IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 64, NO. 10, OCTOBER 2017 4287 RF Behavior of a 220/251.5-GHz, 2-MW, Triangular Corrugated Coaxial Cavity Gyrotron S. Yuvaraj, Student Member, IEEE , M. V. Kartikeyan, Senior Member, IEEE , and M. K. Thumm, Fellow, IEEE Abstract — In this paper, RF behavior studies of a dual regime coaxial cavity gyrotron designed for electron cyclotron resonance heating and current drive of magnet- ically confined plasmas in the future fusion reactors are presented. Considering all the design constraints, the mode pair is chosen as TE 48,30 and TE 55,34 for operation at 220 and 251.5 GHz, respectively. The interaction circuit is initially designed through the cold cavity design. A triangular corru- gated insert offers good mode selection and also reduces the localized heating problem. Single mode computations are carried out to optimize the beam parameters for the max- imum efficiency of the chosen mode pair. Time-dependent multimode simulations are carried out to conform the power in the desired mode pair and the possibility of power in the competing modes. Start-up analyses are performed before and after space-charge neutralization with nonuni- form magnetic field using nominal electron beam parame- ters obtained from magnetron injection gun calculations. These studies ensure that the continuous wave operation of a coaxial cavity gyrotron is possible with the output power of ≈2 MW with the chosen mode pair. Index Terms— Coaxial cavity gyrotron, dual regime gyrotron, space-charge neutralization, time-dependent mul- timode calculation, triangular corrugated insert. I. I NTRODUCTION G YROTRONS are the high-power millimeter wave sources that find application in fusion reactors, material processing, medical spectroscopy, and also in defense applica- tions, like active denial systems. One of the major applications of the gyrotrons is in electron cyclotron resonance heating and current drive of plasmas in magnetically confined fusion reactors. Experimental fusion reactors, like International Ther- monuclear Experimental Reactor (ITER), require gyrotrons operating at frequencies 100–200 GHz with the long pulse power of 1–2 MW [1], [2]. One of the major design goals in Manuscript received May 19, 2017; revised July 10, 2017; accepted August 16, 2017. Date of publication September 1, 2017; date of current version September 20, 2017. The review of this paper was arranged by Editor L. Kumar. (Corresponding author: M. V. Kartikeyan.) S. Yuvaraj and M. V. Kartikeyan are with the Department of Electronics and Communication Engineering, IIT Roorkee, Roorkee 247667, India (e-mail: yuvas.dec2014@iitr.ac.in; kartik@ieee.org). M. K. Thumm is with the Karlsruhe Institute of Technology, Institut fuer Hochleistungsimpuls und Mikrowellentechnik, D-76344 Karlsruhe, Germany. 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.2017.2743342 the development of megawatt class gyrotrons is to keep the ohmic wall losses of the cavity within its limit. According to the present available cooling facilities, ohmic wall loading of the gyrotron cavity should be lower than 2 kW/cm 2 for the continuous wave (CW) operation. Higher order modes are chosen for megawatt class gyrotrons, so that for the given frequency the radius of the cavity would be large, which can limit the peak wall loading of the cavity. Higher order operating mode poses severe mode competition problems and this can be overcome by introducing an insert in the conventional weakly tapered cavity resonator. The insert in the coaxial cavity also reduces the voltage depression of the electron beam and consequently increases the limiting current. Corrugations on the insert offer a better tradeoff between the mode selection and the ohmic wall loading of the insert [3]. Thus, coaxial gyrotrons would be a better choice for the future fusion reactors requiring megawatt of power at frequencies above 200 GHz. Short pulse operation of a 170-GHz coaxial cavity gyrotron for ITER has been reported by KIT with the output power of 2.2 MW and an interaction efficiency of 33% [4], [5]. Recently, studies are going on for the development of mul- tifrequency gyrotrons, since these offer a wide flexibility in experimental conditions of fusion reactors without significant increase in the construction costs [6]. A dual frequency gyrotron operating at the frequencies of 140 and 105 GHz was installed in the ASDEX-U (German Tokamak) [7]. Step tunable operation of the D band gyrotron operating in the frequency range 124–169 GHz was performed at KIT [6]. The JT 60SA (Japan Tokamak) uses a 110-/138-GHz dual frequency gyrotron, which also has an additional operational frequency of 82 GHz [8]. This paper focuses on the possibility of the dual regime operation of a megawatt class coaxial cavity gyrotron. This proposed gyrotron can be used for plasma heating, current drive, and plasma stabilization in the future fusion reactors. As the fusion community is in quest to fix the power- frequency-efficiency of the gyrotrons used in the future toka- maks, one can explore the operation of the gyrotron at 220 GHz. Another exciting area is the space power grid, where enormous solar power will be collected in the space (low earth orbits) and distributed to the earth stations using solar power satellites [9]; 220 GHz is selected for the downward transmission as the atmospheric absorption has a relative 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.