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

The Assessment of Fundamental Skills Involved in Reading Comprehension

192 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1973.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Parallel Processing Experiments on an SCI-based Workstation Cluster

This paper describes the SCI-based workstation cluster system being developed at the HCS Research Lab and the parallel processing and network experiments that have been conducted and the results achieved. Using several different input sizes and degrees of partitioning and granularity for the parallel processing algorithms employed (i.e. matrix multiply and data sorting) and experimenting with both ring- and switchbased topologies and other permutations (e.g. number of workstations), significant reductions in execution time have been achieved. These results help to illustrate what types of parallelism can be achieved today on SCI-based workstation clusters. 1. Introduction Parallel processing has emerged as a key enabling technology for a wide variety of current and future applications spanning both the commercial and military sectors, however these applications continue to rely heavily on the speed of the interconnection system used for processor communication. For years, advancement..

Modeling and Simulation of Processors and Networks for Smart Navy Aircraft Components and Systems

ing bus activity to this high-level is designed to save time in simulation. Latency and throughput measurement are made at the functional level where individual message transmission can be viewed for verification. 0 5 10 15 20 25 30 35 40 45 50 15 20 30 40 50 Offered Load (Mbps) Effective Throughput (Mbps) Figure 5. Effective throughput vs. Offered load A series of experiments are currently being conducted with basic processor models and the HSDB model. Initial results have helped to verify and validate the basic structure of the model. By setting values of base data rate, queue sizes, offered load per station, number of stations, etc., we can estimate the effective throughput and one-way latency of transfers on the network. For example, using the base data rate of 50-Mbps associated with HSDB, a series of two-second simulation runs were executed for a scenario with the following parameters: ten stations; packet size of 32,840 bits based on 32768 bits of data and 72 bits of overhead..

An Integrated Simulation Environment for parallel and distributed system prototyping

The process of designing parallel and distributed computer systems requires predicting performance in response to given workloads. The scope and interaction of applications, operating systems, communication networks, processors, and other hardware and software lead to substantial system complexity. Development of virtual prototypes in lieu of physical prototypes can result in tremendous savings, especially when created in concert with a powerful model development tool. When high-fidelity models of parallel architecture are coupled with workloads generated from real parallel application code in an execution-driven simulation, the result is a potent design and analysis tool for parallel hardware and software alike. This paper introduces the concepts, mechanisms, and results of an Integrated Simulation Environment (ISE) that makes possible the rapid virtual prototyping and profiling of legacy and prototype parallel processing algorithms, architectures, and systems using a networked cluster of workstations. Performance results of virtual prototypes in ISE are shown to faithfully represent those of an equivalent hardware configuration, and the benefits of ISE for predicted performance comparisons are illustrated by a case study