Masayuki Dairaku

Burnout experiments on the externally-finned swirl tube for steady-state and high-heat flux beam stops

An experimental study to develop beam stops for the next generation of neutral beam injectors was started, using an ion source developed for the JT-60 neutral beam injector. A swirl tube is one of the most promising candidates for a beam stop element which can handle steady-state and high-heat flux beams. In the present experiments, a modified swirl tube, namely an externally-finned swirl tube, was tested together with a simple smooth tube, an externally finned tube, and an internally finned tube. The major dimensions of the tubes are 10 mm in outer-diameter, 1.5 mm in wall thickness, 15 mm in external fin width, and 700 mm in length. The burnout heat flux (CHF) normal to the externally finned swirl tube was 4.1 ± 0.1 kW/cm2, where the Gaussian e-folding half-width of the beam intensity distribution was about 90 mm, the flow rate of the cooling water was 30 l/min, inlet and outlet gauge pressures were about 1 MPa and 0.2 MPa, respectively, and the temperature of the inlet water was kept to 20 °C during a pulse. A burnout heat flux ratio, which is defined by the ratio of the CHF value of the externally-finned swirl tube to that of the externally-finned tube, turned out to be about 1.5. Burnout heat fluxes of the tubes with a swirl tape or internal fins increase linearly with an increase of the flow rate. It was found that the tube with external fins has effects that not only reduce the thermal stress but also improve the characteristics of boiling heat transfer.

Improvement of beam performance in the negative-ion based NBI system for JT-60U

The injection performance of the negative-ion based NBI (N-NBI) system for JT-60U has been improved by correcting beamlet deflection and improving spatial uniformity of negative ion production. Beamlet deflection at the peripheral region of the grid segment due to the distorted electric field at the bottom of the extractor has been observed. This was corrected by modifying the surface geometry at the extractor to form a flat electric field. Moreover, beamlet deflection due to beamlet–beamlet repulsion caused by space charge was also compensated for by extruding the edge of the bottom extractor. This resulted in a reduction of the heat loading on the NBI port limiter. As a result of the improvement above, continuous injection of a 2.6 MW H0 beam at 355 keV has been achieved for 10 s. Thus, long pulse injection up to the nominal pulse duration of JT-60U was demonstrated. This has opened up the prospect of long pulse operation of the negative-ion based NBI system for a steady-state tokamak reactor. So far, a maximum injection power of 5.8 MW at 400 keV, with a deuterium beam, and 6.2 MW at 381 keV, with a hydrogen beam, have been achieved in the JT-60U N-NBI. Uniformity of negative ion production was improved by tuning the filament emission current so as to direct more arc power into the region where less negative ion current was extracted.

Performance of jaeri electron beam irradiation stand

Abstract The electron beam facility was constructed to test and develop plasma facing components. Maximum beam power reaches 400 kW. The facility has a plasma electron gun which is the largest one in the world. The power supply of the facility utilizes high-frequency invertors as a beam switching system. The plasma electron gun and power supply system were developed on the basis of neutral beam injectors for JT-60. Heating time ranges from 1 ms to continuous operation. Maximum head flux is over 2000 MW/m2, and heating area is up to 1800 cm2. The heating area can be changed by a number of grid apertures. The facility can produce intense heat loads at off-normal events such as disruption, and steady-state heat loads on the divertor and the first wali at normal operations.

The JT-60 neutral beam injection system

The JT-60 neutral beam injection system has been designed to inject a neutral hydrogen beam power of 20 MW at energies of 75–100 keV for 10 s. The system consists of 14 beamline units, 14 power supply units for the ion sources, a liquid helium and liquid nitrogen cryogenic system for the beamline cryopumps, a demineralized cooling system for heat dump materials, an auxiliary pumping system, and a computer aided control system. Each beamline unit is made with essentially the same geometry as that of the prototype injector unit, which was constructed in 1981 and tested from 1981 to 1983 to confirm unit performance. Each power supply unit provides a voltage regulated output of 100 kV, 90 A. The helium refrigerator has a cooling capacity of 3000 W at 3.6 K. Beam energy and the pulse timing of each unit can be set up independently. Since April 1984, each beamline unit has been tested and conditioned up to 75 keV, 70 A, 10 s at the prototype injector facility. Beamlines have been installed on JT-60 and completion of the total system is scheduled for July 1986.

