Motivation: Centimeter scale magnetically-confined plasma devices encounter a unique set of design challenges when compared to their larger counterparts. For decades, researchers have studied the macroscopic behavior of cusp-confined devices on larger scales due to their high performance, but as the surface area to volume ratio increases with miniaturization, detailed knowledge of the plasma transport behavior become increasingly important. A better understanding of these phenomena will enable a new class of efficient miniature plasma devices suitable for electric propulsion and other plasma applications.

Objective: The current research investigates transport of plasma species across and along magnetic field lines in cusp confined plasma. The simultaneous diffusion of heavy and light species is generally understood, however, there have yet to be models that investigate the inherently three dimensional magnetic field structure encountered in miniature plasma devices. Additionally, the plasma structure is altered from traditional devices due to the increased proximity to the magnets. Through a combination of analytical, computational, and experimental work, this study is developing a detailed understanding of the plasma behavior in these fields, and ultimately revises discharge design guidelines for plasma devices on the centimeter scale.

Motivation: Intermediately-ionized plasmas are common in plasma thrusters, such as ion and Hall-effect thrusters used for deep space missions. Improving our understanding of heavy species collisions in intermediately-ionized plasma is necessary for furthering the development and use of plasma devices. The 3-D Improved Concurrent Electromagnetic Particle-In-Cell (ICEPIC) code developed by AFRL has simulated collisionless electron plasmas from high-power microwave devices, and is being expanded to include heavy species for intermediately-ionized plasmas. The addition of the partially ionized regime to ICEPIC capabilities will enable the use of detailed models to investigate important plasma phenomena, and will ultimately help determine the viability of implementing this type of code for full plasma thruster simulations. Therefore, precision measurements from a simple, well-characterized plasma experiment are needed to provide data to systematically extend and validate ICEPIC for multi-species plasma modeling that includes both heavy species and electrons.

Objective: The objective of this investigation is to acquire detailed measurements from a well-characterized, simplified experiment that can precisely capture the behavior of heavy species collisions. Detailed measurements of plasma behavior reveal ion transport mechanisms in a collisional environment that other experiments are not specifically designed to isolate and analyze. These canonical experiments supply benchmark data for the development and validation of analytical techniques and computational codes that model plasma behavior, including ICEPIC . Another objective is to understand the plasma dynamics within the experiment utilizing complementary computational and experimental efforts. This method tests the validity of applying existing plasma concepts to this specific test setup, assists in interpreting laboratory measurements, and provides insight into future experiment modifications and design. Approach: The experiment is designed to examine fundamental heavy species collisions with precise low-current measurements of an ion beam accelerated into a neutral target gas. Beam steering and conditioning components and manual and motor-driven beam scanning diagnostics are found along the length of the facility as pictured below. The ion beam enters the axisymmeteric test cell region where the ions are scattered to the walls of the test cell if they experience a collision (largely elastic scattering or charge exchange collision) or exits the domain unscattered. Featured below are example experimental data set compared to results from a multi-species hybrid-PIC model developed by Samuel Jun Araki at UCLA.

Motivation: Smaller cusp confined plasma discharge have higher surface-to-volume ratio where the plasma largely occupies a region that is greatly affected by the small scale magnetic field structures and the related interactions of plasma species near the surfaces. A significant improvement in the understanding of the plasma structure within the near-surface magnetic cusp region will enable the development of efficient micro-scale discharges (~1 cm) and assist in improving the efficiency of conventionally sized current cusp confined plasma discharges.

Objective: Investigate important effects (e.g., collisions, space charge, drifts, sheath) for cusp confined plasma species through progressively more complex experiments with well-defined domains and conditions while comparing data with results from computational and analytical analysis. Use these results and understanding to develop a highly efficient and stable micro discharge suitable for both thruster and plasma processing applications.

Find more details on this project at: http://www.wirz.seas.ucla.edu/AFOSR/

Motivation: To develop a computational model that properly treats the behavior of plasma very near the wall for a cusp confined plasma. The model will also serve as a tool to reveal important plasma mechanisms in the cusp region and will aid in the development of analytical description of particle motion in this region due to particle drifts, collisions, and electric and magnetic self fields. The preliminary model employs a particle-in-cell method, treating high energy electrons, ions, and secondary electrons as particles. To simulate quasi-neutral conditions, the model will employ a hybrid approach which will treat ions and slow electrons using a two-fluid approximation in addition to the particle tracker.

