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8-D Fusion2017/10/02

Significant progress towards the realization of a fusion reactor
One-of-a-kind ultra-high voltage bushing has been developed for the ITER plasma heating system.

  • QST succeeds in manufacturing an innovative ultra-high voltage bushing that introduces multiple voltages of up to 1 MV simultaneously into vacuum environment from that highly pressurized by insulating gas.
  • The integration of the component technologies including manufacture of the largest ceramic ring in the world, so far developed to secure 1 MV insulation in vacuum environment, enables to produce this bushing.
  • This bushing realizes supply of 1 MV voltage to the high current electrostatic accelerator operating in the vacuum environment for the ITER neutral beam injection system, which will heat the plasma to several hundreds of millions of degrees. The successful development of this bushing is an important step towards fusion plasma experiments.
The National Institutes for Quantum and Radiological Science and Technology (QST) has developed an unconventional and unprecedented high-voltage bushing to be used in the equipment that will heat the plasma to several hundreds of millions of degrees in ITER currently under construction in the south of France. This bushing enables to supply voltages of up to 1 MV transmitted in the environment highly pressurized by the insulating gas to the high current electrostatic accelerator operating in the vacuum environment for the ITER neutral beam injection (NBI) system. This engineering feat is an important step towards ITER fusion plasma experiments. This achievement can contribute to the development of NBI system in a future fusion DEMO reactor as well as accelerators in the medical, physics and materials fields.
The high current electrostatic accelerator of the ITER NBI employs a five-stage acceleration system, with 0.2 MV for each stage, applying voltage of up to 1 MV. Conventional accelerators are placed inside insulating gas highly pressurized to ensure insulation by preventing peripheral discharge. However, in case of ITER, since the insulating gas would be ionized by radiation, causing a loss of insulation capability, it is necessary to place the accelerator in the vacuum environment in ITER. QST was required to develop a completely new bushing which consists of five conductors, each supplying 0.2 MV up to maximum voltage of 1 MV from transmission lines in environment filled with insulating gas to the accelerator placed in vacuum environment.
For the development of this new bushing, the manufacture of an insulator was the first key engineering milestone. For this milestone, the world’s largest ceramic ring, having an outer diameter of 1.56 m and standing 0.3 m tall was developed. Since the insulator also needs to function as a vacuum vessel, the braze technologies to the ceramic ring with thick Kovar was established by QST to form a seal and maintain vacuum environment.
As ordinary bushings used for leading high voltage lines into transforming stations have a single conductor passing through the center of one bushing, and five conductors would be normally required five bushings. However, in ITER it was required to pass five conductors through a single bushing due to the limited space for installation. Therefore, QST devised a compact bushing with the cylindrical conductors arranged coaxially in five layers. It was the next engineering milestone to achieve both keeping the compact size and ensuring insulation performance in vacuum environment.
QST has established the technologies to ensure the insulation of 1 MV in vacuum environment with this five coaxial cylindrical layers. Compared with conventional vacuum insulation research, the ITER bushing must insulate an exceptionally larger surface area (at least 100 times) of conductors at higher voltage (at least 10 times). Under such conditions deterioration of insulation performance was of great concern. QST investigated insulation performance in detail by varying the size of surface area and distance between the coaxial cylindrical conductors. Based on the results, insulation calculation and design of the conductor shapes were repeatedly conducted to minimize the surface area where the distance between conductors is small and finally the conductor arrangement which enables insulation of 1 MV was decided.
The bushing manufactured by integrating the insulator and five cylindrical layers of coaxial conductors successfully demonstrated 1 MV insulation, the required value of ITER. The 1 MV insulation in the vacuum environment was considered as the highest hurdle among all the components of the ITER NBI. This bushing is the fruit of many years of exhaustive R&D in engineering design achieved by integrating various new technologies and clearing this hurdle. QST transported the bushing to Europe in May of this year to be used in the Neutral Beam Injection Test Facility (NBTF). It has passed the acceptance test by ITER Organization and has officially completed the delivery.

                                   After passing acceptance test at the NBTF site in Italy:
              The QST team with IO responsible officers, local NBTF staff, and staff of the
              bushing manufacturer (Hitachi, Ltd).