Microwave-driven breakdown: from dielectric surface multipactor to ionization discharge^†

J. P. Verboncoeur¹, J. Booske², R. Gilgenbach³, Y. Y. Lau³, A. Neuber⁴, J. Scharer², R. Temkin^5
1University of California-Berkeley, ²University of Wisconsin, ³University of Michigan, ⁴Texas Tech University, ⁵Massachussets Institute of Technology

High-power microwave-driven breakdown in the vicinity of a dielectric window is examined across a wide range of conditions using theoretical and experimental treatments. At low pressures, a single-surface multipactor absorbs about 2% of the microwave energy and has a mean energy of hundreds of eV. At 10-50 Torr for L-band radiation, a transition occurs from a single surface multipactor to a detached ionization discharge. Above 50 Torr, the multipactor disappears and the discharge forms a typical sheath, with mean electron energy below 10 eV. Simple scaling laws fit results in the low and high pressure regimes for several gases. Experimental results demonstrate a variable long statistical delay time, followed by a rapid breakdown. UV illumination of the dielectric surface reduces the statistical delay time, making onset of breakdown more consistent. Experiments recently demonstrated arrays of plasma filaments aligned along electric field lines, spaced ≤ ¼ wavelengths at low pressure, with filaments coalescing into more continuous diffuse plasmas at higher pressure. A 1D drift-diffusion fluid model combined with an analytic model for EM wave propagation though plasma slabs of arbitrary profile was able to demonstrate the propagation and spacing mechanisms, including decreasing spacing with increasing microwave power, as well as the diffuse plasma transition at higher pressure.

^†Research supported by the US AFOSR via a grant on the basic physics of plasma discharges.

John P. Verboncoeur received a B.S. (1986) from the University of Florida and a M.S. (1987) and Ph.D. (1992) in nuclear engineering from the University of California at Berkeley (UCB). He currently serves as Acting Associate Dean for Research in the College of Engineering at Michigan State University (MSU).

Following appointments as a postdoctoral researcher at UCB and Lawrence Livermore National Laboratory, and as a Research Engineer at UCB, he joined the UCB Nuclear Engineering faculty in 2001, where he chaired the Computational Engineering Science Program 2001-2010. In 2011, he was appointed Professor of Electrical and Computer Engineering at MSU. His research interests are in theoretical and computational plasma physics and applications. He has authored/coauthored over 300 journal articles and conference papers, with over 2500 citations, and has taught 13 international workshops and mini-courses on plasma simulation.

Prof. Verboncoeur is a fellow of the IEEE, a member of the American Physical Society Division of Plasma Physics, and presently serves as an Associate Editor for Physics of Plasmas. He served as the Technical Program Co-Chair for the PPPS 2013, and is President-Elect of the IEEE Nuclear and Plasma Sciences Society.