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A scaling technique has been developed in order to predict the voltage holding capability of the world-largest electrostatic accelerator for fusion application having a long gap vacuum insulation around 1 m with large surface area of more than 6 m 2 . One of critical issues for design of such large-size accelerator is the development of the prediction model based on the experimental results. For this purpose, the 1/5 scale mockup has been developed, which is composed of the 5-stage accelerator with the high voltage bushing in the vacuum vessel. Direct comparison of the sustainable voltages in the 1/5 scale and full-scale high voltage busing only shows that a voltage scaling in 1/5 scale size is 55%. It was found that the voltage scaling is consistent with the prediction based on the empirical scaling according to the surface area and gap length. As a result, the voltage holding capability of the 1/5 scale mockup is 400 kV in the experiment, which indicates an estimation of 720 kV in full-scale with a real configuration. This is the first experiment-based prediction of the voltage holding capability for the largest accelerator for ITER. The improvement of the voltage holding capability has been already planned by using multi-layer electrostatic shields. In such experiment, the developed scaling technique is useful to predict the voltage holding capability for large vacuum components which can not be easily tested.

The Large Helical Device is the world's largest heliotron/stellarator as well as the largest superconducting fusion device. Its construction started in 1990, and the plasma experiments started in March 1998 as planned, owing to strongly supported by concentrated R&D activities for superconducting magnets, heating devices and diagnostics. In 15 years since the initial operation, considerable efforts have been devoted to technology R&D activities in the areas of cryogenic engineering, plasma-facing components, heating devices, and diagnostics. These efforts have contributed to the advancement of plasma parameters toward the target values. The obtained results as well as the experiences should contribute to ITER and the next fusion reactors.