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A thermal beam emittance measurement method is developed specifically to measure high power density beams for the ITER neutral beam injection system (NBI)’s. A one-dimensional carbon-fiber composite (CFC) target is utilized to withstand the high current densities of the negative hydrogen beam making the beam emittance measurement possible. Schemes for the emittance measurement were discussed since beam instabilities are present in various stages of the beam phase. The allowable time for the measurement will be less than 2 seconds and it’s necessary to monitor the CFC temperature to not exceed 2000°C. The proposed measurement scheme to minimize beam instabilities is through the constant operation of the arc discharge and pulsed beam extraction. Mechanical structures are also being designed for rapid measurements to select different phases of the beam and for implementing the measurement system to long pulse beam operation. [ABSTRACT FROM AUTHOR]

In order to evaluate deposition of cesium (Cs) in the plasma grid (PG) and investigate the origin of the gradual degradation of the negative ion current in a 100 s long pulsed operation in the JT-60SA negative ion source, Cs behavior in the source is model. To do this, Cs adsorption/desorption coefficient with a wide range of temperature is experimentally measured by a surface ionization detector and a quartz crystal microbalance. As a result, both of the adsorption and desorption are drastically varied at temperature range from room temperature to 100 °C. Based on these data, the model calculates the amounts of the Cs mass in the chamber wall and PG. The calculation result shows that, in the operation, an increase of temperature in the chamber wall (Twall) causes the evaporation of the huge amount of Cs in it. This leads to over pilling-up of Cs in the PG and the slight degradation of the negative ion production. Therefore, it is found that the constant temperature operation is needed to sustain the thickness of the Cs monolayer in the PG, and the Twall is desired to be less than 60 °C to suppress the evaporation of Cs in the chamber wall. According to this model, it is showed that there are many combinations of the temperatures to achieve it with low Cs consumption. [ABSTRACT FROM AUTHOR]

In order to understand the physics mechanism of a negative ion extraction in negative ion sources, an emission surface of the negative ions around an aperture at a plasma grid, so-called a meniscus, has been analyzed by an inverse calculation of the negative ion trajectory in a two dimensional beam analysis code. In this method, the meniscus is defined as the final position of the negative ion trajectories which are inversely calculated from the measured beam profile to the plasma grid. In a case of the volume-produced negative ions, the calculated meniscus by the inverse calculation was similar to that obtained in conventional beam simulation codes for positive ion extractions such as BEAMORBT and SLACCAD. The negative ion current density was uniform along the meniscus. This indicates that the negative ions produced in the plasma are transported to the plasma grid uniformly as considered in the transportation of the positive ions. However, in a surface production case of negative ions, where the negative ions are generated near the plasma grid with lower work function by seeding cesium, the current density in the peripheral region of the meniscus close to the plasma grid surface was estimated to be 2 times larger than the center region, which suggested that the extraction process of the surfaceproduced negative ions was much different with that for the positive ions. Because this non-uniform profile of the current density made the meniscus shape strongly concave, the beam extracted from the peripheral region could have a large divergence angle, which might be one of origins of so-called beam halo. This is the first results of the determination of the meniscus based on the experiment, which is useful to improve the prediction of the meniscus shape and heat loads based on the beam trajectories including beam halo. [ABSTRACT FROM AUTHOR]

A high power density and a long pulse negative ion beam accelerator has been developed in QST to realize the ITER neutral beam accelerator. A main target was the H- ion beam acceleration up to 1 MeV with 200 A/m² for 60 s. After the achievement of 1 MeV at a short pulse, the pulse length has been gradually extended by modifications of grid configurations to suppress grid heat loads due to negative ions and electrons. However, the beam energy was limited up to 0.7 MeV at 60 s operation. One issue was an unstable negative ion production for a long time due to arcing and excess temperature rise of a plasma grid for optimum negative ion production. To suppress arcing, a cathode filament shape was modified. The plasma grid was modified to have a large heat capacitance in order to suppress temperature rise for 60 s operation at high discharge power required for 1 MeV operation. Another issue is to optimize beam optics more precisely. Fine tuning of the 1st acceleration stage was carried to improve beam optics for high current density beam. With these modifications, high power density beam acceleration of 0.97 MeV, 190 A/m² has been achieved for 60 s. This is the first long pulse acceleration of ITER class high power density beam. [ABSTRACT FROM AUTHOR]

