1 FIP/2-2Rb Development of dual frequency gyrotron and launcher - PDF document

1 FIP/2-2Rb Development of dual frequency gyrotron and launcher for the JT-60SA ECH/ECCD system T. Kobayashi 1 , S. Moriyama 1 , K. Yokokura 1 , M. Sawahata 1 , M. Terakado 1 , S. Hiranai 1 , K. Wada 1 , Y. Sato 1 , J. Hinata 1 , K. Hoshino 1 , A. Isayama 1 , M. Saigusa 2 , K. Kajiwara 1 , Y. Oda 1 , R. Ikeda 1 , K. Takahashi 1 , K. Sakamoto 1 1 Japan Atomic Energy Agency (JAEA), Naka, Ibaraki 311-0193 Japan 2 Ibaraki University, Hitachi, Ibaraki 316-8511 Japan E-mail contact of main author: kobayashi.takayuki@jaea.go.jp Abstract . The development of a gyrotron and a launcher operated at two frequencies, 110 GHz and 138 GHz, has made a significant progress toward electron cyclotron heating (ECH) and current drive (ECCD) in JT-60SA. Oscillations of 1 MW for 100 s were achieved at both frequencies, for the first time in the world as a dual- frequency gyrotron, by optimizing electron pitch factor using a triode electron gun resulting in the fulfillment of the target output power and pulse length for JT-60SA. In high-power experiment, low diffraction loss and cavity Ohmic loss enabling 1.5 - 2 MW for several seconds were confirmed. Oscillations at 82 GHz were also demonstrated providing an additional frequency for the use of fundamental harmonic waves at the toroidal magnetic field of ~ 2.3 T. In addition to the above results of the gyrotron development, the ECH/ECCD launcher and polarizer developments toward dual-frequency operations were carried out. Quasi-optical design of the launcher showed not much difference in the poloidal beam width for these frequencies. Prototype test of a wide- band polarizer for operations at the above frequencies at low power (< 1 mW) showed that almost arbitrary elliptical polarization can be generated. The Ohmic loss of the prototype polarizer obtained in a high-power (~0.25 MW, 3s) test qualitatively agreed with the design. 1. Introduction High-power, long-pulse gyrotrons are required for the JT-60SA ECH/ECCD system which has the total injection power of 7 MW and the pulse duration of 100 s using 9 gyrotrons [1]. The millimeter wave frequency in the original specification is 110 GHz, which is effective for off-axis ECH/ECCD to sustain a high-beta plasma at the toroidal field, B t , of ~ 1.7 T. On the other hand, the higher frequency waves at 130 ~ 140 GHz enable ECH/ECCD in the core plasma region at the maximum B t of 2.3 T in JT-60SA. An application of a dual-frequency gyrotron has an advantage as demonstrated in ASDEX-U [2, 3] since it enables to realize a dual-frequency ECH/ECCD system, in which the number of gyrotron (including transmission lines and power supplies etc.) and the total injection power are unchanged from the original single-frequency system. However, a dual-frequency gyrotron that can be operated at the target output power and pulse length of JT-60SA (1 MW for 100 s) at two frequencies did not exist [4]. Simultaneous realization of a high oscillation efficiency to obtain high output power and low diffraction loss to achieve long pulse at both frequencies is a key to develop a dual- frequency gyrotron. In order to extend the operation regime of the ECH/ECCD system in JT-60SA, we have developed a dual-frequency gyrotron (110 GHz, 138 GHz) equipped with a triode type electron gun, which enabled to obtain high oscillation efficiency at both frequencies. In parallel to the gyrotron development, a quasi-optical characteristic of the ECH/ECCD launcher to be installed into the JT-60SA tokamak was numerically studied in order to clarify

2 FIP/2-2Rb the poloidal beam focusing property operating at two frequencies. Moreover, the injected wave needed to have an elliptical polarization suitable for the efficient coupling to the pure X-mode in target plasma. In order to produce arbitrary elliptical polarizations at two frequencies, a development of a wide band polarizer was started in collaboration between Japan Atomic Fig. 1. Progress in high-power, long-pulse operations of the Energy Agency (JAEA) and newly developed dual-frequency gyrotron for JT-60SA. The Ibaraki University. target value for JT-60SA (1MW, 100 s) was achieved at both frequencies (110 Hz, 138 GHz). In this paper, we describe the results of the dual-frequency gyrotron development, design and mock-up tests of the ECH/ECCD launcher and the wide band polarizer. 2. The dual-frequency gyrotron development An application of a dual-frequency gyrotron for the JT-60SA ECH/ECCD system was discussed in 2010 [5]. Then, the detail design of the dual-frequency gyrotron was carried out in JAEA in 2011 [6], and the gyrotron was fabricated in Toshiba Electron Tube and Devices Co., Ltd. in 2011-2012. An initial short pulse commissioning result in early 2012 was reported in the last IAEA FEC [7]. Since the last conference, optimizations of gyrotron operation parameters and conditioning operations toward high-power, long-pulse oscillations have been carried out as shown in Fig .1. As a result of this work, the target values for the dual- frequency gyrotron (110 GHz and 138 GHz, 1 MW, 100 s) of the JT-60SA ECH/ECCD system have been fully satisfied. 2.1. Design of the dual-frequency gyrotron In the dual-frequency gyrotron design, the selection of TE mn modes and frequencies are one of the important issues, because design parameters of the electron gun, cavity, mode convertor, and output window are closely related to each other through eigenvalue of the mode  ’ m , n and  m -1,1 and the frequency, f , or the wavelength,  c / f . Here  mn is an n -th root of derivative of the m -th order Bessel function, and c is the speed of light. The key parameters on the dual- frequency gyrotron design to be satisfied at both frequencies simultaneously are the cutoff frequency of the cavity, f cutoff = c  mn / 2  a , the thickness of the output window, d w = n w  w / 2, the electron beam radius in the cavity to get maximum coupling efficiency, r b =  m -1,1  / 2  , the transverse radiation angle in the mode convertor,  = 2 cos -1 ( m /  mn ), where a , n w , and  w are the radius of the cavity, an integer and the wavelength in the window material, respectively. We found that the parameters shown in Table 1 were the best combination for operation at the frequencies of 110 GHz and 138 GHz for this gyrotron by careful consideration of each parameter and numerical calculations [6]. The shape of cavity resonator was optimized for chosen operating modes. The output power of higher than 1 MW