Browsing by Author "Wilbur, Paul J., advisor"
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Item Open Access Ion extraction from a plasma(Colorado State University. Libraries, 1980) Aston, Graeme, author; Kaufman, Harold R., advisor; Wilbur, Paul J., advisor; Fairbank, William M., Jr., committee member; Sites, James R., committee memberAn experimental investigation of the physical processes governing ion extraction from a plasma is presented. The screen hole plasma sheath of a multi-aperture ion accelerator system is defined by equipotential plots for a variety of accelerator system geometries and operating conditions. A sheath thickness of at least fifteen Debye lengths is shown to be typical. The electron density variation within the sheath satisfies a Maxwell-Boltzmann density distribution at an effective electron temperature dependent on the discharge plasma primary-to-Maxwellian electron density ratio. Plasma ion flow up to and through the sheath is predominately one dimensional and the ions enter the sheath with a modified Bohm velocity. Low values of the screen grid thickness to screen hole diameter ratio give good ion focusing and high extracted ion currents because of the effect of screen webbing on ion focusing.Item Open Access Performance and lifetime simulation of ion thruster optics(Colorado State University. Libraries, 2007) Farnell, Cody Coffman, author; Williams, John D., advisor; Wilbur, Paul J., advisorA simulation code, ffx, was developed to study various aspects of ion thruster optics. Information concerning sheaths, impingement limits, perveance, electric potential, charge exchange, and sputtering is covered. Electron backstreaming and pit and groove wear are discussed in detail as two grid failure mechanisms. The code was used to study the effects of several parameters on grid performance and lifetime, including grid spacing, aperture diameter, and grid thickness. An evolutionary algorithm was used with the ffx code to design grid sets, utilizing net accelerating voltage and current density as primary input parameters. Validation of the ffx code was accomplished through comparison to other ion optics codes and to experimental data obtained from both gridlet and full thruster testing. Gridlet test comparisons included simulations of finite aperture grid sets. The NSTAR thruster was studied in detail with regard to lifetime. The methods used for accurate and efficient optics simulation are discussed, including the multigrid method for solving for electric potential.Item Open Access Plasma flow field measurements downstream of a hollow cathode(Colorado State University. Libraries, 2007) Farnell, Casey Coffman, author; Williams, John D., advisor; Wilbur, Paul J., advisorThe focus of the research described herein is to investigate and characterize the plasma produced downstream of a hollow cathode with the goal of identifying groups of ions and possible mechanisms of their formation within a plasma discharge that might cause erosion, especially with respect to the hollow cathode assembly. In space applications, hollow cathodes are used in electrostatic propulsion devices, especially in ion thrusters and Hall thrusters, to provide electrons to sustain the plasma discharge and neutralize the ion beam. This research is considered important based upon previous thruster life tests that have found erosion occurring on hollow cathode, keeper, and ion optics surfaces exposed to the discharge plasma. This erosion has the potential to limit the life of the thruster, especially during ambitious missions that require ultra long periods of thruster operation. Results are presented from two discharge chamber configurations that produced very different plasma environments. Four types of diagnostics are described that were used to probe the plasma including an emissive probe, a triple Langmuir probe, a remotely located electrostatic analyzer (ESA), and an ExB probe attached to the ESA. In addition, a simulation model was created that correlates the measurements from the direct and remotely located probes.