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.
Plasma chemistry employs energetic electrons to achieve chemical reactions in gases that produce novel neutral atoms and molecules and electrically charged ions. The energetic electrons allow many reactions to take place that are not possible by conventional techniques. The charged ions gain energy from electric fields in non-neutral sheaths adjoining surfaces in contact with the gases. Plasma etching and deposition depend on the ion energy and on the novel ion species. In "low temperature" plasmas with electron temperatures the order of 20,000 times room temperature, neutral particle densities are much greater than ion densities (i.e., low fractional ionization) and ions undergo collisions with neutrals. For over 60 years it has been theorized that in weakly collisional plasmas containing a single positive ion species, ions needed to be accelerated to a velocity known as the ion sound velocity at the plasma sheath boundary. In previous experiments we were the first to experimentally verify this prediction. Subsequent experiments suggested that both ion species in two ion species plasmas were lost at the system sound velocity rather than at their individual sound velocities. The most significant advance during the grant period was the first experiment providing agreement with a new theory which argued that instability enhanced collisional friction between two ion species having comparable densities resulted in a common loss velocity in agreement with our past experiments but when species densities were not comparable, they were lost at their individual sound velocities. The agreement between theory and experiment was excellent. Plasma sheaths are characteristic of all bounded plasmas. Their characteristics are critical to the operation of many devices that depend on plasmas. Examples are found in materials processing plasmas, fusion plasmas, plasma used in thrusters for spacecraft, and plasmas occurring naturally in space. Most of the visible light in the universe comes from plasmas. Another important contribution was to evaluate several different techniques for determining the electric potential in low temperature plasmas using emissive probes. Emissive probes are hot wires which emit electrons when they are biased below the plasma potential. They were invented in the 1920s and are still a valuable and widely used diagnostic tool in plasma research. We determined that the limit of zero emission technique, invented 35 years ago but not widely used today, is the most accurate method of determining the plasma potential with emissive probes. This result will advance the understanding of plasma processing, of ion confinement in plasmas, and of ion thrusters in spacecraft. Plasma potentials are usually positive with respect to the boundaries. We made use of emissive probes to study two novel situations in which plasma potentials were negative. One was a consequence of strong emission of electrons from insulating layers on the boundary. The other was the result of operating at very low neutral pressure, where there was very low fractional ionization. We also began a study of an instability which has prevented the widespread use of the "Maxwell Demon," a biased grid immersed in a plasma. The "Maxwell Demon" is an inexpensive technique for increasing electron temperature which was invented in 1970 but, because of the instability, has been disregarded. It is important to be able to control the electron temperature in plasma both for basic scientific studies and also for most practical plasma applications. We are developing a better understanding of the limitations of the "Maxwell Demon," which will allow it to be used more in the future. Overall, our work will aid future basic plasma science experiments and will improve industrial plasma applications such as semiconductor device fabrication. Our research has also significantly contributed to efforts at other laboratories to develop ion thruster technology, which will one day allow mankind to walk on Mars and also to produce a new generation of space probes which will travel to the outer limits of the solar system.
