Collaborative Research: Understanding Sheath Formation in Electronegative and Electropositive Multiple Ion Species Plasma

The common states of matter on Earth are solids, liquids and gases. Putting energy into gases can produce the fourth state of matter known as plasma, which can consist of electrically charged particles and neutral atoms and molecules, all acting collectively. While uncommon on the Earth, plasma makes up more than 99% of the visible matter in the universe.

Where plasma comes in contact with other material boundaries, a thin region called the plasma sheath usually forms near the plasma boundary to balance charge particle flow from the plasma to the boundary. Experiments to be performed under this collaborative award will improve our understanding of plasma sheaths and associated phenomena that is fundamental to understanding all bounded plasmas.

Plasmas to be studied in this work are similar to those used in 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, a primarily undergraduate institution; Morgan State University will support undergraduate students to collaborate with the University of San Diego and will begin developing an in-house plasma related research laboratory.

The award will also support outreach activities to promote engagement of students with diverse backgrounds in science, technology, engineering and mathematics disciplines.

Experiments to be carried out principally at the University of San Diego 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, anomalous sheath edge velocities were discovered 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, and measurements that determine the presheath and sheath plasma potential profiles with emissive probes. Ion velocity distribution functions associated with ion acceleration will be determined with tunable diode lasers using the laser-induced fluorescence technique. The award will also support transfer of cutting edge research techniques to enhance investigation of atmospheric pressure micro-plasma effects on nanoparticle fabrication at Morgan State University, to improve plasma diagnostics for use by the wider plasma science research community, and to stimulate teaching and learning in classrooms and teaching laboratories for undergraduate and graduate students at the participating institutions.

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.