Y. Fujiwara

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.

Negative hydrogen ion source for TOKAMAK neutral beam injector (invited)

Intense negative ion source producing multimegawatt hydrogen/deuterium negative ion beams has been developed for the neutral beam injector (NBI) in TOKAMAK thermonuclear fusion machines. Negative ions are produced in a cesium seeded multi-cusp plasma generator via volume and surface processes, and accelerated with a multistage electrostatic accelerator. The negative ion source for JT-60U has produced 18.5 A/360 keV (6.7 MW) H− and 14.3 A/380 keV (5.4 MW) D− ion beams at average current densities of 11 mA/cm2 (H−) and 8.5 mA/cm2 (D−). A high energy negative ion source has been developed for the next generation TOKAMAK such as the International Thermonuclear Experimental Reactor (ITER). The source has demonstrated to accelerate negative ions up to 1 MeV, the energy required for ITER. Higher negative ion current density of more than 20 mA/cm2 was obtained in the ITER concept sources. It was confirmed that the consumption rate of cesium is small enough to operate the source for a half year in ITER-NBI without...

Negative ion sources for neutral beam injection into fusion machines

Plasma heating and current drive in future machines for magnetically confined thermonuclear fusion will require neutral beam injectors based upon negative ion sources. Although the technology of filament arc ion sources is considered adequately mature for large scale injectors, such as for ITER, ion source efficiency and reliability need to be improved. The power loading of the source and accelerator are especially problematic. This is particularly important for long pulse (1000 s) operation. Investigations with the Kamaboko source, a small scale model of the ITER arc source, illustrate the importance of the conditions of the plasma grid for reducing electron loading of the accelerator.

Neutral beams for the International Thermonuclear Experimental Reactor

Receiving higher emphasis on the neutral beam (NB) off-axis current drive, the NB system is being highlighted for the steady state operation of the International Thermonuclear Experimental Reactor (ITER). To fulfill the physics requirement of heating and current drive, the NB system delivers ∼50 MW of D0 beams at 1 MeV into the ITER plasmas. The NB injector was designed so as to minimize the axial length, to avoid cost impact on the building. It was estimated by nuclear analyses that the insulation gas around the beam source would cause radiation induced conductivity, which would result in a power dissipation of >100 kW in the gas itself. As a result the present design utilizes vacuum insulation around the beam source. Since the vacuum pressure inside/outside the beam source ranges 10−1–10−2 Pa, both gas (glow) and vacuum arc discharges are taken into account in the design.

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...

Beamlet–beamlet interaction in a multi-aperture negative ion source

Beamlet–beamlet interaction in an acceleration region of a multi-aperture hydrogen negative ion source was experimentally studied. Deflection angle of beamlets was measured in the case of both five-row beamlets and three-row beamlets at the beam energy ranging from 86 to 178 keV. The deflection angle of beamlets at the edge of a beam was about 3 mrad larger than that at the center of the beam. The deflection angle was independent of beam energy at the same perveance. Repulsive force due to the beamlet–beamlet interaction proved to be inversely proportional to the square of distance approximately. From a beam trajectory calculation, shaping of a grid was confirmed to be effective to compensate the beamlet–beamlet interaction.

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.

Hydrogen negative ion beam acceleration in a multiaperture five-stage electrostatic accelerator

To develop a high power negative ion source/accelerator for 1 MeV class neutral beam injector, hydrogen negative ion beam acceleration has been studied using a five-stage, multiaperture electrostatic accelerator. After conditioning each accelerator stage, the negative ions are accelerated to 1 MeV successfully with a drain current of 25 mA for 1 s. Cs was introduced into the ion source to produce higher current density. The highenst acceleration current density of 15 mA/cm2 was successfully accelerated up to an energy of 700 keV for 1 s, keeping the optimum perveance. The total acceleration current of 200 mA was extracted from nine central apertures 14 mm diameter each. A preliminary measurement of the heat load in the accelerator showed that the direct interception of the beam for the first grid and the third grid was negligibly small. The highest heat load was 4.5% of the input power at the second grid.

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.