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In JAEA, research and developments to realize high power accelerator (1 MeV, 40 AD- ion beams for 3600 s) for ITER have been carried out experimentally and numerically utilizing a five stage MAMuG (Multiaperture, Multigrid) accelerator. In this paper, the extension of the gap length, which is required to improve the voltage holding capability, is examined in two dimensional beam optics analyses and also from view point of stripping loss of ions. In order to suppress excess power loadings due to the direct interception of negative ions, which is issued in long pulse tests, the beamlet deflection is analyzed in three dimensional multibeamlet analyses. The necessary modifications shown above are applied to the MAMuG accelerator for coming long pulse tests in JAEA and ITER. [ABSTRACT FROM AUTHOR]

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 gas neutralization to 65%. The maximum neutralization efficiency for argon plasma increased from 51% of the gas neutralization to 55%. The line densities maximizing the plasma neutralization efficiencies decreased to 65% of those for the gas neutralizations. Neutralization efficiencies in the plasmas and the gases were in good agreement with those calculated from the cross sections. It is concluded that a practical plasma neutralizer with substantial dimensions can increase neutralization efficiency at a low line density as predicted from the numerical analysis based on the cross section data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports R&D progress of the MeV class accelerator, beam focussing technique and a high voltage (HV) bushing toward the ITER Neutral Beam system at JAEA. A H− ion beam of 320mA (current density: 140A/m2) was successfully accelerated up to 796keV by fixing air leak in the ion source chamber caused by heat loading due to backstream positive ions. From analyses of a three-dimensional beam trajectory, a countermeasure against beamlet deflection due to space charge repulsion among beamlets was identified to compensate the deflection by proper aperture offset in the electron suppression grid. For development of the HV bushing, a half-size (Ø800mm) mockup single stage bushing was manufactured and tested, that demonstrated the dc voltage holding capability of up to 220kV in a condition with vacuum inside. Electrostatic analyses and design of the HV bushing are in progress, in which some design improvements are suggested to reduce electric field strength on the surface of ceramic insulator. [Copyright &y& Elsevier]

For the ITER neutral beam system, reasearch and development of a vacuum-insulated MeV accelerator has been carried out at Japan Atomic Energy Research Institute. After a successful 1 MV insulation in vacuum for more than 2 h, H--ion-beam acceleration test is in progress to fulfill the ITER requirement of the current density of 200 A/m2 at 1 MeV, with good beam optics quality of <7 mrad divergence. For such a high-current-density H--ion-beam acceleration, the Kamaboko negative-ion source was operated at high input power, with increased filament number and magnetic filter strength. The optimum perveance was investigated by scanning the extraction and acceleration voltages, together with the beam divergence measurement. As a consequence, H--ion beams of 146 A/m2 (total ion-beam current: 206 mA) were obtained at 836 keV, of which power density is in the “ITER relevant” level. Such high-power-density beams with a divergence as low as 5 mrad were obtained at the optimum perveance corresponding to that of the ITER (200 A/m2 at 1 MeV). [ABSTRACT FROM AUTHOR]

Focuses on development of intense negative ion source producing multimegawatt hydrogen/deuterium negative ion beams. Neutral beam injector in TOKAMAK thermonuclear fusion machines; Long pulse negative ion source; Beam optics.

Heat load on acceleration grids by secondary particles such as electrons, neutrals, and positive ions, is a key issue for long pulse acceleration of negative ion beams. Complicated behaviors of the secondary particles in multiaperture, multigrid (MAMuG) accelerator have been analyzed using electrostatic accelerator Monte Carlo code. The analytical result is compared to experimental one obtained in a long pulse operation of a MeV accelerator, of which second acceleration grid (A2G) was removed for simplification of structure. The analytical results show that relatively high heat load on the third acceleration grid (A3G) since stripped electrons were deposited mainly on A3G. This heat load on the A3G can be suppressed by installing the A2G. Thus, capability of MAMuG accelerator is demonstrated for suppression of heat load due to secondary particles by the intermediate grids. [ABSTRACT FROM AUTHOR]

To realize stable negative ion beams for 100 s required in the neutral beam injector of JT-60SA, a physical model to control cesium (Cs) distribution inside the negative ion source has been developed in order to maintain the stable negative ion production at the plasma grid (PG) surface with Cs. In this work, to quantitatively evaluate Cs coverage on the PG, a three-dimensional Cs transportation code was introduced to consider the spatial Cs distribution in the source. The spatial temperature distribution of the chamber wall was also introduced in this model. As a result, the reasonable variation of the Cs coverage for 100 s was obtained, compared to that in the initial model. Based on the modified model, the operational temperature of the chamber wall was proposed to be less than 60 °C to suppress the desorption of Cs in the chamber wall and to sustain the stable negative ion production. In addition, it was also suggested that a slightly higher wall temperature before the operation leads to a decrease in the amount of Cs stored at the chamber wall, resulting in suppression of Cs consumption in the ion source. [ABSTRACT FROM AUTHOR]

