3152 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES,VOL. 53, NO. 10, OCTOBER2005 -Band Resonant Ring for Testing Components for a High-Gradient Linear Accelerator Alexandr Bogdashov, Gregory Denisov, Dmitry Lukovnikov, Yury Rodin, and Jay Hirshfield Abstract—A new millimeter-wave resonant ring using a trav- eling wave in an oversized cylindrical waveguide has been developed for high-power tests of accelerator components. Novel low-loss miter bends with flat mirrors that utilize mode mixing to minimize losses were used in the resonant ring. Low-power mea- surements show a maximum effective power gain factor that ex- ceeds 35 : 1 at the operating frequency of 34.272 GHz. Total quality factor is approximately 21 400, and the reflection coefficient from the input to the ring resonator is less than 1%. Index Terms—Accelerator RF systems, electromagnetic coupling, electromagnetic diffraction, resonators, traveling-wave devices. I. INTRODUCTION A NUMBER of components for a future -band linear accelerator are under development for use in a test accel- erator at very high powers in the range 0.1–1 GW depending on the component. This study is directed toward development of structures that can sustain an acceleration gradient approaching 200 MV/m, three times that of current -band designs. How- ever, there are currently no -band sources available at power levels approaching the gigawatt level. The most promising can- didate at this frequency, a 34-GHz magnicon, is currently under development and test [1]. This tube is designed to amplify radiation to the 40–50-MW level; it has already reached over 10 MW in 200-ns pulses after a conditioning campaign of only 2 10 pulses. It is clear that this power is much lower than that required for a full-scale component test. Major RF components of interest have high transmission coefficients. These components include barrier windows, mode converters, tapers, waveguides, miter bends, phase shifters, pumping ports, compensators of thermal expansion, and others. Transmission coefficients for these components vary from 90% to 99.5%. Such components require high power tests in order to determine their reliability, and/or to introduce corrections in their design. When a component has a high transmission coefficient, it can be tested in a microwave resonant ring fed by an available source such as the magnicon. In a resonant ring, an effective high-power operating regime can be realized with a running wave having field strengths cor- responding to a power much higher than the power available Manuscript received January 26, 2005; revised May 26, 2005. This work was supported by the U.S. Department of Energy, Office of High Energy Physics. A. Bogdashov, G. Denisov, D. Lukovnikov, and Y. Rodin are with the Institute of Applied Physics, Russian Academy of Sciences, 603950 Nizhny Novgorod, Russia (e-mail: bogdash@sandy.ru; den@appl.sci-nnov.ru). J. Hirshfield is with the Department of Physics, Yale University, New Haven, CT 06520 USA and also with Omega-P Inc., New Haven, CT 06520 USA (e-mail: jay.hirshfield@yale.edu). Digital Object Identifier 10.1109/TMTT.2005.855357 Fig. 1. Schematic of the resonant ring. from the source [2]. The goal of the research described in this paper is the development of a resonant ring, which provides a power “gain” factor in the range of 10–20 with the capability of approaching the gigawatt level at 34.272 GHz. Power gain is limited by RF losses in the ring itself, and by losses in the par- ticular component under test. The layout of the millimeter-wave resonant ring is shown in Fig. 1. The operating mode of the ring is the mode in an oversized circular waveguide. This mode has zero elec- tric field at the waveguide walls and, consequently, very low ohmic losses. This mode is a natural choice for the transmis- sion line to feed millimeter-wave accelerator structures. Fur- thermore, the most advanced window designs are based on the use of this mode. Recently a low-loss miter bend operating with this mode was developed. Experimental tests described below show the total loss in the miter bend to be approximately 1%, including ohmic and diffraction losses. The resonant ring con- sists of a directional coupler and straight waveguide runs joined by four miter bends, the latter equipped with vacuum pumping ports. For adjusting the resonant frequency of the ring to be equal to the magnicon operating frequency, a bellows-type ad- justable waveguide section is used. For the total wave run around the cavity of length as great as 6 m, the total relative loss of wave power per round-trip (including coupling losses) should not exceed approximately 10%. The factor of the resonator 0018-9480/$20.00 © 2005 IEEE