Fig. 2-10 A view of the 170 GHz, short pulse gyrotron length : 3 m, weight : 750 kg

Fig. 2-11 The output performance of the developed Gyrotron The oscillation with the higher resonance mode TE31, 8 (31 and 8 cycles in the azimuthal and radial directions, respectively). The acceleration voltage is 87 kV and the pitch constant alpha of the electron beam is 1. The magnetic field is optimized for each beam.

The electro-magnetic wave that resonates with electron cyclotron motion has remarkable features such as a relatively high energy density for the acceleration and/or heating of a plasma. However, there are some difficulties with the intense oscillator in the range of sub-millimeter waves. One of them is that the higher resonance modes mix with neighboring modes, impeding high power performance, when the higher resonance modes are used to protect the cavity wall from overheating. So one of the milestones for a good gyrotron is the achievement of a pure high resonance mode with both high power and high efficiency. A view of the 170 GHz, short pulse gyrotron developed for ITER is shown in Fig. 2-10, and the results of oscillation tests are shown in Fig. 2-11. A power of 1.13 MW for a time duration of 0.4 ms has been achieved with an efficiency of 29%, satisfying the requirements of ITER. This result is due to the high quality of the electron beam, which was obtained by optimizing the shape of the cavity and the magnetic field, and by installing a SiC ring to protect against parasitic modes. The next milestone is to get a high power for a longer time. So far the maximum capability has been limited to 525 kW for a period of 0.6 s, due to the increase in the window temperature. We are currently developing a window with small electro-magnetic wave losses and a high performance mode converter, to realize a flat power profile inside the window.

Reference K. Sakamoto et al., Stable Single-mode Oscillation with High-order Volume Mode at 1 MW, 170 GHz Gyrotron, J. Phys. Soc. jp/english/index.cgin., 65 (7), 1888 (1996).

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