IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 47, NO. 5, MAY 2019 2039 Design, Fabrication, and Cold Testing of a Ka-Band kW-Class High-Bandwidth Dielectric-Loaded Traveling-Wave Tube Evgenya I. Simakov , Senior Member, IEEE , Bruce E. Carlsten , Fellow, IEEE , Frank L. Krawczyk, Kimberley E. Nichols, William P. Romero, and Muhammed Zuboraj, Member, IEEE Abstract—This paper describes the design, fabrication, and initial testing of a Ka-band dielectric-loaded traveling-wave tube (TWT) with very high bandwidth. Applications for high-power mm-wave sources span radar (including imaging), biology, medicine, communications, national security, and other areas. Specifically, to achieve long-range, cm-scale imaging using synthetic aperture radar techniques, a radio-frequency (RF) source with an average power level of 1 kW and a bandwidth of 10 GHz will be required. We developed a novel Ka-band TWT architecture that approaches these requirements. To achieve a very wide bandwidth, we proposed to use a dielectric-lined waveguide slow-wave structure. A dielectric constant of larger than 13 is needed for the resonance with a 20-keV electron beam. In our design, we have used ε = 20 magnesium–calcium– titanate (MCT) ceramics. The two halves of the dielectric are shaped to ensure that the TM 11 -like mode has a flat electric field profile along the beam slot to accommodate transport of a 5-A sheet electron beam. The gain for the structure peaks at 33.25 GHz and is predicted to be 2.3 dB/cm with a total gain of 30 dB. The structure was fabricated and cold tested. Although the results of the cold test were inconclusive, we discuss possible reasons for discrepancies between simulations and measurements and propose simplifications to the tube’s geometry for future studies. Index Terms—Electron tubes, high-power microwave gener- ation, K-band, microwave amplifiers, millimeter-wave devices, vacuum electronics. I. I NTRODUCTION N OVEL higher frequency, traveling-wave tubes (TWTs) are achieving new levels of power [1]–[5]. However, further power increases require increases in beam current that leads to space charge effects, limiting beam transport at W-band and higher frequencies. Meanwhile, there is a growing interest in improving the resolution of sensing systems to cen- timeter (cm) scale at millimeter-wavelengths, which requires high power (at least 1 kW) and bandwidth of the TWTs in excess of 10 GHz, which is especially challenging at lower millimeter-wave frequencies such as Ka-band. While helix Manuscript received October 12, 2018; revised February 21, 2019; accepted March 27, 2019. Date of current version May 8, 2019. This work was supported by the Department of Energy through Laboratory Directed Research and Development at Los Alamos National Laboratory. The review of this paper was arranged by Senior Editor S. J. Gitomer. (Corresponding author: Evgenya I. Simakov.) The authors are with the Los Alamos National Laboratory, Los Alamos, NM 87545 USA (e-mail: smirnova@lanl.gov). 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/TPS.2019.2911903 Fig. 1. (a) Geometry of the Ka-band dielectric-loaded TWT structure. (b) Cross section of the TWT structure with relevant dimensions. TWTs provide the required bandwidth at Ka-band, their output power is limited to below 100 W. The coupled-cavity TWTs and folded waveguide TWTs may achieve the required power levels, but their bandwidths are typically below 10 percent (3 GHz). In order to simultaneously achieve high power and broadband operation, multi-beam approaches and cascaded TWTs were proposed [6]. We propose a novel Ka-band TWT architecture that may approach the high-power and large-bandwidth requirements for the high-resolution imaging. In this new architecture, the slow-wave structure is represented by a dielectric-lined waveguide, which naturally makes it a very wide-bandwidth structure. This paper is organized as follows. In Section II, we present the electromagnetic design of the structure including the beam interaction section and the couplers. In Section III, we describe gain computations with CST Particle Studio [7]. Section IV outlines the steps of the fabrication process. Section V reports the results of the cold testing. II. ELECTROMAGNETIC DESIGN OF THE DIELECTRIC- LINED TRAVELING-WAVE STRUCTURE The geometry of the proposed TWT structure is illus- trated in Fig. 1. The dielectric cladding which is close to 1 mm thick is placed inside of a double-ridged waveguide. The ceramic material is chosen with a relative permittivity of 20 to support a slow radio-frequency (RF) wave with a phase velocity of 0.223c, which can efficiently interact with a 20-keV electron beam. The structure is designed such that the dielectric sections are raised on both sides to ensure a flat profile of the TM 11 -like mode, so a sheet electron beam could be used to provide more power than would be possible 0093-3813 © 2019 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.