Fig. 2-24 Cutaway view of the cross section of a gyrotron

	Fig. 2-25	Outside view of the gyrotron and the output window The total length of the gyrotron is 3 m. The effective diameter of the output window is 80 mm, and its thickness is 2.23 mm. The window is made of diamond and has the water cooling structure at the peripheral edge.

From the physics point of view, high power electromagnetic waves have excellent features for heating plasmas. However, there were difficulties to generate 1 MW output per tube (wave generator). The issues are; electrode damage due to a concentration of parasitic oscillations, and heat damage of a window due to the high frequency losses in coupling the tube to the system. A high efficiency mode converter is needed to change the oscillation mode to a propagating mode of smaller loss. The heat load on the electrodes was decreased with a beam depression electrode that recovers the DC components of the electron beam for the power of the external circuit. Furthermore, we employed a developed synthetic diamond as the window material (96 mm in outer diameter and 2.23 mm in thickness). The oscillator (gyrotron) was fabricated by integrating these new technologies. Figure 2-24 shows a cross section view of the gyrotron. The gyrotron and the window are shown in Fig. 2-25. We achieved 520 kW in 6.2 seconds, and in another case, 450 kW in 8 seconds at a frequency of 170 GHz (~ 2 mm wavelength). The temperature of the central part of the window reached 150 degrees cent. This temperature is rather high. If the purity of the diamond window is increased, it is possible to generate 1 MW in continuous mode operations.

Reference K. Sakamoto et al., High Power 170 GHz Gyrotron with Synthetic Diamond Window, Rev. Sci. Instrum., 70 (1), 208 (1999).

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