Collaborative Research (RUI): Understanding Potential Structures and Ion Dynamics Near Sheaths in Electronegative and Electropositive Multiple Ion Species Bounded Plasma

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

The common states of matter on Earth are solids, liquids and gases. Putting energy into gases can result in the 4th state of matter known as plasma, which consists of electrically charged particles (electrons and ions, both positive and negative) and neutral atoms and molecules. While uncommon on the Earth, plasma makes up more than 99% of the visible matter in the universe. Where plasma comes in contact with material boundaries, a thin region near the plasma boundary usually forms to isolate the plasma from the boundary. This thin region is called the plasma sheath. Experiments to be performed under this award will improve our understanding of plasma sheaths and associated phenomena as fundamental to understanding all bounded plasmas. Plasmas studied in this work are similar to those used in plasma applications such as the etching of semiconducting materials to make computer chips, in plasma space propulsion satellite engines, and in experimental plasma fusion devices. The majority of experimental studies will be carried out at the University of San Diego (USD), which is primarily an undergraduate institution. The award will also support outreach activities to promote engagement of students with diverse backgrounds in science, technology, engineering and mathematics (STEM) disciplines. Experiments to be carried out at USD, in collaboration with University of Wisconsin - Madison, will address important questions associated with sheaths and the Bohm Criterion in multiple ion species plasma, both electropositive and electronegative, many for the first time. In previous work, this collaborative group discovered anomalous sheath edge velocities while testing the Bohm Criterion in two ion species plasma. This effect has now been theoretically explained via the introduction of ion-ion streaming instabilities. As a result, many different aspects of sheath formation must be reevaluated in light of the new theory. Moreover, there are as yet no such corresponding measurements for electronegative plasma with multiple ion species. This project will pursue experiments to perform such measurements, measurements that determine the presheath and sheath plasma potential profiles with emissive probes. Ion velocity distribution functions (IVDFs) associated with ion acceleration will be determined with tunable diode lasers using the laser-induced fluorescence (LIF) technique. The award will also support transfer of cutting edge research techniques to the classrooms and teaching laboratories for undergraduate and graduate students at the University of San Diego, and at the University of Wisconsin-Madison. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.