A new gyrotron operation technique to increase oscillation efficiency was developed using the JT-60 electron cyclotron range of frequency (ECRF) system. In order to increase the efficiency without a significant increase in the anode leakage current by trapped electrons that limits the efficiency of gyrotrons with a collector potential depression technology, we actively optimized the electron pitch factor by controlling only the anode voltage within ∼0.1 s after the start of the gyrotron operation, and a high-efficiency oscillation in the so-called hard-self-excitation region was achieved from the start of the oscillation. As an application of this technique in the JT-60 ECRF system, the gyrotron output power of 1.5 MW for 4 s, which is the longest pulse length in the world at an output power of 1.5 MW, was recorded with a successful reduction in the collector heat load by 20% as compared with the conventional operation. The reduced collector heat load at the 1.5 MW operation was acceptable for a steady-state operation. Further progress was made with respect to the expansion of the long-pulse capability of the ECRF system. A new gyrotron with an improved mode converter was developed in order to demonstrate a reduction in the stray radiation in the gyrotron; such radiation has thus far hindered long-pulse operations by causing an unacceptable heat load. We confirmed that the stray radiation was reduced to 1/3 of that of the original gyrotron; this reduced heat load is acceptable for steady-state operation. A conditioning operation of the improved gyrotron proceeds up to 31 s at 1 MW. This progress significantly contributes to an enhancement in the high-power and long-pulse capability of the ECRF system used in JT-60SA, where a total output power of 9 MW for 100 s is planned.
A gyrotron enabling high-power, long-pulse oscillations at both 110 and 138 GHz has been developed for electron cyclotron heating (ECH) and current drive (CD) in JT-60SA. Oscillations of 1 MW for 100 s have been demonstrated at both frequencies, for the first time in the world as a gyrotron operating at two frequencies. The optimization of the anode voltage, or the electron pitch factor, using a triode gun was a key to obtain high power and high efficiency at two frequencies. It was also confirmed that the internal losses in the gyrotron were sufficiently low for expected long pulse operation at the higher power level of ∼1.5 MW. Another important result is that an oscillation at 82 GHz, which enables use of fundamental harmonic waves in JT-60SA while the other two frequencies are used as second harmonics waves, was demonstrated up to 0.4 MW for 2 s. These results of the gyrotron development significantly contribute to enhancing the operation regime of the ECH/CD system in JT-60SA.
Development of an electron cyclotron (EC) wave system was conducted in an effort to achieve the capability required in JT-60SA. Pulse duration at 1 MW output was extended to 31 s. Transmission line components with a diameter of 60.3 mm were installed in 2011 to reduce temperature rise during a gyrotron oscillation. Development of a dual-frequency gyrotron was started to enable heating and current drive in the core region of the JT-60SA plasma for the toroidal field of 2.3 T. The EC wave frequency was chosen to be 138 GHz to meet the requirements of physics experiments and gyrotron design. Fabrication of the gyrotron began in 2011.
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