N. Miyamoto

High power negative ion sources for fusion at the Japan Atomic Energy Research Institute (invited)

Technologies producing high power negative ion beams have been highly developed over the years at Japan Atomic Energy Research Institute for use in neutral beam injectors for heating the thermonuclear fusion plasmas. At present, it is possible to produce multiampere H−/D− ion beams quasicontinuously at energies of more than a few hundred keV with a good beam optics of beamlet divergence of a few mrad. Based on these technologies, two research and development projects have been initiated; one is to develop a 22 A/500 keV/10 s D− ion source for the neutral beam injector for JT‐60U, and the other is to develop a 1 A/1 MeV/60 s H− ion source to demonstrate high current negative ion acceleration up to the energy of 1 MeV, the energy required for the neutral beam injector for the International Thermonuclear Experimental Reactor.

Development of a 500 keV, 22 A D− ion source for the neutral beam injector for JT‐60U

The first results of the performance test of the large negative ion source for a JT‐60U negative‐ion‐based neutral beam injector (N‐NBI) are presented. The ion source consists of a cesium seeded multicusp plasma generator, where negative ions are produced via volume and surface processes, a 110 cm×45 cm multiaperture extractor, and a three‐stage electrostatic accelerator. After negative ion production and voltage holding tests in test stands, the ion source was installed in the N‐NBI system and the full power test began. Up to now, the ion source has produced 400 keV, 5.9 A (2.4 MW) D− ion beams, the world highest D− current and beam power, with a pulse duration of 0.1 s.

Grid power loading in a multiaperture, multigrid negative ion accelerator

Grid power loading in a multistage, multigrid accelerator was calorimetrically measured using the Japan Atomic Energy Institute 400 keV accelerator. This is the first result on the grid power loading in the multistage accelerator. It is turned out that the extracted electrons are effectively suppressed in an extractor. The leakage of the electrons extracted from the source plasma to the first acceleration grid, the second grid and the grounded grid were no more than 0.8%, 0.2%, and nearly zero of the extracted electron current, respectively. Under the optimum beam optics that gives the lowest beam divergence, the power loadings of the acceleration grids were dominated by acceleration of the stripped electrons. At a typical source operating pressure of 0.3 Pa, the power loadings of a first acceleration grid, a second acceleration grid, and a grounder grid were 3.4%, 5.0%, and 2.8% of the total beam power, respectively. From the comparison with the calculated grid power loading, it is found that the strippe...

Influence of radiation on insulation gas at the ITER–NBI system

Abstract In neutral beam injection (NBI) systems for next generation tokamaks such as the International Thermonuclear Experimental Reactor (ITER), insulation gas around beam sources will be irradiated with neutrons and gamma rays generated by nuclear fusion reactions. To evaluate the influence of radiation, irradiation experiments were performed using the 60Co gamma rays. Ionization current and voltage-holding characteristics of various gases, such as air, SF6, C2F6, CO2, and mixing gas of air and SF6, were investigated up to an absorbed dose rate of 0.45 Gy/s. Saturation current flowing through the gases proved to increase linearly with gas volume, gas pressure, an absorbed dose rate. Saturation current also increased with molecular weight of the gases. During irradiation, voltage-holding capability was degraded by about 10%; the degree of the degradation did not depend on the absorbed dose rate. Mixture of a small quantity of SF6 gas and air proved to be effective from the viewpoint of suppressing ionization current, since air containing a small amount of SF6 gas has lower ionization current and higher voltage-holding capability. An experimental formula for estimating saturation current was obtained on the basis of experimental results. Using the experimental formula, ionization current at the ITER-NBI system was estimated to be higher than 2 A; such a high ionization current is not acceptable. Instead of gas insulation, other insulating method like vacuum insulation will be necessary.

TEMPERATURE CONTROL OF PLASMA GRID FOR CONTINUOUS OPERATION IN CESIUM-SEEDED VOLUME NEGATIVE ION SOURCE

A new-concept plasma grid, named frame-cooling-type plasma grid, was developed for a long pulse operation of a cesium-seeded volume H− ion source. The frame-cooling-type plasma grid was tested using a long pulse H− ion source. The grid has a kind of bellows structure that acts as thermal insulator to keep the surface temperature at an optimum value for H− ion production. It was confirmed that the temperature was kept at over 250 °C, an optimum temperature for the cesium effect, in steady-state operations with the plasma grid cooled by room-temperature water. Using the plasma grid, a maximum H− ion beam of 40 keV, 600 mA (16 mA/cm2) for a long pulse operation of 90 s was generated successfully.

