A fusible metal pebble divertor system named the REVOLVER-D2 is newly proposed here, based on the molten tin jet divertor REVOLVER-D and the pebble divertor concepts. In the REVOLVER-D2, solid tin pebble flows are used as the divertor target. Molten tin pools receive the falling tin pebbles to mitigate the impact of the heavy metal shower. All pebbles are melted in the molten tin pool and recycled into the tin pebbles again. The solid tin pebbles are formed inside the vacuum vessel by the shot tower method using silicone oil pools. This concept enables simultaneous achievement of a high heat load tolerance and a high evacuation efficiency, while mitigating the MHD effects. A high heat load can be removed by the pebble flow if the pebble size is large enough and the velocity is fast enough as with the molten tin jet. Although the charge up of pebbles affects the pebble motion due to the Lorentz force and the Coulomb force, the Lorentz force is negligible if the injection velocity is fast enough. For example, center-to-center pebble distance due to the Coulomb force will be ∼ 4 mm in the case of the injection velocity of 10 m/s if the molten tin pool is placed 2 m under the point where pebbles enter the ergodic layer.
This paper presents the progress of performance tests of ITER gyrotrons developed at QST and the design of dual-frequency (170 GHz and 104 GHz) gyrotrons to enhance various operation scenarios in ITER, such as characteristics studies of H-mode/ELM at low magnetic fields. The major achievements in developing the ITER gyrotrons are as follows: (i) manufacturing of eight ITER gyrotrons and their auxiliary components has been completed. Performance tests at QST have been progressing and four of the eight gyrotrons achieved the required test criteria-1 MW/300 s/50%, 5 kHz modulation with 0.8 MW. Excitation of LP 01 mode having a purity of 95% was also successfully demonstrated using a 50 mm-diameter waveguide transmission line, satisfying the specification. (ii) The design of dual-frequency gyrotrons capable of 1 MW continuous-wave operation, was successfully completed.
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