of EP devices, sputter resistant materials such as molybdenum (Mo) are generally used for these surfaces. However, unique mechanical properties of carbon nanotubes (CNTs) have triggered tremendous curiosities in their applications, particularly for EP thrusters. Vertically aligned multi-walled carbon nanotubes (VA-MWNTs) are evaluated as protective coatings against ion erosion in hopes of finding a superior material for EP applications. More information is available at http://www.phy.mtu.edu/yap/

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Zero-Gravity Student Research

Michigan Tech students regularly perform experiments on-board NASA's zero-gravity simulation aircraft through the NASA Reduced Gravity Student Flight Opportunity Program http://microgravityuniversity.jsc.nasa.gov/. The C9 aircraft, which flies a parabolic path consisting of a steep climb followed by a steep dive, can provide 30-second periods of simulated zero gravity for participants. MTU student research teams conducted experiments on the C9 in 2004, 2005, and 2007. In the most recent set of tests, researchers evaluated a proposed scheme for removing accumulated lunar dust from solar panels in conditions that simulated a lunar outpost (1/6th Earth gravity). In these tests a clear glass substrate was patterned with transparent electrodes and placed over a solar cell. By applying high-voltage radio-frequency signals to the electrodes, accumulated dust was repelled from the surface due to electrostatic forces. For more information on the Aerospace Enterprise view their web page at http://www.enterprise.mtu.edu/aerospace/

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Oculus: a nanosat for Space Situational Awareness

Satellites, having very predictable orbits, are increasingly vulnerable to attack by both enemy countries and terrorists. Many nations and groups either have, or will have soon, access to space that they can exploit to gain an advantage during conflict. For the future of the US space force it is imperative to (1) protect US-flagged spacecraft from hostile disruption as well as (2) have an accurate knowledge of our adversaries' space capabilities. Currently, the US spacecraft tracking system relies on ground-based radar to detect satellites as they pass over monitoring stations. This system is slow to detect and respond to spacecraft which may have changed their orbits between passes over the ground stations. Also, the ground-based radar is sufficient to determine the relative size of an object, however it is unable to provide information on vehicle configuration, capability, or intent. The Air Force Space Command's Strategic Master Plan has identified the need for space-based surveillance systems, including inspector satellites, that are capable of providing details of space objects unattainable by ground-based systems.

The MTU nanosatellite Oculus will serve as a test-bed for space-based surveillance technologies used to characterize resident space objects. The vehicle, weighing less than 30 kg, will carry a sophisticated imaging system developed by Raytheon Missile Systems of Tucson, AZ. Oculus will be capable of highly accurate three-axis control, allowing it to provide exquisite optical characterization of other spacecraft. For more information on the Aerospace Enterprise view their web page at http://www.aerospace.mtu.edu/aeroweb/

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Michigan Tech Isp Lab
R.L. Smith Building B007 C
1400 Townsend Drive
Houghton, MI 49931

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