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To realize dc 1 MV power supply system for neutral beam injector in ITER, long-pulse dc ultrahigh-voltage (HV) insulation technologies have been developed. For a dc 1 MV insulating transformer, a composite bushing has been newly developed to overcome the manufacturing difficulty of a conventional condenser bushing (CB). The composite bushing has a small-sized CB and fiber reinforced plastic tube to sustain 1 MV stably inside and outside of the bushing. The mock-up of the insulating transformer has fulfilled the ITER requirement of dc 1.2 MV insulation for 3600 s. An HV bushing to transmit multiple voltages simultaneously has also been developed through a careful experimental study on vacuum insulation characteristics with multiple gaps between large electrodes. It has been found that the voltage holding is dominated not by number of the electrodes but by a total cathode surface area of electrodes. Based on this result, the vacuum insulation inside the HV bushing with five electrostatic screens has been designed to realize a stable vacuum insulation of 1 MV. [ABSTRACT FROM AUTHOR]

In the framework of the program for the construction of 1 MeV–16 MW negative neutral beam injector (NNBI) for ITER, a research and development activity on voltage holding in vacuum has been initiated since 2009, aimed at supporting the design, construction, and development of the NNBI accelerator. For this purpose, the voltage holding prediction model (VHPM) previously developed has been updated. In the VHPM, the effect of the anodic electric field and the cathodic electric field on the probability of breakdown is evaluated by means of two exponents: $\alpha $ and $\gamma $. On the basis of the experimental results from different test stands and of detailed 3-D numerical simulation of the corresponding electric-field configurations, the predictions of the VHPM numerical code have been benchmarked. New exponents, $\alpha $ and $\gamma $ , have been proposed to obtain a more precise location of the weak point of the system and a better prediction of the maximum withstanding dc voltage in high vacuum. [ABSTRACT FROM AUTHOR]