Vacuum insulation on a large size negative ion accelerator with multiple extraction apertures and acceleration grids for fusion application was experimentally examined and designed. In the experiment, vacuum insulation characteristics were investigated in the JT-60 negative ion source with >1000 apertures on the grid with the surface area of ∼2 m2. The sustainable voltages varied with a square root of the gap lengths between the grids, and decreased with number of the apertures and with the surface area of the grids. Based on the obtained results, the JT-60SA (super advanced) negative ion source is designed to produce 22 A, 500 keV D- ion beams for 100 s. [ABSTRACT FROM AUTHOR]

Abstract: Neutral beam (NB) injectors for JT-60 Super Advanced (JT-60SA) have been designed and developed. Twelve positive-ion-based and one negative-ion-based NB injectors are allocated to inject 30MW D0 beams in total for 100s. Each of the positive-ion-based NB injector is designed to inject 1.7MW for 100s at 85keV. A part of the power supplies and magnetic shield utilized on JT-60U are upgraded and reused on JT-60SA. To realize the negative-ion-based NB injector for JT-60SA where the injection of 500keV, 10MW D0 beams for 100s is required, R&Ds of the negative ion source have been carried out. High-energy negative ion beams of 490–500keV have been successfully produced at a beam current of 1–2.8A through 20% of the total ion extraction area, by improving voltage holding capability of the ion source. This is the first demonstration of a high-current negative ion acceleration of >1A to 500keV. The design of the power supplies and the beamline is also in progress. The procurement of the acceleration power supply starts in 2010. [Copyright &y& Elsevier]

Abstract: In order to investigate the effect of radiation damage on hydrogen behavior in tungsten, tungsten samples with radiation damage of up to 3.5dpa were irradiated by a mixed hydrogen–carbon ion beam. The radiation damage was produced with 700keV negative hydrogen ion beam irradiation. The number density of blisters produced by the mixed ion beam irradiation decreased with increasing radiation damage. This was especially observed for blisters with diameters of 20μm or less. This result showed that radiation damage produced by high-energy particle irradiation suppresses blister formation on tungsten surfaces. [Copyright &y& Elsevier]

A multiaperture multigrid accelerator called “MeV accelerator” has been developed for neutral beam injection system of international thermonuclear experimental reactor. In the present work, long pulse H- ion beam acceleration was performed by the MeV accelerator equipped with new water-cooled grids. At present, the pulse length was extended to 5 s for the beams of 750 keV, 221 mA, and 10 s for the beams of 600 keV, 158 mA. Energy density, defined as products of beam energy (keV), current (mA), and pulse (s) divided by aperture area (m2), increased more than one order of magnitude higher compared with original MeV accelerator without water cooling in its grids. At higher energy and current, the grid was melted by beam deflection. Due to this grid melting, breakdowns occurred between the grids, and hence, the pulse length was limited. Beam deflection will be compensated by aperture displacement in next experiment. [ABSTRACT FROM AUTHOR]

Voltage holding capability of JT-60 negative ion source that has a large electrostatic negative ion accelerator with 45 cm×1.1 m acceleration grids was experimentally examined and improved to realize 500 keV, 22 A, and 100 s D- ion beams for JT-60 Super Advanced. The gap lengths in the acceleration stages were extended to reduce electric fields in a gap between the large grids and at the corner of the support flanges from the original 4–5 to 3–4 kV/mm. As a result, the voltage holding capability without beam acceleration has been successfully improved from 400 to 500 kV. The pulse duration to hold 500 kV reached 40 s of the power supply limitation. [ABSTRACT FROM AUTHOR]

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 accelerator generated H- ion beams of 70 mA at 1 MeV, and 110 mA (80 A/m2) at 900 keV stably for several hundreds shots (pulse length, 0.5–1 s limited by the target). The dark current flowed inside/on the surface of the FRP insulator due to photoelectron effect was estimated to be less than μA in case of 0.1 A electron beam acceleration (maximum current observed in the experiment). This is in the same order to that due to conductivity of the insulator material. So far, high voltage breakdown due to photoelectron has never been observed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines hydrogen negative ion beam acceleration using a five-stage, multiaperture electrostatic accelerator. Development of a high power negative ion source/accelerator for a neutral beam injector; High current density beam acceleration; Heat load in the accelerator.