Transitions Between Gas Breakdown and Electron Emission Mechanisms at Nanoscale
Abstract: Predicting gas breakdown is critical for applications requiring plasma formation, such as plasma assisted combustion and plasma medicine, and avoiding plasma formation, such as microelectromechanical systems (MEMS) and vacuum electronics. The continuing reduction in device size to nanoscale makes this characterization more important. Paschen’s law, driven by Townsend avalanche, is well known in plasma physics. For micro- and nanoscale gaps, the increased electric fields strip electrons from the cathode by field emission. These electrons ionize gas atoms near the cathode to create ions that enhance the surface electric field (and field emission current) and secondary emission. Instead of scaling with the product of pressure and gap distance, breakdown voltage instead decreases linearly with decreasing gap distance at lower gap distances.
This seminar reviews theoretical and experimental efforts to characterize gas breakdown and electron emission from microscale to nanoscale. Specifically, we will apply a matched asymptotic analysis to a theory unifying field emission and Paschen’s law to derive analytic equations showing that the breakdown voltage scales linearly with gap distance when field emission drives breakdown. Furthermore, we will use these equations to assess the transition from field emission to the classical Paschen’s law and the implications on the well-known Paschen’s minimum. We report experimental results showing that while surface roughness does not impact initial breakdown voltage, it dramatically alters breakdown voltage for subsequent breakdown events due to electrode cratering. We also show that at atmospheric pressure, nanoscale breakdown voltage decreases linearly with gap distance until ~200 nm, when the slope changes. Finally, we demonstrate that breakdown in this nanoscale regime may occur from a space-charge dominated condition rather than directly from field emission. We discuss these results in the context of “nexus theory,” which is a theoretical framework linking various electron emission mechanisms, including space-charge limited, field, thermionic, photo-, and quantum space-charge limited emission.
Bio: Dr. Allen L. Garner received the B.S. degree (with high honors) in nuclear engineering from the University of Illinois, Urbana-Champaign, in 1996. He received an M.S.E. in nuclear engineering from the University of Michigan in 1997, an M.S. in electrical engineering from Old Dominion University in 2003, and a Ph.D. in nuclear engineering from the University of Michigan in 2006. He was an active duty Naval officer from 1997 to 2003 and is currently a Captain in the Navy Reserves. From 2006 to 2012, he was an electromagnetic physicist at GE Global Research Center. He joined Purdue University in 2012, where he is currently an Associate Professor and Undergraduate Program Chair of Nuclear Engineering.
Prof. Garner received a University of Michigan Reagents’ Fellowship and a National Defense Science and Engineering Graduate Fellowship. He has been awarded two Meritorious Service Medals, the Navy and Marine Corps Commendation Medal, and five Navy and Marine Corps Achievement Medal. He also received the 2016 IEEE NPSS Early Achievement Award, 2013 Best Teacher Award, 2019 Outstanding Faculty Mentor of Engineering Graduate Students, and 2021 School of Nuclear Engineering Outstanding Research Award.