NanoPhysics of electron dynamics near surfaces in high power microwave devices and systems

Abstract

Recently, a consortium comprising the above indicated universities have been awarded a five year Multidisciplinary University Research Initiative (MURI) program aimed at providing a fundamental understanding of RF breakdown phenomena and developing novel, high current density cathodes. The start date is June 1, 2004 and this talk will focus on both the initial activities and long term directions. The above goals will be accomplished by addressing two underlying issues: "pulse shortening" caused by RF breakdown and the lack of suitable, robust high current density cathodes. RF breakdown is a phenomenon that is poorly understood with respect to both the "nano-physics" of its inception as well as the techniques to minimize its effects. Understanding it and preventing it require coordinated experimental and simulation programs. The cathodes required for so-called high power microwave (HPM) sources must provide in excess of 100 A/cm and conventional thermionic cathode technology falls short of that current density by about a factor of 5. Cathode studies will employ laser deposition techniques will the goal of achieving up to 200 A/cm emission from oxide and scandate based thermionic cathodes at relevant lifetimes. Both advanced field emitter array (FEA) and carbon nanotube (CNT) based field emitter arrays will also be developed, along with integrated active control elements, for higher current density cathodes. We will employ two kinds of nanotube structures for this purpose: high-density vertically aligned nanotube towers and gated individual nanotube field emitters. We will develop triode-like gated individual nanotube field emitters by performing controlled synthesis to grow individual vertical nanotubes at desired sites. To investigate the RF breakdown issue, we will implement a comprehensive experimental and computational program. RF breakdown experiments will be conducted in a resonant single-cell cavity, configured as a "windowtron" and fed directly by a 50-MW X-band SLAC klystron. This experimental arrangement was used in a previous MURI and was found to be much more effective than an earlier method of inserting the cavity in a resonant ring to provide the required sustained power. Computational studies will include computer modeling of the breakdown dynamics in the actual cavity geometry using a newly developed parallel processing version of MAGIC 3D. To gain clearer insight into the physics of RF breakdown, a microscopic model will be developed.