970 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 32, NO. 3, JUNE 2004 Prebunching of Electrons in Harmonic-Multiplying Cluster-Cavity Gyro-Amplifiers Yingyu Miao, Thomas M. Antonsen, Jr., Senior Member, IEEE, Gregory S. Nusinovich, Fellow, IEEE, Alexander N. Vlasov, Senior Member, IEEE, Hezhong Guo, and Victor L. Granatstein, Life Fellow, IEEE Abstract—The use of a cluster of cavities in frequency multi- plying gyro-amplifiers is described. An analytical theory has been developed to maximize the second harmonic current and optimize the drift section length for the case of a single low- input cavity, operating at the fundamental cyclotron harmonic, and bunching clustered cavities operating at the second-harmonic. MAGY simu- lations have been conducted to benchmark the theory and further study the detailed characteristics of cluster-cavity gyro-amplifiers. The theory and MAGY code simulations agree. In the small signal regime, the bandwidth of a cluster-cavity device (with a pair of cav- ities in the cluster) is twice that of a single cavity device, while both have the same peak bunching. With a gyro-TWT output section, a peak power of 247 kW, efficiency of 24.2% and bandwidth of 1.08% has been simulated using a cluster of cavities as a buncher. In ad- dition, the power-bandwidth product is 10 kW MHz, which is double that of the single cavity buncher case. We also investigate the effect of coupling between the cavities of a cluster, and the per- formance of a three-cavity cluster. Index Terms—Clustered cavities, harmonic-multiplying gyro-amplifier, MAGY, point-gap model. I. INTRODUCTION T HERE HAS been considerable interest in the develop- ment of high-power gyro-amplifiers, driven mainly by millimeter-wave radar applications. However, existing ( or 2) mode gyroklystron amplifiers have a bandwidth of less than 1%, which is limited by the cavity factor. For example, the -band gyroklystrons used in the WARLOC radar have a bandwidth of 0.64% [1]. The bandwidth of gyro-traveling wave tubes (TWTs) is greater than that of gyroklystrons, but their efficiency is lower. The Ka-band gyro-TWT developed at the Naval Research Laboratory (NRL) has an efficiency of 17% with a dB bandwidth of 1.11 GHz (3.3%) [2]. For the two-stage harmonic-multiplying Ka-band gyro-TWT developed at the University of Maryland (UMD), College Park, the predicted saturation efficiency is less than 20% [3]. Thus, there is a need to improve the bandwidth of gyroklystrons without sacrificing efficiency. Manuscript received September 30, 2003; revised December 5, 2003. This work was supported by the Department of Defense and by the Multidisciplinary University Research Initiative (MURI) for innovative vacuum electronics under Air Force Office of Scientific Research Grant F4962001528306. Y. Miao, T. M. Antonsen, Jr., G. S. Nusinovich, H. Guo, and V. L. Granat- stein are with the Institute for Research in Electronics and Applied Physics, Uni- versity of Maryland, College Park, MD 20742 USA (e-mail: miao@umd.edu; antonsen@umd.edu). A. N. Vlasov is with Science Applications International Corporation, McLean, VA 22102 USA. Digital Object Identifier 10.1109/TPS.2004.828804 Given the this background, a new gyro-device interaction circuit with clustered cavities was proposed by Guo, et al. [4], based on the concept originally developed for linear beam klystrons by Symons [5]. It is anticipated that the cluster-cavity approach will improve the bandwidth of all cavity-related gyro-amplifiers such as gyroklystrons, gyrotwystrons, and inverted gyrotwystrons (phigtrons). The use of clustered cavities in fundamental gyro-amplifiers was theoretically studied by Nusinovich, et al. [6]. In that study, an analytical theory of a cluster-cavity gyroklystron was devel- oped. The analysis showed that, using the cluster-cavity con- cept, the bandwidth of gyroklystrons can be broadened signif- icantly while the efficiency of the device can be the same as or even higher than that in conventional gyroklystrons. In ad- dition, the gain of a device of given drift section length can be increased. Alternatively, for a given gain the drift section can be shortened. In this paper, we consider harmonic-multiplying gyro-ampli- fiers. Harmonic-multiplying gyrotrons operate at lower mag- netic fields and require lower frequency drive sources than fun- damental operated gyro-amplifiers. Moreover, a previous study showed that for a multicavity gyro-device, in which the first cavity operates at the fundamental, and the second cavity op- erates at the second harmonic, the harmonic content in the elec- tron beam current and the efficiency of operation in the output circuit are higher than in the case in which the first two cavi- ties operate at the fundamental harmonic [7]. For these reasons, our study here focuses on harmonic-multiplying cluster-cavity gyro-amplifiers. The aim of this paper is to analyze the effect of the clustered cavities in a gyro-amplifier. The development of the analytical theory allows the maximizing of the harmonic current and opti- mizing of the drift section lengths. MAGY code simulations are conducted to benchmark the theory and further study the de- tailed characteristics of cluster-cavity gyro-amplifiers. II. CLUSTER-CAVITY CONCEPT The original cluster-cavity concept was proposed by Symons [5]. The diagram of a cluster-cavity device is shown in Fig. 1. Bunching is accomplished by two or more short cavities that are clustered together. The spacing of the gaps of the individual subcavities in each cluster is as close as possible without pro- ducing significant coupling between adjacent cavities. The res- onant frequencies of cavities in a cluster can be the same or dif- ferent. If they are different, frequency bands of adjacent cavities should overlap. 0093-3813/04$20.00 © 2004 IEEE