M. Taniguchi

XPS and UPS studies on electronic structure of Li2O

Abstract The adsorption behavior of H2O on Li2O was studied by X-ray photo electron spectroscopy (XPS) and ultraviolet photo electron spectroscopy (UPS). XPS and UPS spectra of Li2O single crystals which were exposed to different pressure of H2O vapor were observed. In O(1s) region, two peaks were observed and they were assigned to O(1s) in precipitated LiOH on the surface and O(1s) in Li2O. After H2O exposure, a peak broadening and an appearance of a new peak were observed at the higher binding energy region than O(1s) in Li2O. They were attributed to surface −OH and H2O molecule adsorbed on the surface. The adsorption behavior of H2O was discussed from the observation of electronic structure in Li2O surface.

High heat load test of CFC divertor target plate with screw tube for JT-60 superconducting modification

Abstract A flat carbon fiber composite (CFC) tile mock-up with screw tubes, which have helical fins like a nut, was fabricated aiming at further improvement of the heat removal performance of the cost-effectively manufactured divert or target for JT-60SC (modified JT-60 as a superconducting coil tokamak). The heat removal performance of the mock-up was successfully demonstrated on the JAERI Electron Beam Irradiation Stand. The estimated heat transfer coefficient of the screw tube at the non-boiling region was roughly three times higher than that of the smooth tube. This corresponds to 1.5 times that of the swirl tube. A heat cycle test of 10 MW/m2 showed that the mock-up with the screw tubes could withstand for 1400-cycles. These results indicate that the divertor target plate with the flat CFC tile and the screw tube can be a promising candidate for the JT-60SC divertor target.

Disruption tests on repaired tungsten by CVD coating

Abstract The chemical vapor deposition (CVD) coating is considered as one of the possible methods for in situ repairing of the tungsten armor. In the present work, CVD coatings on eroded tungsten specimens were prepared to investigate the applicability of this method to repair the eroded tungsten surface. To simulate the damaged surface relevant to the disruption erosion, specimens were irradiated by an electron beam at a heat flux of 1250 MW/m2 before the CVD repairing procedure. From metallographic results, no pores or cracks were observed at the interface between the CVD layer and the eroded layer seemed to be successfully repaired by the CVD coating. However, the CVD layer was delaminated by thermal shock tests which simulate disruption heat loads. It was found that complete removal of the resolidified area before CVD coating is effective to repair the eroded surface.

Development of negative ion extractor in the high-power and long-pulse negative ion source for fusion application.

High power and long-pulse negative ion extractor, which is composed of the plasma grid (PG) and the extraction grid (EXG), is newly developed toward the neutral beam injector for heating and current drive of future fusion machines such as ITER, JT-60 Super Advanced and DEMO reactor. The PG is designed to enhance surface production of negative ions efficiently by applying the chamfered aperture. The efficiency of the negative ion production for the discharge power increased by a factor of 1.3 against that of the conventional PG. The EXG is also designed with the thermal analysis to upgrade the cooling capability for the long pulse operation of >1000 s required in ITER. Though the magnetic field for electron suppression is reduced to 0.75 of that in the conventional EXG due to this upgrade, it was experimentally confirmed that the extracted electron current can be suppressed to the allowable level for the long pulse operation. These results show that newly developed extractor has the high potential for the long pulse extraction of the negative ions.

Voltage holding study of 1 MeV accelerator for ITER neutral beam injector

Voltage holding test on MeV accelerator indicated that sustainable voltage was a half of that of ideal quasi-Rogowski electrode. It was suggested that the emission of the clumps is enhanced by a local electric field concentration, which leads to discharge initiation at lower voltage. To reduce the electric field concentration in the MeV accelerator, gaps between the grid supports were expanded and curvature radii at the support corners were increased. After the modifications, the accelerator succeeded in sustaining -1 MV in vacuum without beam acceleration. However, the beam energy was still limited at a level of 900 keV with a beam current density of 150 A∕m(2) (346 mA) where the 3 × 5 apertures were used. Measurement of the beam profile revealed that deflection of the H(-) ions was large and a part of the H(-) ions was intercepted at the acceleration grid. This causes high heat load on the grids and the breakdowns during beam acceleration. To suppress the direct interception, new grid system was designed with proper aperture displacement based on a 3D beam trajectory analysis. As the result, the beam deflection was compensated and the voltage holding during the beam acceleration was improved. Beam parameter of the MeV accelerator was increased to 980 keV, 185 A∕m(2) (427 mA), which is close to the requirement of ITER accelerator (1 MeV, 200 A∕m(2)).

