Kristina M. Lemmer

Using a helicon source to simulate atmospheric re-entry plasma densities and temperatures in a laboratory setting

The purpose of this research is to develop a plasma system capable of reproducing plasma densities found during atmospheric re-entry of a capsule. We developed a 150mm diameter helicon source at the University of Michigan Plasmadynamics and Electric Propulsion Laboratory (PEPL) and used a Langmuir probe to characterize plasma properties downstream. The helicon source was operated with argon gas at a background pressure of 0.6mTorr. We used a commercial RF-compensated single Langmuir probe to measure ion number density and electron temperature in the region downstream of the helicon source where we want to create conditions similar to those found during hypersonic flight within the atmosphere. We measured these values with and without the presence of a large 450mm wide by 550mm long surface downstream in the horizontal plane to simulate a vehicle surrounded by plasma in order to determine how the downstream body affects plasma properties. We found that the presence of a surface downstream of the helicon source lowers the downstream plasma density range from between 1.7 × 10 17 and 3.3 × 10 17 m −3 down to 0.55 × 10 17 and 1.3 × 10 17 m −3 . In addition, the peak plasma potential decreases from 65 to 55V, but the electron temperature remains unchanged ranging between 1.5 and 6.5eV.

Mission Analysis for a Micro RF Ion Thruster for CubeSat Orbital Maneuvers

The performance of a micro RF ion thruster is analyzed for several CubeSat orbital maneuver strategies. Mission simulations are performed for various types of trajectory control, including thrust at perigee only; intermittent thrust for a passive, magnetically stabilized spacecraft; and constant and pulsed thrust along the velocity vector. For each case, the propulsion system efficiency and maximum orbit change are evaluated. Constraints on power, fuel mass, and mission duration are considered. The most effective combinations of thruster operational modes and trajectory control strategies are discussed.

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.

Experimental results for communications blackout amelioration using crossed electric and magnetic fields

As a vehicle reenters the atmosphere or travels at hypersonic speeds within it, a bow shock forms around the leading edge of the vehicle. The air is superheated as it passes through the shock wave and becomes ionized. This plasma layer prevents the transmission of radio frequency communications to or from the vehicle, causing what is know as communications blackout. In this paper, we present results from experiments performed to evaluate the use of crossed electric and magnetic fields to lower the plasma density in a region surrounding an antenna. Plasma number density, plasma frequency, and signal attenuation measurements were made with a Langmuir probe, hairpin resonance probe, and S2-1 probe, respectively. The hairpin resonance probe and the S2-1 probe measured frequency responses for input frequencies ranging from 200 up to 4000 MHz. Results show that this approach is a viablemethod for communications blackout amelioration.We found that the plasma number density decreases by as much as 70%with the operating conditions used in this work, and the plasma frequency dropped by asmuch as 75%. The increased reduction in the plasma frequency, as compared to the plasmanumber density, was due to the addition of greater magnetic field strength when the frequency measurements were made. In addition, frequencies that were previously attenuated by more than 10 dB have almost no attenuation after the application of the electric and magnetic fields.

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.

An Experimental Study of Hydrogen Production by Dissociation of Water Vapor in a Helicon Plasma Source

Summary form only given. This study explores the option of using a helicon plasma source to dissociate water vapor for hydrogen production. Fossil fuels currently provide 86% of energy in the United States and much of the world. The dependency of the world on fossil fuels as the main energy source leads to an unsustainable environmental and energy future. Hydrogen has been proposed as an alternative source of energy carrier, but it is a sustainable energy source only if it is produced from renewable resources. Currently, 96% of hydrogen is produced by non-renewable methods. Electrolysis is the predominant renewable method of hydrogen production, contributing to 3.8% of total hydrogen production. However, its efficiency, the ratio of the energy value of hydrogen and the energy input to produce hydrogen, is only 25%. In this experiment, a helicon plasma source is used to dissociate water vapor into hydrogen and oxygen. Hydrogen partial pressure is measured using a residual gas analyzer. An evaluation of this hydrogen production method is presented based on hydrogen partial pressure measurement and estimated efficiencies. A characterization of the quantity of hydrogen produced in various operating conditions is also presented. A comparison between this approach and electrolysis is discussed.

An experimental study of hydrogen production by dissociation of water vapor in a radio frequency plasma source

This experiment investigates a novel technique of hydrogen production by dissociating water molecules in a radio-frequency (rf) plasma. This plasma source has a helicon antenna, capable of operating in capacitive, inductive, or helicon mode when operating conditions match those required to excite these modes. Hydrogen is produced by injecting water vapor into the plasma source. The species identified in the plasma from data obtained via a residual gas analyze are hydrogen, oxygen, water, hydroxyl, and nitrogen. Partial pressures of these gases are also obtained from the residual gas analyzer. In this paper, we show that this plasma source is capable of dissociating water molecules into their constituent species when operating in inductive mode.

Distributing regionally, distinguishing locally: examining the underlying effects of local land use on airborne bacterial biodiversity: Airborne microbes and land use

Airborne bacteria are abundant and can vary with land use. Urban expansion is increasing rapidly at a global scale, altering natural sources of airborne bacterial biodiversity, as soils and native plants are replaced by pavement and managed yards. Urbanization homogenizes the biodiversity of larger organisms, but its effects are understudied with respect to microbes. This study uses categorical and gradient approaches to examine airborne bacterial communities in southwest Michigan (USA). Airborne communities carried a gut-microbial signature and were equally homogenous above urban and rural sites, despite greater homogeneity of soil communities at urban sites. Ruminococcaceae were abundant, the source of which is likely wildlife. Beyond the gut-microbial signature, there were underlying effects of land use, which were evident in the shared airborne taxa across urban and rural sites. Bacillales, Burkholderiales, Alteromonadales and Pseudomonadales were shared more across urban sites, while Xanthomonadales, which contains crop-plant pathogens, were shared across rural agricultural sites. These results suggest that taxa which may distribute globally, coupled with localized sources, contribute to urban communities, while regional rural activities drive rural composition. We determined that soils were unlikely to contribute to broad distribution of some plant-associated taxa, but may be a source for distribution of others.

Mission Analysis for CubeSats with Micropropulsion

The orbital maneuver capabilities of several CubeSat propulsion systems are analyzed using trajectory simulations. Properties of several types of developmental micropropulsion systems are reviewed, and ΔV capabilities are compared. Mission simulations are used to analyze the relationship between thrust arc length and orbit change capability in a low-thrust spiral trajectory. Constraints on power, fuel mass, and mission duration, as well as system-level constraints, are considered. Feasible CubeSat architectures and mission designs are developed for three electric propulsion systems. The most effective combinations of thruster operational modes and trajectory control strategies are discussed.

Integration of micro electric propulsion system for cubesat orbital maneuvers

Attitude and trajectory control strategies are developed for a CubeSat with electric propulsion. The thrust capabilities of a micro RF ion thruster are characterized using ion source measurements to estimate thrust vectoring performance. Trajectory simulations are used to determine feasible orbit-raising maneuvers from low-Earth orbit. A PID controller is developed to determine attitude control torques for circular and elliptical orbit cases. Thrust vectoring to allow thruster pointing at a variable angle with respect to the CubeSat body is simulated.