M. Araki

Experiments on heat transfer of smooth and swirl tubes under one-sided heating conditions

To design the divertor plate for the next generation fusion machines which is subjected to high heat loads on its one side by the plasma, it is essential to evaluate performances of heat transfer efficiency. However there is little in the literature for predicting of heat transfer coefficients under such one-sided heating conditions. To establish the heat transfer correlation for water under one-sided heating conditions, the authors have performed heat transfer experiments on smooth circular and swirl tubes in the regions from non-boiling to high subcooled partial nucleate boiling. Based on the experimental results, it is confirmed that the existing heat transfer correlations can be applicable at the non-boiling region. For the subcooled partial nucleate boiling region, they cannot be available so that a new heat transfer correlation has been proposed under one-sided heating conditions.

High energy negative-ion based neutral beam injection system for JT-60U

Abstract On the basis of recent progress in the research and development of a high current and high energy negative-ion source, the construction of a 500 keV negative-ion based neutral beam injection (NBI) system for JT-60U has begun to demonstrate a mega-amp level NB current drive at high plasma density and to study high energy beam heating in reactor-grade plasmas. The specification of the NBI system is as follows: a beam energy of 500 keV, an injection power of 10 MW, a beam duration time of 10 s, beam species of deuterium or hydrogen. The neutral beam of 10 MW is injected in a tangential codirection with a single beamline that has two negative ion sources. The construction of the negative-ion based NBI system will be completed in 1996, and NB current drive and plasma core heating experiments will start immediately in JT-60U.

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.

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.

Beam stops of JT-60 neutral beam injector

The JT-60 neutral beam injector consists of 14 beam line units and injects a rated power of 20 MW with an energy level of 70–100 keV for beam pulses up to 10 s. The total handling power of ion and/or neutral beams in the beam line unit amounts to as much as 8 MW per unit for a beam extraction of 100 keV/80 A, though each unit delivers a neutral beam power of about 1.4 MW. Accordingly, the beam stop components have to receive a high heat load. Another difficulty is that the beam stops must receive quasi continuous heat loadings for up to 10 s. The design procedures and the measured characteristics of the beam stop components irradiated with the beam are described. In the maximum rated operation of a 100 keV/80 A beam extraction for a beam pulse up to 10 s, the incident power to the beam stop components in the beam line unit has roughly reached the design value, and every component works well now.

Critical-heat-flux experiment on the screw tube under one-sided-heating conditions

Development of high-heat-flux components such as the divertor plate of fusion experimental machines is essential for removal of high heat loads with heating on one side. For this purpose, the authors machined a tube with an inside wall like a nut, namely, a screw tube, to enhance heat transfer efficiency and simplify the machining process. The screw tube is compared with a swirl tube, originally developed by Oak Ridge National Laboratory, and the Hypervapotron, developed by Joint European Torus (JET). The spirally machined inside wall can enlarge the heat transfer area and make a little vortex flow only close to the wall. The performance of the screw tube is characterized by a critical-heat-flux experiment that uses water flow velocities ranging from 4 to 20 m/s with a water inlet pressure of 1.0 MPa. As a result, the screw tube has a higher incidence of CHFs compared with the smooth tube and the Hypervapotron and performs similarly to the swirl tube at identical flow velocities. 15 refs., 10 figs., 2 tabs.

Dimensional analysis of critical heat flux in subcooled water flow under one-side heating conditions for fusion application

Critical heat flux (CHF) experiments have been carried out to design a high performance cooling device with pressurized subcooled water flow under one-side heating conditions for plasma facing components. Swirl, screw and hypervapotron tubes are the most efficient geometries to remove high incident heat fluxes. A dimensional analysis shows that CHF phenomenon can be featured by dimensionless groups that are the critical Boiling, the Eckert and the Reynolds numbers, the mass enthalpic quality and the ratio between liquid and vapor densities. CHF for uniformly heated smooth tubes, the maximum wall heat flux for smooth and for swirl tubes under one-side heating conditions are predicted with a reasonable accuracy by correlating these five dimensionless groups. An enhancement factor featuring the difference between one-side and uniform heating is also introduced. CHF occurrence under one-side heating is characterized by a normalized temperature, ratio between the maximal wall temperature and the onset of nucleate boiling temperature, which only depends on the pressure. According to finite element calculations performed for smooth and swirl tubes, temperature and heat flux are distributed along the inner wall of the cooling channel under one-side heating conditions. These distributions are characterized by two dimensionless numbers for each geometry: a peaking factor, which is the ratio between the maximal heat flux at the inner wall of the cooling channel and the incident heat flux, and a full width angle at half maximum of wall heat flux at the inner wall of the cooling channel. This study draws the dimensionless groups featuring CHF phenomenon not only under uniform heating, but also under one-side heating, which involve additional normalized parameters.

Multi-ampere negative hydrogen ion source for fusion application

Abstract We are developing a magnetically filtered multicusp source as one of the best candidates of the volume production type negative ion sources for fusion applications. Three types of magnetic filters, i.e., the rod filter, the external filter, and the electromagnetic filter, were studied from a viewpoint of extracting a high current H− beam from a wide extraction area. A H− current density of 22 mA/cm2 was extracted with the external filter. It seems that the H− current density is enhanced by applying a magnetic field on the extraction surface. Corresponding to the formation of a uniform magnetic filter field, the deviation of the H− current densities over a 12 cm × 26 cm extraction area were suppressed below 15% in the rod filter, and below 7% in the electromagnetic filter. A total H− current of 1.6 A was obtained from the whole extraction area in the external filter.

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.