Thermal shock tests on various materials of plasma facing components for FER/ITER

Development of plasma facing components and materials is a key element in the R&D program for the Fusion Experimental Reactor (FER), which has been designed at JAERI, and the International Thermonuclear Experimental Reactor (ITER), which has been designed under international collaboration. In these next-step tokamak devices, the plasma facing components and materials will be exposed to severe heat load and incident particle flux. The concern is especially acute that the extremely high thermal shock due to plasma disruption could cause material fracture. Efforts on developing the first wall and divertor have been energetically undertaken at JAERI. The present paper describes recent experimental and analytical results on thermal shock characteristics of various materials.

Disruption erosions of various kinds of tungsten

Abstract Thermal shock experiments on CVD-W, powder sintered tungsten (P-W), monocrystal tungsten (M-W) and brush tungsten (B-W) have been performed under the heat flux of 1000–2400 MW m −2 with the pulse length of 2 ms in JAERI electron beam irradiation system (JEBIS). The samples were exposed to a single pulsed beam with the electron energy of 70 keV and preheated up to 1000°C. In these experiments, the following information was obtained: (1) CVD-W is the most promising from an erosion point of view; (2) the increased weight loss was mainly caused by severe particle emission; (3) weight loss for every kind of tungsten was greater at 1000°C than at room temperature; (4) no cracks were observed at 1000°C for every sample by SEM observation; (5) secondary cracks were observed along the rolled direction at room temperature and may cause the exfoliation of a piece of tungsten surface during normal operation.

High heat flux experiment on B4C-overlaid C/C composites for plasma facing materials of JT-60U

High heat flux experiments (5–40 MW/m 2 , 5 s and 550 MW/m 2 , 5–10 ms) in the JAERI electron beam irradiation stand (JEBIS) have been carried out on three kinds (conversion, CVD and LLPS) of B 4 C-overlaid C/C composites, on which B 4 C is overlaid with a thickness of 100–250 μm. Measurements were made with respect to the weight loss, changes of the surface morphology and of the surface atomic composition, and the surface temperature. As a result of these experiments, it is found that B 4 C layers of all samples have no damages except small weight losses up to 12 MW/m 2 heat loads, which are estimated at the divertor tiles of JT-60U in normal plasma operation, and that the conversion method is the best of the three methods applied in the present tests, since no exfoliation has occurred even under the disruption conditions.

Design and experimental results of a new electron gun using a magnetic multipole plasma generator

A new electron gun utilizing a magnetic multipole plasma generator was designed and fabricated as the heat source of the high heat flux test facility, called JEBIS (JAERI electron beam irradiation stand). By changing the acceleration grids, this electron gun is able to produce a pencil to a sheetlike electron beams up to 4 A at 100 keV for 1 ms to continuous mode. In this electron gun, magnetic lens system is not adopted to focus the electron beam, but the space charge neutralization effect by the beam plasma produced downstream of the electron gun is utilized to prevent the blow‐up of the electron beam. In addition, high permeability metal is embedded in the first and the second grids to magnetically shield the earth field and the stray field from the beam bending magnet. It was experimentally demonstrated that wide range of heat flux from 0.2 MW/m2 to over 2000 MW/m2 can be realized at the test sample position about 1.7 m downstream of the electron gun.

100‐kV test of the prototype neutral beam injector for JT‐60

A prototype neutral beam injector for JT‐60 has demonstrated extraction of 100‐kV, 70‐A, 10‐s ion beams, delivering neutral beam power of 1.43 MW into the target chamber. The power‐flow measurements showed that all beam line components, including the ion sources, were operated successfully. This verified the validity of the design work related to the ion source and neutral beam cooling devices. No significant change in the beam divergence during the pulse has been observed up to the maximum rated beam extraction of 40 A at 100 kV for 10 s from each ion source. The measurement also indicated that the power distribution to the beam line components agreed well with independently obtained ion species ratio and gas pressure distribution. Efficiencies of 28% and 20% were obtained for the neutralization and neutral injection into the target, respectively, for 100‐kV, 70‐A, 10‐s operation.

Sputtering yields of carbon based materials under high particle flux with low energy

Abstract A new ion source which can produce high particle flux beams at low energies has been developed. This paper presents preliminary results on the sputtering yield of the carbon fiber reinforced composites (CFCs) measured with the new ion source. The sputtering yields of 1D and 2D CFCs, which are candidate materials for the divertor armour tiles, have been measured by the weight loss method under the hydrogen and deuterium particle fluxes of 2 ∼ 7 × 10 20 /m 2 s at 50 ∼ 150 eV. Preferential sputtering of the matrix was observed on CFCs which included the matrix of 40 ∼ 60 w%. The energy dependence of the sputtering yields was weak. The sputtering yields of CFCs normally irradiated with deuterium beam were from 0.073 to 0.095, and were around three times larger than those with hydrogen beam.