Shinzaburo Matsuda

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.

Quasi–dc extraction of 70 keV, 5 A ion beam

An ion source that produces a high energy ion beam in quasi‐dc mode has been fabricated and tested. The significant features of the source are effectively cooled accelerator grids with properly shaped apertures and a carefully cooled douPIGatron plasma generator. To opeate the source quasi‐continuously, the heats dissipated in the accelerator grids and in the back plate of the plasma generator were suppressed below permissible values by attaining low pressure in the arc chamber. Under such condition, a 70 keV, 5 A, 10 s ion beam was obtained repeatedly without any deleterious problems. The measurement by photo beam monitor indicates no change of beam divergence during 10 s. The pressure change in the beam drift region during the pulse has also been investigated.

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.

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.

Production of 75‐keV, 70‐A, 10‐s ion beams

High‐power long pulse ion sources were fabricated and tested at a prototype injector unit for JT‐60. Ion beams of 70 A at an energy of 75 keV were extracted repeatedly for up to 10 s. The heat loadings to each grid were within our design values and each grid turned out to be thermally stable during 10 s pulse. The neutral beam power deposited to the beam target was over 1.43 MW, which corresponds to the design value of the JT‐60 neutral beam injector. The e‐folding half‐width beam divergence angle was about 1.0° at optimum beam current and a proton ratio of about 80% was obtained. It was also confirmed that other beam line components, such as the ion beam dump and the cryopump, were sufficiently reliable.

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.

A New Method of Protecting Ion Source Accelerators against Deterioration Due to Source Breakdown

A new countermeasure against the surge current due to capacitive stored energy during source breakdown is proposed. The effectiveness is confirmed both numerically and experimentally.

Magnetic Island Formation Due to Error Field in the JFT-2 Tokamak

A formation of islands in magnetic surfaces due to the error field in resonance with the rotational transform of magnetic lines of force in the tokamak device is described. The size of the islands has been obtained analytically so far, and is shown to be proportional to the square root of the intensity ratio of the error field to the azimuthal field and also inversely proportional to the square root of the shear. This theory is ascertained by a simple analysis which gives a physical picture more clearly. Islands which may appear due to the resonant components of the pulsed vertical field provided in the JFT-2 tokamak device are examined numerically, following the magnetic lines of force by the R. K. G, method and then are compared with the theory.

Hole Geometry Optimization of the Extraction Electrode in a Duopigatron Ion Source

The optimum shape of the extraction aperture in the positive electrode is investigated experimentally by using a duopigatron ion source with single hole extraction electrodes. We find that a slightly chamfered aperture gives the best beam divergence without decreasing the optimum perveance. This result is applied to the ion sources of the JFT-2 neutral beam injector and the injection power to the torus can be improved appreciably. The computational result about the optimum emitting surface by the ion beam simulation code indicates that a slight chamfering is preferable for beam optics.