Fig.3-19 Millimeter wave plasma heating device; Gyrotron
Fig.3-20 Repetitive gyrotron operation
A gyrotron is a device that can generate a high power microwave (Fig.3-19). A rotating electron beam is generated inside the gyrotron, which has a very high vacuum. The generated beam is accelerated up to 80keV and goes through a cavity in a high magnetic field region, thereby converting its energy to rf power. A 7T super conducting magnet is used in order to generate 170GHz rf power for ITER.
At JAEA, many advanced gyrotron techniques have been developed for ITER. One of the most important developments is the energy recovery technique, which dramatically improves the energy conversion efficiency. Another breakthrough is the 1MW oscillation using an artificial diamond output window.
With these advanced techniques, in 2007, we achieved the ITER requirement, i.e., 1MW oscillation, 50% energy conversion and 500s pulse width. This proves the advantages of the gyrotron as a heating device for ITER and fusion reactors in general.
We had focused on the long pulse operation (longer than 400s). However, repetitive operation becomes also important, because cyclic operation with a pulse width of 400s at 30min intervals is planned in ITER. In the repetitive operation, the operational state of the gyrotron, such as vacuum pressure and electron gun, must recover within 30min. In particular, the electron gun requires a long time to recover, because its surface temperature varies when it releases the electrons for the rf generation. Therefore, it is important to keep the temperature change as small as possible in order to shorten the recovery time. For that purpose, active control of the magnetic field, acceleration voltage and heater current of electron gun during the oscillation was performed. As a result, as shown in Fig.3-20, 5 hours of repeated operation with a 400s/30min cycle was successfully achieved. This is the world's first long pulse repetitive gyrotron operation, and proves the feasibility of ITER.
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