The temperature control system of the large-size plasma grid has been developed to realize the long pulse production of high-current negative ions for JT-60SA. By using this prototype system for the JT-60SA ion source, 15 A negative ions has been sustained for 100 s for the first time, which is three times longer than that obtained in JT-60U. In this system, a high-temperature fluorinated fluid with a high boiling point of 270 degree Celsius is circulated in the cooling channels of the plasma grids (PG) where a cesium (Cs) coverage is formed to enhance the negative ion production. Because the PG temperature control had been applied to only 10% of the extraction area previously, the prototype PG with the full extraction area (110 cm x 45 cm) was developed to increase the negative ion current in this time. In the preliminary results of long pulse productions of high-current negative ions at a Cs conditioning phase, the negative ion production was gradually degraded in the last half of 100 s pulse where the temperature of an arc chamber wall was not saturated. From the spectroscopic measurements, it was found that the Cs flux released from the wall might affect to the negative ion production, which implied the wall temperature should be kept low to control the Cs flux to the PG for the long-pulse high-current production. The obtained results of long-pulse production and the PG temperature control method contributes the design of the ITER ion source. [ABSTRACT FROM AUTHOR]

R&D of high power density negative ion beam acceleration has been carried out at MeV test facility in JAEA to realize ITER neutral beam accelerator. The main target is H- ion beam acceleration up to 1 MeV with 200 A/m² for 60 s whose pulse length is the present facility limit. For long pulse acceleration at high power density, new extraction grid (EXG) has been developed with high cooling capability, which electron suppression magnet is placed under cooling channel similar to ITER. In addition, aperture size of electron suppression grid (ESG) is enlarged from 14 mm to 16 mm to reduce direct interception on the ESG and emission of secondary electron which leads to high heat load on the upstream acceleration grid. By enlarging ESG aperture, beam current increased 10% at high current beam and total acceleration grid heat load reduced from 13% to 10% of input power at long pulse beam. In addition, heat load by back stream positive ion into the EXG is measured for the first time and is estimated as 0.3% of beam power, while heat load by back stream ion into the source chamber is estimated as 3.5 ~ 4.0% of beam power. Beam acceleration up to 60 s which is the facility limit, has achieved at 683 keV, 100 A/m² of negative ion beam, whose energy density increases two orders of magnitude since 2011. [ABSTRACT FROM AUTHOR]

In large negative ion sources of the neutral beam injector (NBI) for large fusion devices, such as ITER and JT-60SA, one of issues is that negative ion beam might be partially intercepted at acceleration grids due to a spatial non-uniformity of negative ion production on large extraction area. Previous experiments have shown that fast electrons emitted from filament cathodes are transported in a longitudinal direction by B × grad B drift and the spatial distribution of electron temperature (Te) is strongly related with the non-uniformity of negative ion production. In this study, a three-dimensional electron transport analysis code including detailed collision processes has been developed to clarify a physical mechanism of non-uniform Te distribution. Electron density and temperature in the analysis agree well with measurements by Langmuir probe in the JAEA 10A ion source. Then, this study has clarified that the non-uniformity of Te distribution is mainly caused by the following reasons; i) fast electrons drifted in the longitudinal direction survive near the end wall with high energy up to E = 25 - 60 eV and ii) they produce thermal electrons by collision with plasma particles there. [ABSTRACT FROM AUTHOR]

Acceleration of a 500 keV beam up to 2.8 A has been achieved on a JT-60U negative ion source with a three-stage accelerator by overcoming low voltage holding which is one of the critical issues for realization of the JT-60SA ion source. In order to improve the voltage holding, preliminary voltage holding tests with small-size grids with uniform and locally intense electric fields were carried out, and suggested that the voltage holding was degraded by both the size and local electric field effects. Therefore, the local electric field was reduced by tuning gap lengths between the large size grids and grid support structures of the accelerator. Moreover, a beam radiation shield which limited extension of the minimum gap length was also optimized so as to reduce the local electric field while maintaining the shielding effect. These modifications were based on the experiment results, and significantly increased the voltage holding from <150 kV/stage for the original configuration to 200 kV/stage. These techniques for improvement of voltage holding should also be applicable to other large ion sources accelerators such as those for ITER. [ABSTRACT FROM AUTHOR]

