Design of a CW 1 THz Gyrotron (Gyrotron Fu Cw III) Using a 20 T Superconducting Magnet La Agusu & T. Idehara & H. Mori & T. Saito & I. Ogawa & S. Mitsudo Received: 25 February 2007 / Accepted: 9 March 2007 / Published online: 28 March 2007 # Springer Science + Business Media, LLC 2007 Abstract Design of a CW 1 THz gyrotron at second harmonic operation using a 20 T superconducting magnet has been described. The mode competition analysis is employed to investigate operation conditions of second harmonic mode, which is being excited at the frequency ranging from 920 GHz to 1014 GHz. The output power up to 250 watt corresponding to the efficiency of 4.16 percent could be achieved by using an electron beam with accelerating voltage 30 kV and current 200 mA. The important advantage of this gyrotron is that the single mode excitation at second harmonic, and extremely high frequency of the radiation, could be maintained even at high currents. It opens possibility to realize a high power radiation source at 1 THz. Such gyrotron is under construction at FIR Center, University of Fukui. Keywords Gyrotron . Terahertz . Submillimeter wave . CW operation . Second harmonic . Mode competition 1 Introduction A gyrotron is an important source of short wavelength coherent radiation. Gyrotron development is being advanced in two ways. The major way is development of high power, millimeter wave gyrotrons for heating and current drive for fusion plasma. It is being pursued worldwide and has achieved around 1MW output power for long pulse operation (longer than several tens second or quasi CW) at the frequency of 170 GHz or 140 GHz [1–4]. On the other hand, medium power, high frequency gyrotrons are being developed in several institutions in the world [5–7]. In the devices belonging to the latter class, high magnetic field and higher harmonic operations are used for increasing the operation frequency. Such gyrotrons have already covered wide frequency range in millimeter to submillimeter wavelength region and have been applied as submillimeter wave radiation sources in many fields including plasma diagnostics [8], electron spin resonance (ESR) spectroscopy [9, 10], nuclear magnetic resonance (NMR) spectroscopy [11], new medical technology [12], and so on. Int J Infrared Milli Waves (2007) 28:315–328 DOI 10.1007/s10762-007-9215-y L. Agusu (*) : T. Idehara : H. Mori : T. Saito : I. Ogawa : S. Mitsudo Research Center for Development of Far-Infrared Region, University of Fukui, 3-9-1 Bunkyo, Fukui-shi 910–8507, Japan e-mail: laagusu@fir.fukui-u.ac.jp