Spacecraft Charge Neutralization During Active Electron Emission

Active (i.e. deliberate) electron emission in space is a powerful scientific technique that often proves problematic due to the spacecraft charging it induces. Indeed, effective spacecraft neutralization during active electron emission in low density space plasmas continues to be a challenge. A charge control technique was recently identified through particle-in-cell (PIC) simulations which promises to deliver this critical capability. The technique is termed the ion emission model and uses ion emission from the surface of a dense, quasi-neutral contactor plasma across a plasma sheath (double layer). This ion emission is shown to balance the electron emission current from the spacecraft without inducing significant spacecraft charging. Before conducting in-space demonstration experiments to validate PIC results, ground-based plasma chamber experiments were needed to help with early validation. This dissertation focuses on Earth-based plasma experiments conducted in a vacuum chamber to validate the ion emission model. These experiments are divided into four experimental campaigns which addressed distinct aspects of the “spacecraft”-plasma system. The campaigns examined: 1) the initial, transient “spacecraft” potential and plasma response to simulated electron beam emission, 2) the steady state plasma response to simulated electron beam emission, 3) the spatial ion emission current (and nearby plasma parameters which define it), and 4) how the peak spacecraft potential during simulated electron emission scales with electron emission current, emitted plasma current, and ion mass. The results from these experiments support the ion emission model and add to the physical understanding of ion emission as it may occur in tenuous space plasmas. Contributions of this work include: Demonstration of “spacecraft” neutralization during simulated electron emission. The “spacecraft” potential and bulk plasma potential were found to react in unison. The “spacecraft” potential was found to reach equilibrium tens of seconds into electron “beam” emission. A measured plasma response to changes in hollow cathode source potential relative to chamber ground. Langmuir probe measurements of the bulk plasma potential, floating potential, electron temperature, electron density, and ion density are presented for two plasma source potentials. The plasma potential was found to stay within a few electron temperatures of the source potential. The electron temperature was found to increase for higher source potentials. Charged particles were found to concentrate near the plasma source for higher source potentials, while the bulk plasma remained quasi-neutral outside of the chamber wall’s plasma sheath. Direct experimental validation of the semi-analytical ion emission model. Plasma measurements of the ion emission region near the chamber wall are presented via Langmuir probe, retarding potential analyzer (RPA), and emissive probe measurements of the bulk electron temperature, ion energy distribution function (IEDF), and spatial plasma potential. These measurements were used as inputs to the ion emission model and an analytical space-charge limit (SCL) expression. The ion emission model and SCL emission current predictions were compared to the measured emission currents and both were found to agree within 50%. A parametric analysis of physical properties that affect ion emission from a quasi-neutral plasma. The “spacecraft” potential scaling with simulated electron emission current was found to follow an exponential function which is likely defined by the electron temperature. The peak “spacecraft” potential during electron emission was found to decrease for both lower simulated electron emission current and ion mass.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/162960/1/gmiars_1.pd

Appendix, DS_10.1177_1941738118775039 – Cardiovascular Screening Practices and Attitudes From the NCAA Autonomous “Power” 5 Conferences

<p>Appendix, DS_10.1177_1941738118775039 for Cardiovascular Screening Practices and Attitudes From the NCAA Autonomous “Power” 5 Conferences by Christopher W. Miars, Andreas Stamatis, Grant B. Morgan, and Jonathan A. Drezner in Sports Health: A Multidisciplinary Approach</p

Book of Abstracts of The Sixth International Conference on Tethers in Space

Abstracts of The Sixth International Conference on Tethers in Space. June, 12-14, 2019. Madri