@misc{oai:repo.qst.go.jp:00082531,
 author = {Murayama, Masamichi and Kojima, Atsushi and Shimabukuro, Yuji and Ichikawa, Masahiro and mae quinones Saquilayan, Glynnis and Hiratsuka, Junichi and Watanabe, Kazuhiro and Tobari, Hiroyuki and Kashiwagi, Mieko and Masamichi, Murayama and Atsushi, Kojima and Yuji, Shimabukuro and Masahiro, Ichikawa and mae quinones Saquilayan, Glynnis and Junichi, Hiratsuka and Kazuhiro, Watanabe and Hiroyuki, Tobari and Mieko, Kashiwagi},
 month = {Oct},
 note = {Five-stage and large beam source (BS) with 3 m x 3 m is designed to generate 1MeV, 40 A negative ion beams for the ITER neutral beam injector (NBI). The unique point of this BS is to be installed in a vacuum vessel with 5 m x 5 m. However, it was observed that the voltage holding capability over 1 m long gap was saturated around 700 kV, and then, inserting the intermediate screens to divide the long gap to the stack of short gap was effective to achieve 1MV. Since vacuum insulation of -1 MV is a critical issue for the ITER BS, it can be expected that inserting the electrostatic screens is the major countermeasure to realize 1MV in the vacuum. To proceed this design, the voltage holding test has been performed by using a 1/5 scale model which can simulate the corners and the surface area effect precisely. 
This time, in order to identify a weak point of the BS as a first step, the breakdown position is investigated by voltage holding tests by using this 1/5 scale model. The voltage holding tests under a vacuum pressure of 0.1 mPa were carried out by increasing the applied voltage in 1kV step in each 10 s up to a breakdown occurs. To identify the breakdown position, the wall as the BS vessel model was divided several parts to measure the dark current separately. As a result, the peaked current at the saturated breakdown voltages was mostly observed between the flat plane at the top of the BS and the backside wall of the BS vessel. The breakdown position was also observed by visible cameras. As a result, it was found that the breakdown position was identified to occur at the flat plane of the BS, which is the largest plane of the BS. According to the empirical scaling developed in the past experiments with simple geometry, the voltage holding capability in this position was predicted to be 424 kV, which agreed within 10\% of the experimental results, 387 kV. Thus, it can be confirmed that this empirical scaling is available to design to insert the electrostatic screens even in 3D geometry. This result strongly contributes to realize the 1MV vacuum insulation for the ITER BS., The 29th International Toki Conference (ITC29)},
 title = {Voltage holding tests to realize the vacuum insulated beam source for the ITER NBI},
 year = {2020}
}