Objective: Magnetic cusp confinement of plasma at conducting surfaces involves interactions between a highly divergent magnetic field, ions, electrons, neutral particles, and the pre-sheath and sheath conditions that develop along the surface boundary. Large plasma devices have benefitted greatly by using permanent magnet cusps for bulk plasma confinement for both terrestrial and space applications; however, the magnetic cusp confinement mechanisms and the associated plasma dynamics very near the conducting surface are poorly understood. This lack of understanding has prevented researchers of micro-scale discharges from fully realizing the benefits of cusp confinement and control at smaller scales, and has also prevented the optimization of larger discharges.

Find more details on this project at: http://www.wirz.seas.ucla.edu/AFOSR/

Motivation: Hollow cathodes can be used to produce plasma for ion thrusters as well as other ion sources, plasma processing, lasers, etc. Energetic ions produced in the plume of these cathodes are likely the cause of erosion issues that have been observed in ion thrusters and that reduce the lifetimes of these systems.

Objective: This work studies the effects of neutral gas injection in or near the cathode plume on the production of energetic ions and on the overall cathode operation. Variables of interest include the number of neutral gas injection sites, the locations of these injection sites, and the orifice size of the injectors. This study will use experimental methods to determine the injection conditions that will yield optimum hollow cathode operation and increased system life.

Motivation: CubeSats are micro-spacecraft built from 10x10x10cm structure cubes, called U’s (units). These spacecraft have been utilized primarily for Earth-science experiments and commonly perform low Earth orbit (LEO) missions. The successful heritage, simplicity, and low cost of CubeSats make them attractive candidates for demanding space science missions reaching beyond LEO.

Objective: The first objective of this research is to present a methodology for developing a top-level satellite and mission design with an emphasis on CubeSat technology. The second aspect of this work applies this methodology to a lunar mission CubeSat (called LuMi) employing the UCLA/JPL developed MiXI (Miniature Xenon Ion) thruster. Successful completion of this effort will offer a first-order design of LuMi and will yield a CubeSat design for demanding scientific missions to the Moon. Additionally, the LEO inclination change capabilities of the spacecraft, which are of interest for Earth-science and military applications, will be assessed.

Motivation: The Miniature Xenon Ion (MiXI) thruster provides unique thrust capabilities that fill an important technology gap for several proposed NASA and DoD missions including precision formation flying. At only 3 cm in diameter, the MiXI thruster (in contrast to much larger traditional ion thrusters) is at a miniature scale where desirable performance requires careful consideration of the plasma stability and a high surface-to-volume ratio of the discharge. An improved understanding of minature plasma discharges is required to ensure that MiXI can deliver consistent performance and high-efficiency throughout the life of its mission.

Objective: The objective of this research is to investigate the physics of miniature discharge chambers. Plasma chamber studies are being conducted to improve plasma stability and at high levels of propellant and power efficiency. This research is being done with a combined computational and experimental methodology, and will culminate in the production of a repeatable and highly-efficient thruster that can be used for a wide range of future space missions.

Motivation: Diffusion theories have been formulated to describe perpendicular motions of weakly or fully ionized plasmas, but the motions in intermediately ionized plasmas still remain obscure. In fusion reactors, dominant mechanisms at play are governed by ions and electrons as the plasma are fully ionized. However, in an electric propulsion devices, densities of ions and neutrals are about the same order of magnitude, thus the plasma is intermediately ionized. The model under development would give a better understanding of how plasmas behave in this regime.

Objective: To develop a self-consistent hybrid model that describes the ion motion across magnetic fields; in this model, ions and low-energy electrons are treated as fluids while high-energy electrons are modeled as particles. Employing a finite volume method, ion density is obtained by solving the ion diffusion equation derived from standard fluid equations for ions and electrons. High-energy electrons ejected from a cathode are tracked using a particle pushing technique to obtain ion generation rate. Magnetic field meshes are used in this model in order to minimize a numerical diffusion.