Steady state operation of an ampere-class hydrogen negative ion source

A cesium-seeded volume negative ion source producing H− ion beams of 800 mA has been operated continuously at a high current density of 20 mA/cm2. The ion source consists of a magnetically filtered multicusp plasma generator and a multiaperture extractor. The ion source has a frame-cooling-type plasma grid, which is continuously able to keep the temperature at optimum using radiation from filaments and arc discharge. The ion source produces about 150 mA of H− in operation without cesium (pure volume operation). The negative ion yield is enhanced by more than a factor of four by injecting 600 mg of cesium. It is important to keep the plasma grid surface temperature at about 300 °C, where the negative ion yield has the maximum. The plasma generator has six tungsten filament cathodes of 1.2 mm in diameter. To estimate a lifetime of the filaments, weight and diameter of the filaments were measured after continuous operation. It was found that evaporation is the dominant wearing-out process, and no significant...

Measurement of beam halo in a 400 keV H− ion source

A preliminary measurement of the beam halo in a high energy negative ion beam has been carried out using a Cs seeded volume production type negative ion source, which has a three‐stage electrostatic accelerator. The beam profile including both neutrals and ions was precisely measured calorimetrically. In the pure volume operation, the power fraction of the beam halo was estimated to be 3∼4% of the total beam power at an optimum perveance. We suggest that the neutrals formed via stripping of negative ions are the major origin of the beam halo in the negative ion acceleration.

Development of a 400 keV multi-stage electrostatic accelerator for neutral beam injectors

A three-stage electrostatic accelerator has been tested up to 400 ke V. The structure of the accelerator is the same as that of the 500 ke V accelerator for the JT-60U negative-ion-based neutral beam injection (N-NBI) system. It was confirmed that the heat loads were mainly due to secondary particles generated by the stripping of H - ions in the accelerator, and suppressed effectively by reducing the operational gas pressure. The heat loads at the source pressure of 0.3 Pa, which is the design pressure of the JT-60U ion source, were evaluated to be 3.4% (the first acceleration grid), 4.3% (the second acceleration grid) and 2.4% (the grounded grid) of the input electric power. A H - beam of 0.18 A has been accelerated successfully up to 400 keV for 1 s from 9 apertures. The accelerated H - current density was 13 mA/cm 2 , the same current density of the JT-60U source. Higher H - beam current of 0.5 A was also obtained at 350 keV from 49 apertures.

Recent progress of high-power negative ion beam development for fusion plasma heating

A negative-ion-based neutral beam injector (N-NBI) has been constructed for JT-60U. The N-NBI is designed to inject 500 keV, 10 MW neutral beams using two ion sources, each producing a 500 keV, 22 A D- ion beam. In the preliminary experiment using one ion source, a D- ion beam of 13.5 A has been successfully accelerated with an energy of 400 keV (5.4 MW) for 0.12 s at an operating pressure of 0.22 Pa. This is the highest D- beam current and power in the world. Co-extracted electron current was effectively suppressed to the ratio of Ie/I D - < 1. The highest energy beam of 460 keV, 2.4 A, 0.44 s has also been obtained. To realize 1 MeV class NBI system for ITER (International Thermonuclear Experimental Reactor), demonstration of ampere class negative ion beam acceleration up to I MeV is an important mile stone. To achieve the mile stone, a prototype accelerator and a I MV, I A test facility called MeV Test Facility (MTF) were constructed. Up to now, an H- ion beam was accelerated up to the energy of 805 keV with an acceleration drain current of 150 mA for 1 s in a five stage electrostatic multi-aperture accelerator.

Beam acceleration test in negative‐ion based NBI system for JT‐60U

Beam extraction and acceleration test in the Negative Ion Based Neutral Beam Injector for JT‐60U has been started using one ion source that is designed to produce a 500 keV, 22 A D− ion beam. Deuterium negative ions are produced in a cesium‐seeded semi‐cylindrical plasma generator and accelerated by a multi‐aperture three‐stage electrostatic accelerator. In the preliminary experiment of beam acceleration, the D− ion beam of 13.5 A was successfully accelerated to 400 keV for a pulse duration of 0.12 s. The negative ion beam power was 5.4 MW. The operating gas pressure in the plasma generator was as low as 0.22 Pa. The highest energy beam of 460 keV, 2.4 A, 0.44 s was also obtained. The ratio of extracted electron current to extracted negative ion current is estimated Ie/ID−<1. It was confirmed that the electron leak from the extractor to the accelerator is suppressed efficiently by the effects of biassing, electron trapping gap and magnetic field.