Collaborative Research: Understanding Sheaths and Pre-Sheaths in Magnetized and Unmagnetized Plasmas

University Of San Diego, San Diego CA

Investigators

Abstract

This project will investigate the physical properties of a near-surface layer of a plasma, a plasma sheath, which forms when a solid body is placed into a plasma. Plasmas are gases consisting of free ions and electrons; they are abundant in the universe, have many industrial applications and are the critical component of controlled fusion research. Nearly a half of the manufacturing steps necessary to fabricate a computer microprocessor employ plasmas. The interaction of plasmas with materials depends on the region close to the surfaces known as the plasma sheath. Better understanding of sheaths will lead to improved microprocessor fabrications and might be essential in achieving controlled fusion. The tabletop experiments employed in this study are also ideal for, and will greatly contribute to, training of students in the field of plasma physics and other STEM fields. Proposed new and continuing experiments are aimed at establishing the basic properties of sheaths and associated presheaths, which accelerate ions to the sheath. The specific questions that will be addressed are: 1. What are the details of the newly recognized instability-enhanced collisional friction? 2. What happens to the anisotropy of ion velocity distribution functions (IVDFs) along the presheath and at the sheath-edge? 3. Why do experimental transverse IVDF results differ from simulations? 4. Are the ions in three species weakly collisional plasmas all lost at the system sound velocity when they have comparable densities? 5. Are there separate presheaths associated with ion-neutral collisions and oblique magnetic fields? 6. How do sheaths and presheaths at dielectric and mixed boundaries differ from those at conductors? 7. Does the sheath disappear or not when a small electrode is biased to the plasma potential? Coordinated studies of these phenomena will be carried out at the University of Wisconsin-Madison and the University of San Diego. Experiments will determine the presheath and sheath plasma potential profiles and the resulting IVDFs associated with ion acceleration. Most of the experiments will be carried out in Madison and led by the PI Prof. Noah Hershkowitz. Laser induced fluorescence (LIF) diagnostic development will be carried out on both campuses and led by the Co-PI Prof. Greg Severn. Plasma parameters will be determined with two or more techniques, e.g. combinations of emissive probes, LIF, Langmuir probes, ion acoustic wave measurements, and optical emission spectroscopy. Experiments will employ hot filament multi-dipole, capacitive, inductive, and helicon sources currently in operation.