Accelerator R&D for JT-60U and ITER NB systems

Abstract High energy accelerator and high current ion source development have been carried out at JAERI for negative-ion based neutral beam (N-NB) systems of JT-60U and ITER. One of R&D issues on the ITER prototype 5-staged electrostatic accelerator was voltage holding capability of the accelerator insulator column. By installing a stress ring, which reduces electric field concentration at the triple junction (interface of metal flange, insulator, and vacuum), 300 kV is held at each stage instead of rated voltage of 200 kV. At present the accelerator insulator column sustains 900 kV stably with the rings in all five stages. In the JT-60U N-NB system, a beamlet deflection by distorted electric field was found at the bottom of the extractor. Correcting the distorted field, reduction in beam divergence was confirmed for the overall beam. As the result, the heat load on the NB port limiter of JT-60U, located about 20 m away from the accelerator, was reduced to less than a half of the previous value before the correction. Consequently, we have succeeded in 10 s continuous injection of H 0 beam with the NB power of 2.6 MW at 355 keV.

Beam optics in a MeV-class multi-aperture multi-grid accelerator for the ITER neutral beam injector.

In a multi-aperture multi-grid accelerator of the ITER neutral beam injector, the beamlets are deflected due to space charge repulsion between beamlets and beam groups, and also due to magnetic field. Moreover, the beamlet deflection is influenced by electric field distortion generated by grid support structure. Such complicated beamlet deflections and the compensations have been examined utilizing a three-dimensional beam analysis. The space charge repulsion and the influence by the grid support structure were studied in a 1∕4 model of the accelerator including 320 beamlets. Beamlet deflection due to the magnetic field was studied by a single beamlet model. As the results, compensation methods of the beamlet deflection were designed, so as to utilize a metal bar (so-called field shaping plate) of 1 mm thick beneath the electron suppression grid (ESG), and an aperture offset of 1 mm in the ESG.

Acceleration of 1 MeV, 100 mA Class H− ion beams in a proof-of-principle accelerator for ITER

Development of an electrostatic accelerator for 1 MeV, 1 ampere class H− ion beams has been carried out as a Proof-of-Principle (PoP) of high current accelerators required in neutral beam system of fusion reactors such as ITER. A unique feature of such fusion oriented accelerators is to utilize vacuum insulation not only inside but also outside of the accelerator for high voltage insulation, since conventional gas insulation is not applicable due to excess radiation induced conductivity in the gas. The PoP accelerator is surrounded by an insulator stack as a vacuum boundary with a vacuum gap of 50 mm all around the accelerator. One of the key technologies to achieve insulation of MV voltage was reduction of electric field strength at triple junction of the insulator (made of FRP, fiber reinforced plastic) stack. By lowering the stress to 1.7 kV/mm, the accelerator sustained 1 MV stably for more than 2 hours. By tuning the ion source with/without cesium under stable accelerator operation, the PoP accelerat...

Development of ITER Divertor Vertical Target with Annular Flow Concept - II: Development of Brazing Technique for CFC/CuCrZr Joint and Heating Test of Large-Scale Mock-Up

Abstract The first fabrication and heating test of a large-scale carbon-fiber-composite (CFC) monoblock divertor mock-up using an annular flow concept has been performed to demonstrate its manufacturability and thermomechanical performance. This mock-up is based on the design of the lower part of the vertical target of the International Thermonuclear Experimental Reactor (ITER) divertor adapted for the annular flow concept. The annular cooling tube consists of two concentric tubes: an outer tube made of CuCrZr and an inner stainless steel tube with a twisted external fin. Prior to the fabrication of the mock-up, brazed joint tests between the CFC monoblock and the CuCrZr tube have been carried out to find the suitable heat treatment mitigating loss of the high mechanical strength of the CuCrZr material. A basic mechanical examination of CuCrZr undergoing the brazing heat treatment and finite element method analyses are also performed to support the design of the mock-up. High heat flux tests on the large-scale divertor mock-up have been performed in an ion beam facility. The mock-up has successfully withstood more than 1000 thermal cycles of 20 MW/m2 for 15 s and 3000 cycles of >10 MW/m2 for 15 s, which simulates the heat load condition of the ITER divertor. No degradation of the thermal performance of the mock-up has been observed throughout the thermal cycle test although in the tile with exposure to the heat flux of 20 MW/m2, the erosion depth has been measured as 5.8 and 8.8 mm at the 300th and 500th cycles.

Non-destructive testing of CFC monoblock divertor mock-ups

Non-destructive examination (NDE) methods for joint interfaces between different materials in high heat flux (HHF) components of divertor should be urgently developed to assure quality and reliability of joining techniques. The purpose of this work is to demonstrate the ability of using ultrasonic wave and thermography NDE techniques to detect the defect in the joining interface (joint defect) of divertor mock-ups with carbon-fiber reinforced carbon monoblock armor tiles brazed on a copper cooling tube. The results of both NDEs are benchmarked with HHF tests and cross-sectional observation of the mock-up to correlate the joint defect size detected with NDEs to the thermal response of the mock-up with initial joint defects. From the results of the HHF tests and the cross-sectional observations, it can be concluded that both NDE techniques have sufficient accuracy to predict the surface temperature of the HHF components.