Voltage holding of -1 MV is an essential issue in development of a multi-aperture multi-grid (MAMuG) negative ion accelerator, of which target is to accelerate 200 A/m2 H- ion beam up to the energy of 1 MeV for several tens seconds. Review of voltage holding results ever obtained with various geometries of the accelerators showed that the voltage holding capability was about a half of designed value based on the experiment obtained from ideal small electrode. This is considered due to local electric field concentration in the accelerators, such as edge and steps between multi-aperture grids and its support structures. Based on the detailed investigation with electric field analysis, accelerator was modified to reduce the electric field concentration by reshaping the support structures and expanding the gap length between the grid supports. After the modifications, the accelerator succeeded in sustaining -1 MV for more than one hour in vacuum. Improvement of the voltage holding characteristics progressed the energy and current accelerated by the MeV accelerator. Up to 2010, beam parameters achieved by the MAMuG accelerator were increased to 879 keV, 0.36 A (157 A/m2) at perveance matched condition and 937 keV, 0.33 A (144 A/m2) slightly under perveance. [ABSTRACT FROM AUTHOR]

This paper describes the development of the neutral beam (NB) systems on JT-60SA, where 30-34 MW D0 beams are required to be injected for 100 s. A 30 s operation of the NB injectors suggests that existing beamline components and positive ion sources on JT-60U can be reused without the modifications on JT-60 SA. The JT-60 negative ion source was modified to improve the voltage holding capability, which leads to a successful acceleration of 2.8 A H- ion beam up to 500 keV of the rated acceleration energy for JT-60SA. [ABSTRACT FROM AUTHOR]

This paper reports the recent activities at JAEA for the development of 1 MeV H- ion accelerator toward the ITER NBI. For the development of MAMuG accelerator, a 320 mA H- ion beam was successfully accelerated as the highest record in the world at the MeV class energy (796 keV). This was achieved by protecting the H- ion source from the high heat load by the backstream positive ions, which was produced during the high current H- ion acceleration. The SINGAP accelerator was tested at JAEA to compare the performance of the SINGAP and the MAMuG. This paper also reports the results of the SINGAP test and the comparison between the SINGAP and the MAMuG is discussed. [ABSTRACT FROM AUTHOR]

IRFM (CEA Cadarache) and JAEA Naka have entered into a collaboration in order to test a SINGAP [1] accelerator at the JAEA Megavolt Test Facility (MTF) at Naka, Japan. Whereas at the CEA testbed the acceleration current was limited to 0.1 A, at JAEA 0.5 A is available. This allows the acceleration of 15 H- beamlets in SINGAP to be tested and a direct comparison between SINGAP and MAMuG [2] to be made. High-voltage conditioning in the SINGAP configuration has been quite slow, with 581 kV in vacuum achieved after 140 hours of conditioning. With 0.1 Pa of H2 gas present in the accelerator 787 kV could be achieved. The conditioning curve for MAMuG is 200 kV higher. SINGAP beam optics appears in agreement with calculation results. A beamlet divergence better than 5 mrad was obtained. SINGAP accelerates electrons to a higher energy than MAMuG. Measurements of the power intercepted on one of the electron dumps have been compared with EAMCC code [3] calculations. Based on the experiments described here, electron production by a SINGAP accelerator scaled up to ITER size was estimated to be too high for comfort [ABSTRACT FROM AUTHOR]

Excess heat loads to accelerator grids limit extension of pulse length in operation of the large negative ion sources with multi aperture accelerator. Part of the heat loads is caused by interception of deflected beamlets due to their space charge repulsion. In this paper, a beamlet steering technique using aperture offset was examined for compensation of the beamlet deflections utilizing a three dimensional beam analysis simulating the D- negative ion source of JT-60 U. The beamlet deflection was analyzed in detail using fifty beamlets, which were extracted from apertures arranged in a lattice pattern of 10×5. The simulation showed successful compensation of the beamlet deflection by aperture offsets defined according to the thin lens theory. Even if the beam energy was changed, the necessary aperture offset would not be changed maintaining the perveance and a ratio of extraction and acceleration voltage. In JT-60 U, it was shown that the aperture offset of less than 1.0 mm would be enough to compensate the repulsion of all beamlets. When the magnetic field was applied for suppression of co-extracted electrons, necessary aperture offset was estimated to be ±0.5 mm for 500 keV D- ion beam in JT-60 U, in addition to the offset for the space charge repulsion. This result showed good agreements with the previous experimental results and design study of the JT-60 U N-NBI. [ABSTRACT FROM AUTHOR]

