M. Kashiwagi

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...

Development of a large volume negative-ion source for ITER neutral beam injector

Development of the negative-ion sources has been conducted to realize a high power neutral beam injector for International Thermonuclear Experimental Reactor (ITER). A high negative-ion current density of 31 mA/cm2 (H−) at a very low pressure of 0.1 Pa has been produced in a cesium seeded multicusp plasma generator which has the same concept of the ITER source. For a vacuum insulated accelerator, a voltage holding experiment of long distance vacuum gaps up to ∼1.8 m has been performed. It was clarified that the transition region of product pressure distance (pd) from the vacuum breakdown to the gas discharge is about 0.2 Pa m which is high enough from the operating region of the ITER source. A prototype vacuum insulated accelerator was fabricated based on the experiment and tested. A high-energy H− beam acceleration up to 970 keV, 37 mA, and 1 s has been successfully demonstrated.

Experimental comparison between plasma and gas neutralization of high-energy negative ion beams

A plasma neutralization efficiency of high-energy negative ion beams was measured in a practical neutralizer with substantial dimensions and compared with a gas neutralization efficiency for fusion application. Hydrogen negative ion beams of 200 keV, 4 mA were neutralized with hydrogen and argon arc plasmas in the plasma neutralizer of 2 m in length and 0.6 m in diameter. To clarify the difference between the plasma and the gas neutralizations, the high-density plasmas were produced at low operating pressure. By means of ∼30 G transverse magnetic fields that were applied locally at openings for beam path, leakage of the primary electrons to the outside of the neutralizer was suppressed sufficiently. The plasma densities were 1011–1012 cm−3 at low operating pressures of 0.002–0.05 Pa for both plasmas while an arc power was ∼40 kW. Neutralization efficiency was increased by using plasmas whose ionization degree was 10%–15%. The maximum neutralization efficiency for hydrogen plasma increased from 55% of the ...

Accelerator R&D for JT-60U and ITER NB systems

Abstract High energy accelerator and high current ion source development have been carried out at JAERI for negative-ion based neutral beam (N-NB) systems of JT-60U and ITER. One of R&D issues on the ITER prototype 5-staged electrostatic accelerator was voltage holding capability of the accelerator insulator column. By installing a stress ring, which reduces electric field concentration at the triple junction (interface of metal flange, insulator, and vacuum), 300 kV is held at each stage instead of rated voltage of 200 kV. At present the accelerator insulator column sustains 900 kV stably with the rings in all five stages. In the JT-60U N-NB system, a beamlet deflection by distorted electric field was found at the bottom of the extractor. Correcting the distorted field, reduction in beam divergence was confirmed for the overall beam. As the result, the heat load on the NB port limiter of JT-60U, located about 20 m away from the accelerator, was reduced to less than a half of the previous value before the correction. Consequently, we have succeeded in 10 s continuous injection of H 0 beam with the NB power of 2.6 MW at 355 keV.

Acceleration of 1 MeV, 100 mA Class H− ion beams in a proof-of-principle accelerator for ITER

Development of an electrostatic accelerator for 1 MeV, 1 ampere class H− ion beams has been carried out as a Proof-of-Principle (PoP) of high current accelerators required in neutral beam system of fusion reactors such as ITER. A unique feature of such fusion oriented accelerators is to utilize vacuum insulation not only inside but also outside of the accelerator for high voltage insulation, since conventional gas insulation is not applicable due to excess radiation induced conductivity in the gas. The PoP accelerator is surrounded by an insulator stack as a vacuum boundary with a vacuum gap of 50 mm all around the accelerator. One of the key technologies to achieve insulation of MV voltage was reduction of electric field strength at triple junction of the insulator (made of FRP, fiber reinforced plastic) stack. By lowering the stress to 1.7 kV/mm, the accelerator sustained 1 MV stably for more than 2 hours. By tuning the ion source with/without cesium under stable accelerator operation, the PoP accelerat...

Development of negative ion sources for the ITER neutral beam injector

Abstract A vacuum insulated beam source (VIBS), where all the components including the negative ion source and the acceleration grids are immersed in the vacuum, has been designed at Japan Atomic Energy Research Institute. The beam source is characterized by high gas flow conductance that reduces stripping losses of negative ions in the accelerator and improves the acceleration efficiency. The gas flow in VIBS was calculated using a 3-D Monte-Carlo code. At a typical operating pressure of 0.3 Pa in the negative ion source, the stripping loss of D − ions in VIBS is about 23%, which is about half of stripping loss in the gas insulated beam source (GIBS) that was designed in the ITER-FDR design. The acceleration efficiency is improved from 84 to 94%. To enhance the acceleration efficiency further, the possibility for reducing the operating pressure was experimentally studied. It was found the negative ion production in the KAMABOKO source adopted for ITER is dominated by surface production, therefore, the operating pressure can be reduced by improving plasma confinement. It is predicted that the operating pressure in KAMABOKO source can be reduced to 0.15 Pa, where the stripping loss and the acceleration efficiency in VIBS are improved to be 13 and 97%, respectively.

