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Success in High-voltage Acceleration in High-current Beam with Large Electrodes for Fusion Reactor
-Huge Contribution to Development of Neutral Beam Injector of ITER-

Dec. 22 , 2009

As a result of research and development over years, JAEA has successfully increased the voltage resistance of the ion source, the heart of the neutral beam injector for the JT-60, and the hydrogen ion beam of 3 amperes has been finally accelerated to 500 kilovolts. This is a world record of accelerating the beam of over 1 ampere to 500 kilovolts, and this not only meets a requirement for the JT-60SA, which is now under construction, but also makes a huge contribution to the development of the neutral beam injector of ITER.

The neutral beam injector is used for heating the plasma to several hundred million degrees Celsius needed for fusion reaction and also for driving current in the plasma. In order to heat and drive current in the center of the plasma efficiently, the high energy beam such as 500 kilovolts is required for the JT-60SA and 1000 kilovolts for ITER and fusion reactors in the future. Accelerating the ions with high voltage is required for generating the high-energy beam, and thus developing an ion source with high voltage resistance had been an urgent task. Although breakdown voltage characteristics in the case of using a small electrode (200 cm2) had been studied, the characteristics in the case of using a large electrode (20000 cm2), essential for generating the high current beam for the JT-60SA and ITER, had not been known well.

For that reason, this time JAEA studied the voltage resistance characteristics in vacuum between the large electrodes in detail in order to increase the voltage resistance. As a result, it was found that keeping a large distance between the electrodes was necessary for increasing the voltage resistance, because the voltage resistance characteristics with the small electrodes were significantly different from the characteristics with the large electrodes. In addition, orbits of the ion beam were calculated and the optimized distance between the electrodes were determined because there was an issue of decrease of the ion beam due to collisions of the beam to the electrodes by the weakened accelerated electric field caused by simply widening the distance between the electrodes. Accordingly, the ions were steadily accelerated to 500 kilovolts that was required for the JT-60SA.

These results help research and development of the neutral beam injector for ITER, which will have similar large accelerating electrodes. Also it is expected to promote an understanding of vacuum discharge phenomena in an academic field and create a spin-off effect for ion implantation equipment, which would be used for manufacturing semiconductor substrates expected to be grown in size.

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