GENERIC COLORIZED JOURNAL, VOL. XX, NO. XX, XXXX 2017 1 High purity mode CW gyrotron covering the sub-THz to THz range using a 20 T super-conducting magnet Toshitaka Idehara, Isamu Ogawa, Dietmar Wagner, Member, IEEE , Manfred Thumm, Fellow, IEEE , Kosuke Kosuga, and Svilen Petrov Sabchevski Abstract — In this paper, we present the current status and both the ongoing investigation and the continuous improvements to the operational performance of a unique gyrotron, which is built using a 20 T superconducting mag- net and holds a world record of 10 W THz wave generation at the highest frequency (1.08 THz) in continuous wave (CW) operation. Additionally, it has demonstrated high- purity single-mode generation on a sequence of modes that cover a wide range from sub-THz to THz frequencies at both fundamental and second-harmonic resonances of the electron cyclotron frequency. As an illustration, the measurements of the observed radiation patterns of eight output modes radiated from a currently used resonant cavity with a linear up-taper are presented and compared with the corresponding patterns simulated by scattering matrix calculations. A new design of an optimized cavity with a nonlinear up-taper, which improves further the mode purity of the generated output radiation has been proposed and is currently being implemented as a replacement of the existing resonator. The overall operational performance and the output characteristics of this gyrotron (called FU CW III in accordance with the nomenclature adopted at FIR UF Center) make it a versatile and appropriate source of coherent CW radiation for many novel applications in the fields of high-power THz science and technologies. Index Terms— Cavity resonators, Electron tubes, Gy- rotrons, High power microwave generation, Submillimeter wave propagation, Submillimeter wave technology, Super- conducting magnets, Terahertz radiation, Vacuum electron- ics. The work was supported partially by the Special Fund for Education and Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan, and SENTAN Project of the Japan Science and Technology Agency (JST). T. Idehara (e-mail: idehara@fir.u-fukui.ac.jp), I. Ogawa (e-mail: ogawa@fir.u-fukui.ac.jp), and K. Kosuge are with the Research Center for Development of Far-Infrared Region at the University of Fukui (FIR UF), Fukui 910-8507 Japan . D. Wagner is with Max-Planck Institute for Plasma Physics, Boltzmannstr. 2, D-85748 Garching, Germany (e-mail: dietmar.wagner@ipp.mpg.de). M. Thumm is with Karlsruhe Institute of Technology (KIT), Institute for Pulsed Power and Microwave Technology (IHM), Kaiserstr. 12, 76131 Karlsruhe, Germany (e-mail: manfred.thumm@kit.edu). S.P. Sabchevski was with the Research Center for Development of Far-Infrared Region at the University of Fukui (FIR UF), Fukui 910- 8507 Japan. He is now with the Institute of Electronics of the Bulgarian Academy of Sciences, Sofia 1784, Bulgaria (e-mail: sabch@ie.bas.bg). I. I NTRODUCTION I N recent years, the advancement of the gyrotrons towards higher frequencies and output powers is accompanied by remarkable improvements in their operational performance (e.g. stable CW operation during long time intervals, con- tinuous and step-wise frequency tunability in wide bands, possibility to modulate both the power and the frequency of the generated radiation, etc.). Nowadays, it is generally recognized and commonly accepted that the gyrotrons are the most-powerful sources of coherent radiation operating in CW regime in the region of the electromagnetic spectrum ranging from the sub-THz to the THz frequencies [1]–[3]. All these advantageous features of gyrotrons have opened an avenue to many novel and emerging applications in the high-power THz science and technologies [4]–[6]. Since the gyrotrons operate at resonances that correspond to the electron cyclotron frequency (which is linearly propor- tional to the intensity of the magnetic field in the resonant cavity– about 28 GHz per Tesla) or its harmonics, in order to exceed a frequency of 1 THz at a fundamental resonance one needs a field intensity as strong as around 36 T. Such high value is beyond the capabilities of the currently available and affordable superconducting magnets. During the previous decade, in order to overcome this severe limitation at FIR UF we used a high-current pulse magnet and/or operation at the second harmonic of the cyclotron frequency. Following such an approach we succeeded to demonstrate a breakthrough reaching a frequency of 1 THz. The output power at the second harmonic operation is about 10 W, and several kW at the fundamental [7], [8]. Another breakthrough crossing of the symbolic 1 THz threshold has been demonstrated by a gyrotron with a 40 T pulsed magnet developed at IAP in N. Novgorod, Russia. Their device produces coherent radiation with a frequency of 1.022THz and an output power of 1.5 kW (energy of 75 mJ in 50 μ sec pulses) [9]. Most of the applications, however, require long-pulse (at least several seconds) or CW radiation. Among them are such advanced spectroscopic methods as DNP-NMR (Nuclear Magnetic Resonance with a signal enhancement through Dy- namic Nuclear Polarization) [10]–[16], ESR (Electron Spin Resonance) spectroscopy [17], [18], pump-and-probe tech- nique based on XDMR (X-Ray Detected Magnetic Resonance)