Development of high performance negative ion sources and accelerators for MeV class neutral beam injectors

The operation of an accelerator at low pressure is an essential requirement to reduce the stripping loss of negative ions, which, in turn, results in high efficiency of the neutral beam systems. For this purpose, a vacuum insulated beam source (VIBS) has been developed at Japan Atomic Energy Research Institute, which reduces the gas pressure in the accelerator by enhanced gas conductance through the accelerator. The VIBS achieves a high voltage insulation of 1 MV by immersing the whole structure of the accelerator in vacuum with a long (~1.8 m) insulation distance. Results of the voltage holding test using a long vacuum gap of 1.8 m indicate that a transition from vacuum discharge to gas discharge occurs at around 0.2 Pa m in the long vacuum gap. So far, the VIBS succeeded in accelerating a 20 mA (H−) beam up to 970 keV for 1 s. It has been demonstrated that the high voltage holding capability of the 1 MV bushing surrounding the VIBS accelerator could be drastically improved by installing new large stress rings that reduces the electric field concentration at the triple junction. After implementing this change, the VIBS sustained 1 MV stably for more than 1200 s. Acceleration of ampere class H− beams at high current density is to be started soon to demonstrate ITER relevant beam optics. The operation of a negative ion source at low pressure is also essential to reduce the stripping loss. However, it was not very easy to attain high current density H− ions at low pressure, since the destruction cross-section of the negative ion becomes large if the electron temperature is >1 eV in low pressure discharge. Using a strong magnetic filter to lower the electron temperature, and introducing higher arc discharge power to compensate for the reduction of plasma density through the filter, an H− ion beam of 310 A m−2 was extracted at a very low pressure of 0.1 Pa. This satisfies the ITER requirement of current density at one-third of the ITER design pressure (0.3 Pa).

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.

Optimization of plasma grid material in cesium-seeded volume negative-ion sources

In cesium-seeded hydrogen negative-ion sources, surface production on the plasma grid plays an important role in negative ion production. To enhance the surface, it is required to use material that would give a lower work function when Cs is absorbed on the surface. In a semicylindrical and cesium-seeded volume negative-ion source, eight materials (W, Cu, Mo, V, Cr, Ni, Ag, and Au) were tested as candidates for the plasma grid material. To avoid deposition of the cathode material on these materials, a filament-free plasma source was used, to fire the microwave (2.45 GHz) discharge in the Kamaboko source. The material surface was examined by measuring the photoelectron current by laser irradiation. It was observed that the discharge enhanced the photoelectron current when the material was biased negatively to the plasma potential during discharge. In the present experiment, Ni, Au, and Ag surfaces with a Cs layer showed a higher photoelectron current than the others. This was 1.5 times larger than that of ...

Optimization of negative ion extractor in a JAERI 400 keV H− ion source

The influence of a dipole magnetic field in the extractor on the beamlet deflection has been investigated using a JAERI 400 keV H− ion source. The beam deflection angle decreased from 10.2 to 7.0 mrad when the integrated value of the magnetic field was changed from 2530 to 910 G cm. The measured deflection angle was larger than the estimated value from the ion trajectory considering the magnetic field. To understand the dominant factor that enhances the beam deflection angle, three-dimensional trajectory simulation was performed. It was confirmed that the axis of the beamlet deflected by the magnetic field in the extractor is displaced from the center of the aperture at the grounded grid (GRG). This displacement enhances the beam deflection angle due to the effect of the electrostatic lens at the GRG. This phenomenon is similar to the beam deflection by aperture displacement of the GRG.The influence of a dipole magnetic field in the extractor on the beamlet deflection has been investigated using a JAERI 400 keV H− ion source. The beam deflection angle decreased from 10.2 to 7.0 mrad when the integrated value of the magnetic field was changed from 2530 to 910 G cm. The measured deflection angle was larger than the estimated value from the ion trajectory considering the magnetic field. To understand the dominant factor that enhances the beam deflection angle, three-dimensional trajectory simulation was performed. It was confirmed that the axis of the beamlet deflected by the magnetic field in the extractor is displaced from the center of the aperture at the grounded grid (GRG). This displacement enhances the beam deflection angle due to the effect of the electrostatic lens at the GRG. This phenomenon is similar to the beam deflection by aperture displacement of the GRG.