Vacuum Insulation in Negative Ion Accelerator with Long Gap and Large Surface for Fusion Application

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 m2. 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.