Progress of high power gyrotron development in JAEA

A 1 MW 170 GHz long pulse gyrotron is required for a power source of the electron cyclotron heating and current drive (EC H&CD) system on ITER (International Thermonuclear Experimental Reactor). In the ITER gyrotron development of JAEA (Japan Atomic Energy Agency), a stable 1 MW 170 GHz oscillation has been achieved at CW-relevant pulse duration (800 s), which is a twice of burning time of ITER plasma. The efficiency was 55% with the depressed collector with the precise optimization of the oscillation parameters in the hard self-excitation region during the oscillation. The power balance is measured calorimetrically, i.e., output power from the window is 1020 kW, power deposition to the collector is 742 kW, and a stray radiation output from the relief windows is 24 kW. Total ohmic-loss power into the inner components of the gyrotron is 63 kW. The power dissipation at the cavity agreed with the design value. The attained maximum efficiency in the long pulse mode is ~60%. The gyrotron has been working since March 2006 without major trouble, and records ~150 GJ of the output energy. The results satisfy the ITER requirement. As the frequency of 170 GHz is expected for the maximum toroidal field of 5.3 T at the plasma center of ITER, it is useful to estimate a frequency tunability of the gyrotron for the operation at lower toroidal filed. A simulation indicates that a similar performance is obtained at ~137 GHz oscillation at TE27,6 mode with the 170 GHz/TE31,8 mode oscillation. Both modes penetrate the diamond window of 1.853 mm in thickness. The triode magnetron injection gun (MIG) generates the rotational electron beam of low velocity divergence for both operation parameters. As for a fast frequency control, which will be useful for profile control in the fusion plasma, a He-free magnet with an additional super conducting sweeping coil was developed. A diameter of a room temperature bore is 240 mm, and the 7 T at the center. Using a commercially available standard DC power supplies, the magnetic field sweeping was demonstrated with a speed of 0.4 T/10 sec at 7 T.