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Achievement of robust high-efficiency 1 MW oscillation in the hard-self-excitation region by a 170 GHz continuous-wave gyrotron

Nature Physics volume 3, pages 411–414 (2007)Cite this article

Abstract

Gyrotrons are a high-power source of coherent microwave radiation1. Their oscillation mechanism is a cyclotron-resonance maser effect, in which a fraction of the rotational kinetic energy of a mildly relativistic magnetized electron beam is converted into electromagnetic energy. The most active area of gyrotron development is their potential use for heating magnetically confined fusion plasmas to the point of thermonuclear ignition. A major obstacle to this endeavour is that during high-power millimetre-wave operation2,3,4,5,6,7,8,9 competing modes and mode shifts seriously degrade a gyrotron’s stability and efficiency10,11,12,13. Here, we show that these problems can be overcome by active control of the electron-beam parameters during the oscillation. In doing so, we successfully demonstrate the robust steady-state operation of a 170 GHz gyrotron producing a continuous 1 MW output power with an unprecedented efficiency of over 55% in a hard-self-excitation region. Moreover, we find that an adjacent resonant mode previously expected to compete with and adversely affect the principal operating mode does not in fact jeopardize but rather helps this mode as a result of nonlinear effects. The result improves the outlook for using these devices for heating and instability control in future experimental fusion reactors, such as ITER14,15,16,17,18,19.

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Figure 1: Output power versus magnetic field in resonator Bc.
Figure 2: Time behaviour of TE31,8 and TE30,8 powers calculated by multi-mode simulation code.
Figure 3: Power contour of the TE31,8 mode with respect to Bc and the voltage between the control anode and cathode Vak.
Figure 4: Time behaviour of gyrotron parameters.

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Acknowledgements

The authors would like to thank T. Kariya, K. Hayashi and Y. Mitsunaka of Toshiba Electron Tubes and Devices for the gyrotron fabrication, and Yu. Ikeda and S. Komori of JAEA for their support of the experiment. We would like to thank T. Yamamoto, R. Yoshino, H. Takatsu and T. Tsunematsu for their encouragement.

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Author notes

  1. Ryutaro Minami

    Present address: Present address: Plasma Research Center, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan,

Authors and Affiliations

  1. Plasma Heating Technology Group, Japan Atomic Energy Agency (JAEA), 801-1 Mukoyama, Naka-shi, Ibaraki 311-0193, Japan

    Keishi Sakamoto, Atsushi Kasugai, Koji Takahashi, Ryutaro Minami, Noriyuki Kobayashi & Ken Kajiwara

Authors
  1. Keishi Sakamoto
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  2. Atsushi Kasugai
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Correspondence to Keishi Sakamoto.

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Sakamoto, K., Kasugai, A., Takahashi, K. et al. Achievement of robust high-efficiency 1 MW oscillation in the hard-self-excitation region by a 170 GHz continuous-wave gyrotron. Nature Phys 3, 411–414 (2007). https://doi.org/10.1038/nphys599

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