Fig. 2-14 Progress of JAERI's R&D on gyrotron and the status of major world developments JAERI's R&D has progressed rapidly over the past few years, approaching a level required for ITER.

Fig. 2-15 Gyrotron with energy recovery system The electron beam, after the generation of high frequency electromagnetic waves, is de-celerated by a decelerating electric field before arriving at the collector, and the de-celerated beam energy is recovered by the power supply system, which results in an improvement of gyrotron efficiency.

R&D on plasma beating and current drive technologies is being carried out extensively world-wide as an indispensable tool for the tokamak fusion reactor. Among the various technologies being developed, the use of high-frequency electromagnetic waves with a range of frequencies of electron cyclotron heating (100 ~ 300 GHz) is expected to be very appropriate for a fusion reactor, because the waves of this frequency range can be transferred using compact-sized waveguides with a high power density, and can also be handled as optical beams. A gyrotron is a source of electromagnetic waves (in this specific range of frequencies) which has been developed for many years, but it has remained very difficult to develop a high-powered gyrotron with a high generating efficiency. At JAERI we have recently found a way to a high-powered and a high efficiency gyrotron by aiming at the recovery of electron beam energy after its use for the generation of electromagnetic waves. The principle is simple, the beam energy is recovered by applying a decelerating electric field to the electron beam as shown in Fig. 2-15. The decelerated electron beam energy is recovered by the power supply system electrostatically, and this improves the overall gyrotron efficiency. The experimental results have shown that the overall efficiency is significantly improved from about 30 % to 50 %. Thus, the use of gyrotrons reduces the power supply and cooling system required, and consequently leads to an optimization of a total plasma heating and current drive system. The energy recovery technique of a gyrotron will contribute to ITER as one of the major R&D results from Japan. The status of development of gyrotrons is summarized in Fig. 2-14.

Reference K. Sakamoto et al., Major Improvement of Gyrotron Efficiency with Beam Energy Recovery, Physical Review Letters, Vol.73, No.26, p.3532-3535 (1994).

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