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Long pulse acceleration of hydrogen negative ion beams with the power density over 70 MW/m2 and the pulse length over 100 s has been demonstrated for the first time by using a multi-aperture 3-stage accelerator. Such long pulse acceleration was achieved by integrating the design of beam optics and voltage holding capability to meet the requirements of JT-60SA. By using the newly designed accelerator for JT60SA, voltage holding at 500 kV with beam acceleration was stably sustained even after 5 g of cesium was seeded, and heat load on each acceleration grid was reduced below the allowable level for long pulse, less than 5% of total acceleration power. As a result, 500 keV, 154 A/m2 for 118 s beam acceleration was achieved, which satisfies the requirement of the negative ion source for JT-60SA. This pulse length of such high-power density beams is longest in the world. In addition, the result contributes to the long pulse acceleration of multi-stage electrostatic accelerators, such as 1 MeV negative ion accelerator for ITER.

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)

This article introduced design work of NBI Protection Plates (NPP) of JT-60SA. The neutral beam is partially re-ionized by collision with environmental gas molecules, and the ionized beam is bended by the magnetic field, resulting in heat flux on the wall of the NBI duct, which includes NBI gate valve, NBI side bellows, port extension, vacuum vessel (VV) side bellows and port. The heat flux is estimated to be 0.5-3MW/m2 in the port and 0.2MW/m2 in the other areas, and its duration is 100s. Considering the long-pulse high heat flux, NPP was installed in the NBI duct as a water-cooled thinner duct. Three types of NPP were designed for three types of NBI in the JT-60SA, i.e., perpendicular NBI (P-NBI), tangential NBI (T-NBI) and negative-ion NBI (N-NBI). The NBI duct was separated into three parts connected loosely with each other by two bellows. The designed NPP was separated into two or three parts along the beam line, which were overwrapped securing a sufficient gap considering the relative displacement with each other. The protection plates in the port area were designed as sets of narrow (~15cm) and long (~2m) cooling plates surrounding the neutral beam, to reduce the electro-magnetic force due to eddy current at the moment of the plasma disruption. The protection plates in other area were designed as sets of conical or columnar copper plates bonded to cooling pipes considering low magnetic field. Even the space in the NBI duct was very limited, the inner diameter of NPP was designed to be sufficiently large not to scrape the neutral beam. Considering heat loads from the neutral and re-ionized beam, electro-magnetic force due to eddy current and acceleration due to earthquake and disruption, numerical analyses were carried out and it was confirmed the design of the NPP was reasonable., 31st Symposium on Fusion Technology (SOFT2020)

ITERやJT-60SAといった、大電流の中性粒子入射加熱装置において、メートル級の大面積電極から負イオンビームを長時間加速する際の課題の一つは、負イオン生成部内の負イオンの非一様性により、各場所での負イオン電流が不均一となり、ビーム発散角や電子電流が増加し、電極での熱負荷が過大となることである。本研究では、QSTにある大型の大電流負イオン源をモデルとし、1次電子軌道シミュレーションを用いて、これらの課題を改善する負イオン生成部の新規フィルター磁場配位の検討を行った。 解析では、既フィルター磁場配位における負イオン不均一性の原因として、負イオンを破壊する高速の一次電子が負イオン引出し部に漏洩するパスがあることを突き止めた。磁場配置検討の結果として、この一次電子の漏洩を改善する磁場配置について報告する。, The 7th international symposium on Negative ions, Beams, and Sources (NIBS' 20)

In multi-beamlet negative ion accelerators for neutral beam injectors, the transverse magnetic field necessary for suppressing the co-extracted electrons induces a deflection of the negative ion beamlets that must be corrected. For the design, particle-tracing simulation codes are used to compute ion trajectories and optical properties of the beamlets in the acceleration stage. In these codes, uniform boundary conditions are normally assumed for the ion current density distribution at the surface (called meniscus), where negative ions are extracted from the plasma and form beamlets, which are accelerated across the apertures of the accelerator grids. Recently, experimental campaigns dedicated to the accurate measurement of the beamlet deflection in the acceleration phase revealed higher deflection than foreseen by simulations. In this work, we demonstrate that an agreement with the experimental data can be obtained by incorporating in the numerical simulations a non-radially symmetric distribution of the ion current density extracted across the meniscus surface. In the first part, the asymmetry of the ion current density at the meniscus is studied in an empirical way, by analysing and fitting the experimental results obtained with different operating parameters. In the second part, we show that the ion current asymmetry estimated by this procedure is well consistent with the flow pattern of H− ions calculated in the meniscus zone using a detailed particle in cell (PIC) model of the ion source in the presence of transverse magnetic field.

The predictions obtained using newly developed numerical models, based on consolidated beam acceleration codes (Opera, COMSOL, SLACCAD, EAMCC3D), have been compared to the experimental measurements obtained on the MeV ion source Test Facility (MTF). MTF is a high voltage negative ion beam accelerator, dedicated to fusion experiments in QST, Naka, Ibaraki, Japan. During 2019, MTF has been operated in conditions comparable to those of the future ITER NBI, for a long duration of the beam (around 100 s), in a three- or five-stage multi-aperture accelerator configuration, with high accelerating energy and good optics. These experiments allowed us to measure the beamlet divergence and deflection as well as the heat loads due to particle impinging on the accelerator grids, over a wide range of operating parameters. In the calculations, a new hypothesis on the ion current density profile at the beamlet meniscus is assumed, as indicated by recent studies that correlate the non-uniform extraction of negative ions to the strength of the applied transverse magnetic field. As a result, when the ion current distribution is non-symmetric with respect to the beamlet axis, the beamlet trajectory changes considerably. The good agreement obtained between measurements and beam optics simulations strengthens the credibility of this hypothesis. The heat loads on the accelerator grids predicted by simulations are also compared to the heat loads measured by water calorimetry during experiments. The differences still existing concerning the heat loads are then discussed in detail.

This paper describes the trans-river migration of the Iban and Bekatan to the Tatau River Basin in Bintulu Division, Sarawak. Until the beginning of the nineteenth century, The Iban and Bekatan did not inhabit the Tatau River Basin, but migrated there from other river basins from the middle of the nineteenth century onward. We describe the diversity of factors, routes, and forms of migration of each community. The data for analysis is based on oral histories documented in five Iban and two Bekatan longhouses in August 2011. We discuss how each longhouse has experienced splits and integration with various groups, and how the current geographical village distribution has been formed.