Proposal Title: Collaborative Research: Understanding Sheaths and Pre-Sheaths in Plasmas Principal Investigator: Hershkowitz, Noah / Severn, Greg Institution: University of Wisconsin-Madison / University of San Diego Proposal No: CBET- 0903783 / CBET- 0903832

This research addresses basic science questions concerning the sheath-presheath physics in multispecies plasmas that are central to many fields of plasma science and technology, including plasma thrusters, plasma processing, divertors of fusion devices, Langmuir probes, and RF plasmas. After more than 80 years, many important questions associated with sheaths will be addressed for the first time.

Proposed experiments 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 San Diego University to determine the presheath and sheath plasma potential profiles and the resulting ion velocity distribution functions associated with ion acceleration. Most of the experiments are to be carried out in the laboratory of Prof. Hershkowitz. However, LIF diagnostic development is being carried out on both campuses and is led by Prof. Severn. Plasma parameters will be determined with two or more techniques; e.g., combinations of emissive probes, Ar and Xe metastable ion LIF, Langmuir probes, the phase velocities of ion acoustic waves, pseudo-wave velocities and optical emission spectroscopy. Agreement of several diagnostics is needed because of the invasive or incomplete nature of the diagnostics due to (e.g.) particle depletion by probes and LIF measurements of metastable rather than ground state ions. Experiments will employ hot-filament multi-dipole, capacitive, inductive, and helicon sources currently in operation in our laboratory. Negative ions will be produced by adding O2, SF6, or Cl2 into Ar and Xe plasmas.

As noted above, many plasma-based technologies would benefit. The results of the proposed research will be disseminated broadly in the scientific community and industry through publications and conference presentations. The panel considered it likely that the results will be used in plasma physics textbooks and will be widely referenced for years to come. Graduate and undergraduate students will participate in the research, and the experiments used in these investigations will be used in student laboratory courses. The relatively inexpensive tabletop experiments used in these investigations are suited to student laboratory courses, permitting students to perform state-of-the-art experiments.

Project Report

One of the principal contributions we've made in this funding cycle is the first experimental test of a new theory of sheath formation in multiple ion species plasmas. Our work showed that speed of the two ion species converge nearly to the ion sound speed of the bulk plasma at the sheath edge. {em This result was new and controversial because it clearly demonstrated the need for revision of the standard theory of sheath formation for general plasma systems.} A new theory was proposed and published during this funding cycle and our work was the first to test it and substantially vindicate it, although much work remains in this regard. This work was the subject of invited talks at national research conferences and two high profile publications during the funded period, with more to come. The result has broad application in plasma physics. Plasma sheaths are characteristic of all bounded plasmas. They are plasma potential barriers that form to provide a balance of electron and ion fluxes to boundaries. Their characteristics are critical to the operation of many devices which depend on plasmas. Examples are found in materials processing plasmas, fusion plasmas, as well in space physics plasmas. We also found a new scheme laser-induced fluorescence scheme (valid and efficient absorption and fluorescent transitions) for Kr+ useful as a krypton ion flow diagnostic in plasmas, suitable for use by diode lasers. The krypton ion flow diagnostic clears the way for a more rigorous test of the new theory of sheath formation and will permit us to study sheath formation in more complex plasmas. One of the fundamental aspects of all plasma processing application is the plasma sheath in both DC and RF plasmas. Increased understanding of sheath formation supports a very wide range of plasma science applications ranging from ultra-large scale integrated circuit manufacture to surface preparation for medical implants such as artificial hips. Further, the ion flow diagnostic for Kr+ ions using diode laser based laser-induced fluorescence (LIF) will aid in basic plasma science experiments for Hall Thruster ion propulsion engines. Such thrusters are already in use in space for satellites, and are currently the favored concept for the first manned mission to Mars. I have been mentoring a doctoral student at UW-Madison in the department of Engineering Physics through most aspects of his thesis work. I have also been training undergraduate students at USD to do research in an area of current research in physics. My most recent student redesigned the LIF collection optics for the plasma confinement device at USD. His work was the subject of a talk and two posters given at national research conferences. But mentoring undergraduates also involves inspiring students to consider research work in physics more broadly. As the result of many conversations about current research topics in physics, this student got interested in high temperature superconductivity and completed a project involving hollow-cylindrical type II superconductors. This work was the subject of a conference presentation this past summer at the ``Beyond the First Year Physics Laboratory Conference", an AAPT/PERC Conference, Philadelphia, PA July 26, 2012. But also this work has led directly to experiments done in our undergraduate physics curriculum, in our jr/sr advanced lab course ``Experimental Modern Physics'', PHYS 480W. We now regularly do a plasma physics experiment studying ion sound waves involving the use of lockin amplifiers. In addition to the mentoring and training cited immediately above, we have been able to fund recruitment efforts for the Physics program at USD, which have been successful at attracting women and minorities to become physics majors, through funds made available to us stemming from the grant.

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University of San Diego
San Diego
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