Collaborative Research: Understanding Ion Losses to Plasma Boundaries Sheaths and Presheaths

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This award is made in response to a proposal submitted to and reviewed under the NSF/DOE Partnership in Basic Plasma Science and Engineering joint solicitation NSF 09-596. The award provides funds to support undergraduate participation in the overall research effort, which is being funded separately by the DOE under contract to the University of Wisconsin - Madison (Grant Number DE-FG02-97ER54437)." Understanding sheaths is fundamental to the understanding of all bounded plasmas. Experiments for this project are aimed at establishing the basic properties of sheaths and associated presheaths for a variety of situations for which theoretical predictions and assumptions have not yet been verified. Coordinated studies will be carried out at the University of Wisconsin and the University of San Diego. Experiments will determine the presheath and sheath plasma potential profiles and the resulting ion velocity distribution functions (IVDFs) associated with ion acceleration. Most of the experiments will be carried out in Madison and led by the P.I., Prof. Noah Hershkowitz. LIF diagnostic development will be carried out on both campuses and led by the other P.I. Prof. Greg Severn. Plasma parameters will be determined with two or more techniques, e.g. combinations of emissive probes, Ar, Xe, and Kr metastable ion LIF, Langmuir probes, the phase velocities of ion acoustic waves, and optical emission spectroscopy. The intellectual merit of this work is that after more than 80 years, many important questions associated with sheaths will be addressed for the first time. Indeed it was during the current funded period that our previous discovery of anomalous sheath edge velocities in multiple ion species plasmas met with theoretical justification by the introduction of a new theoretical element: ion streaming instabilities. The new theory, which is the freshest thinking about medium like behavior in connection with sheath formation in 80 years, was applied directly to our experimental results with excellent agreement. It follows then that many different aspects of sheath formation must be reevaluated in light of the new theory. Our research activities will also have a broader impact. Results will be disseminated widely to the scientific community through refereed publications and talks by the P.I.'s and their students at scientific meetings, as well as in seminars at universities and national Labs, and through patents. For example, previous work on electron sheaths resulted in a patent application and STTR support from NASA to develop an electron source to replace hollow cathode electron sources as neutralizers of ion-thrusters on long-range space exploration missions. Further, the relatively inexpensive tabletop experiments used in the investigations we are pursuing in this project are suited to student laboratory courses, permitting students to perform state-of-the-art experiments. Prof. Hershkowitz has developed three new plasma-based courses offered several times to a very inter-disciplinary audience. Several student lab experiments were adopted to train semiconductor-processing technicians at Portland Community College. New ion acoustic wave experiments have been introduced by Prof. Severn in the upper division experimental modern physics course at the University of San Diego. In the past, several undergraduates who worked with Prof. Hershkowitz have been first authors on published papers, have gone on to receive Ph.D.s in plasma physics and other areas, and to work in a variety of fields, including one who is now a reporter for the New York Times. Further, students have served as tutors in community colleges and secondary schools to increase awareness in careers in science. A student who performed undergraduate research work with Prof. Severn was one of six students honored for outstanding undergraduate research in plasma physics at the 51st American Physical Society, Division of Plasma Physics meeting in Atlanta, GA, in 2009. This student was a first generation college attendee. Our activities will continue to have a positive impact on diversity in STEM fields in the US.