Negative ion beams are assuming increasing significance in the field of controlled fusion research. The first generation of large high current negative ion sources has now operated successfully on the JT-60U tokamak for some years, producing negative ions of hydrogen or deuterium at energies up to 400 keV for conversion to several megawatts of neutrals which are used for plasma heating and current drive. We report an unexpected intensification phenomenon, and some of the improvements made or planned. In the realm of inertial confinement fusion, interest has grown in the use of heavy negative ion driver beams formed from halogens such as bromine, iodine, or chlorine. We discuss the advantages of negative ions as heavy ion fusion drivers, and assess the technical feasibility of a halogen negative beam system. We also discuss the potential for photodetaching the negative ions to produce atomically neutral beams and reduce beam space charge effects within the target chamber. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

Negative Cu ions of 60 keV, at high dose rates, have been applied for nanoparticle fabrication in insulators, using both conventional implantation and dynamic negative ion mixing (DNIM). Intense ion beams cause significant inbeam rearrangements of implants, i.e., spontaneous precipitation and depth-directional atomic migration. These dynamic processes are applicable for spatial control of nanoparticle structures. The spatial control is also possible by changing substrates, i.e., amorphous and crystalline SiO[sub 2] and a spinel crystal of MgAl[sub 2]O[sub 4]. The other approach for the spatial control is to use a DNIM method with dynamic growth, which presents a thick uniform structure of nanoparticles. [ABSTRACT FROM AUTHOR]

The negative ion based neutral beam system now operating for plasma heating and current drive on the large JT-60U tokamak marked the first application of negative ion source technology to the production of high current, high voltage beams for conversion to neutral atomic beams. This pioneering system has demonstrated the technical feasibility of negative ion based neutral beams for future fusion devices. Because this was a very large advance in the state of the art with respect to all system parameters, the principal physical processes governing the performance of the ion source and accelerator were somewhat different than had been the case with earlier much smaller negative ion sources. We have explored the physical mechanisms limiting the power and pulse length capability of these large sources, and have implemented ameliorating changes to reduce power loading on the accelerator grids and increase the transmitted power fraction. [ABSTRACT FROM AUTHOR]

A high-current heavy-ion accelerator system has been developed to realize intense particle fluxes for material modification. The facility of a tandem accelerator attained 1 mA-class ion current both for negative low-energy ions and positive high-energy ions. A negative ion source of the key device is of plasma-sputter type, equipped with multi-cusp magnets and Cs supply. The intense negative ions are either directly used for material irradiation at 60 keV or further accelerated up to 6 MeV after charge transformation. Application of negative ions, alleviating surface charging, enables us to conduct low-energy high-current irradiation to insulating substrates. Since positive ions over MeV are irrelevant to the Coulomb repulsion, the facility as a whole meets high-current irradiation into insulators over a wide energy range. Application of high flux ions provides technological merits not only in efficient implantation but also in different material kinetics. Other characteristics of the system are co-irradiation of intense laser and in-situ detection of kinetic processes. For the material modification, we present nanoparticle fabrication in insulators, and synergistic phenomena by co-irradiation of ions and photons. [ABSTRACT FROM PUBLISHER]

The 500keV Negative-ion based neutral beam injector for JT-60U started the operation in 1996 and has injected so far a neutral beam power of 5.2MW at 350keV for 0.7sec. However there are some issues to be conquered for increasing furthermore the beam power and beam pulse duration time. One of those is source plasma non-uniformity in the arc chamber. Improving the issue, three countermeasures have been conducted. The first is to change an arc discharge distribution through changing arc current resistors which connects in series to each of eight filament cathode groups. The second is masking both ends of the plasma grid with thin molybdenum plates to block beam extraction at the edges. The third is changing arc discharge mode from space charge limited discharge to emission limited which is relatively independent on the plasma condition in the arc chamber. The first two countermeasures have been determined to be effective against the source plasma non-uniformity, and the third is being examined. [ABSTRACT FROM PUBLISHER]