High Power Long Pulse Injection (0.84 MW, 5 s per unit) of ECH in JT-60U
The electron cyclotron heating (ECH) system is an effective tool for the MHD instability suppression and the current profile control by local heating and current drive. In JT-60U the ECH system (110GHz, 4 gyrotrons) was equipped as shown in Fig.1 to achieve high performance plasmas. However, the overheat of electron gun of the gyrotron by parasitic RF oscillation and the large power loss in the transmission line have limited the power and pulse length of ECH operation (so far, less than 0.6 s for 0.8 MW and 2 s for 0.6 MW per unit).
Two major improvements were made in the JT-60U ECH system; (1) the overheat of electron gun was suppressed by installing RF absorber in the gyrotron. (2) the transmission efficiency of 60-70% was improved to 80% by adopting small radius corrugated waveguides, which could be evacuated. As a result, high power (0.84 MW) and long pulse injection (5 s) per unit (the world highest record) were achieved as shown in Fig.2, corresponding to the requirement for ITER ECH system (0.8 MW per unit).
The total injected power of 3 MW was also maintained for 2.7 s, which allows the world highest level central electron temperature of about 26 keV for a reversed shear plasma.
Fig. 1 Outline of JT-60U ECH System.
Fig. 2 Comparison of ECH performance.
Fig. 3 Te profile with and w/o ECH