4686 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 64, NO. 11, NOVEMBER 2017 Study of Electronic Switching Between Multiple Backward-Wave Modes in a W-Band Extended Interaction Oscillator Liangjie Bi, Yong Yin, Changpeng Xu, Zhang Zhang, Zhiwei Chang, Fanbo Zeng, Ruibin Peng, Wen Zhou, Abdur Rauf, Safi Ullah, Bing Wang, Hailong Li, and Lin Meng Abstract — The performance of electronic switching between multiple backward-wave modes is studied in a designed extended interaction oscillator (EIO) based on a ladder circuit with finite number of periods to overcome electronic tuning range limits of EIOs operated in standing- wave mode. The dispersion characteristic of the circuit with finite number of periods, which is constructed by a series of discrete modes, is investigated. The mode separation is analyzed and reduced to support continuous switch- ing between multiple different modes by increasing the number of periods as compared with the standing-wave EIO approach. An output circuit is designed to extract the power of backward wave. The electronic switching between nine backward-wave modes has been achieved by changing the beam voltage from 4.1 to 10.5 kV, where the maximum output power over 58 W is obtained at 5.3 kV from the sim- ulation prediction. The EIO can operate over an electronic tuning range of 3.53 GHz from 89.65 to 93.18 GHz in ensuring the output power no less than 20 W. This technique can be extensively applied to increase operating band for extended interaction klystrons (EIKs) and electronic tuning range for EIOs, making them more suitable for many potential applications. Index Terms— Backward-wave mode, electronic switch- ing, electronic tuning range, extended interaction oscilla- tor (EIO), vacuum electronics. I. I NTRODUCTION M ILLIMETER wave and Terahertz (THz) vacuum elec- tronic device (VED) sources are of great interest in many potential applications including high data rate commu- nications, high-resolution imaging, airborne, and deep space research [1]. As a kind of high-frequency VED source, Manuscript received June 11, 2017; revised July 29, 2017, August 24, 2017, and August 31, 2017; accepted September 1, 2017. Date of publication September 14, 2017; date of current version Octo- ber 20, 2017. This work was supported in part by the National Natural Sci- ence Foundation of China under Grant 61671116, in part by the National Key Basic Research Program of China under Grant 2013CB933603, and in part by the Fundamental Research Funds for the Central Universities under Grant ZYGX2015J037 and Grant ZYGX2015J039. The review of this paper was arranged by Editor L. Kumar. (Corresponding author: Yong Yin.) The authors are with the Vacuum Electronic National Labora- tory, School of Physical Electronics, University of Electronic Sci- ence and Technology of China, Chengdu 610054, China (e-mail: yinyong@uestc.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2017.2749878 extended interaction oscillators (EIOs) combine the merits of klystrons and coupled-cavity traveling-wave tubes (TWTs) to achieve high peak power, efficiency, and reliability [2]. Furthermore, an EIO has a low requirement for quality of the electron beam. It can be driven by an electron beam generated by not only a thermionic cathode but also pseudospark dis- charge [3]–[6]. These features make the EIOs widely studied and used in a variety of radars [7], [8] and scientific instru- mentation applications, as in the case of acting as a driver to test the performance of a sheet beam EIK [9]. An EIO incorporates a slow wave resonant circuit to extend the interaction between an electron beam and circuit. To give more details, the EIKs [9]–[13] and EIOs are typically based on a ladder circuit with finite number of periods because of its high interaction impedance and high peak power capability. Such circuits were usually designed to operate in standing- wave modes, such as π mode [13], [14] and 2π mode. The 2π mode was commonly chosen as the operating mode in conventional EIOs [15]–[22], because it requires a large period of the circuit for the same operating voltage and frequency [21]. However, the output power of the EIO operated in the 2π mode drops suddenly to 0 with the increase in the voltage. This is essentially due to the change of the operating mode from 2π to 2π - 1 mode [20], [21]. It results in a fairly narrow electronic tuning range. Communications and Power Industries in Canada has manufactured the VKB 2445 series of pulsed W-band EIOs and each of them has a typical electronic tuning range of 200 MHz [23]. Berry et al. [2] showed because the EIO cavity contains multiple gaps, the frequency separation between circuit res- onances (modes) is not large, permitting the possibility for electronic switching between resonances. Moreover, the EIK bandwidth can be increased by designing and tuning the output cavity to operate with several cavity resonances. A 3-dB bandwidth of 2.25 GHz has been achieved in a W-band EIK designed for operation with multiple reso- nances [2]. By combination of mechanical tuning and elec- tronic switching between two operating modes, the EIO can operate over 12 GHz from 258 to 270 GHz with over 1 W of continuous wave power [24]. More recently, the mode- switching phenomenon in an EIO has been preliminarily studied in [25]. In this paper, we describe a designed W-band EIO based on a ladder circuit with finite number of periods to explore the 0018-9383 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.