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In order to realize negative ion sources and accelerators to be applicable to International Thermonuclear Experimental Reactor and JT-60 Super Advanced, a large cesium (Cs)-seeded negative ion source and a multi-aperture and multi-stage electric acceleration have been developed at Japan Atomic Energy Agency (JAEA). Long pulse production and acceleration of the negative ion beams have been independently carried out. The long pulse production of the high current beams has achieved 100 s at the beam current of 15 A by modifying the JT-60 negative ion source. The pulse duration time is increased three times longer than that before the modification. As for the acceleration, a pulse duration time has been also extended two orders of magnitudes from 0.4 s to 60 s. The developments of the negative ion source and acceleration at JAEA are well in progress towards the realization of the negative ion sources and accelerators for fusion applications. [ABSTRACT FROM AUTHOR]

To understand the physics of the cesium (Cs) recycling in the large Cs-seeded negative ion sources relevant to ITER and JT-60SA with ion extraction area of 45-60 cm × 110-120 cm, the time evolution of the negative ion profile was precisely measured in JT-60SA where the ion extraction area is longitudinally segmented into 5. The Cs was seeded from the oven at 180 °C to the ion source. After 1 g of Cs input, surface production of the negative ions appeared only in the central segment where a Cs nozzle was located. Up to 2 g of Cs, the negative ion profile was longitudinally expanded over full ion extraction area. The measured time evolution of the negative ion profile has the similar tendency of distribution of the Cs atoms that is calculated. From the results, it is suggested that Cs atom distribution is correlated with the formation of the negative ion profile. [ABSTRACT FROM AUTHOR]

Long pulse beam extraction with a current density of 120 A/m² for 100 s has been achieved with a newly developed plasma grid (PG) for the JT-60SA negative ion source which is designed to produce high power and long pulse beams with a negative ion current of 130 A/m² (22 A) and a pulse length of 100 s. The PG temperature is regulated by fluorinated fluids in order to keep the high PG temperature for the cesium-seeded negative ion production. The time constant for temperature controllability of the PG was measured to be below 10 s, which was mainly determined by the heat transfer coefficient of the fluorinated fluid. The measured decay time of the negative ion current extracted from the actively temperature-controlled PG was 430 s which was sufficient for the JT-60SA requirement, and much longer than that by inertial-cooling PG of 60 s. Obtained results of the long pulse capability are utilized to design the full size PG for the JT-60SA negative ion source. [ABSTRACT FROM AUTHOR]

The purpose of this study is to calculate atomic (H0) density distribution in JAEA 10 ampere negative ion source. A collisional radiative model is developed for the calculation of the H0 density distribution. The non-equilibrium feature of the electron energy distribution function (EEDF), which mainly determines the H0 production rate, is included by substituting the EEDF calculated from 3D electron transport analysis. In this paper, the H0 production rate, the ionization rate, and the density distribution in the source chamber are calculated. In the region where high energy electrons exist, the H0 production and the ionization are enhanced. The calculated H0 density distribution with-out the effect of the H0 transport is relatively small in the upper region. In the next step, the effect should be taken into account to obtain more realistic H0 distribution. [ABSTRACT FROM AUTHOR]

The modeling and analysis of a negative ion source is proceeding by using a 2D particle-in-cell simulation. The effect of the H- ion production on the plasma grid (PG) surface is investigated. It is shown that with the increase of H- ions per time step, the H- ion current density is enhanced, while the electron current density decreases with increasing the H- production rate on the PG surface. These results agree well with the experimental results observed in typical negative ion sources. Moreover, it is found that plasma quasi-neutrality is held mainly by both H+ and H- ions in the bulk plasma around the PG. [ABSTRACT FROM AUTHOR]

The physical mechanism of the formation of the negative ion beam halo and the heat loads of the multi-stage acceleration grids are investigated with the 3D PIC (particle in cell) simulation. The following physical mechanism of the beam halo formation is verified: The beam core and the halo consist of the negative ions extracted from the center and the periphery of the meniscus, respectively. This difference of negative ion extraction location results in a geometrical aberration. Furthermore, it is shown that the heat loads on the first acceleration grid and the second acceleration grid are quantitatively improved compared with those for the 2D PIC simulation result. [ABSTRACT FROM AUTHOR]

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]

Abstract: The origin of the beam non-uniformity, that is one of the key issues in JT-60 negative ion sources, was experimentally examined by measuring correlations between the intensity of the H− ion beam and plasma parameters, such as an electron temperature and plasma density in the JAERI 10 A negative ion source. The ion source was operated without cesium. Although a uniform filter field was applied, the electron temperature near plasma grid (PG) was higher than 1eV in the upper half region of the longitudinal direction, where the spatial beam intensity decreased rapidly. This indicates that the beam non-uniformity is caused by a higher electron temperature than 1eV, namely, by a leakage of the high-energy electrons toward the PG. To investigate the path of high-energy electrons leaking into the PG, a metal plate of 50mm×50mm was placed in the plasma. By placing the plate in a weak magnetic field of <30G leading to a driver region near the upper wall, the electron temperature near the PG was sufficiently reduced to 1eV in the upper half region, where the beam intensity was simultaneously increased. As a result, a 20% gain of the beam current was obtained. Further, the source of the high-energy electrons leaking toward the PG was examined by changing the relative filament position to the upper wall. It is found that the filaments closest to the upper wall, i.e., at 80mm from the upper wall is one of the sources of the high-energy electrons leaking into the PG. [Copyright &y& Elsevier]

Heat loading of acceleration grids has been investigated using a JAERI 400 keV negative hydrogen ion source, which has a three-stage multiaperture accelerator. To investigate the effect of beamlet steering on the heat loading, the apertures in the grounded grid were displaced up to 5 mm. The steering angle increased linearly with the displacement distance of the grid. The obtained steering angle was 7.5 mrad at the displacement distance of 5 mm. It was confirmed that the steering angle agrees well with the thin lens theory. Degradation of beam optics due to the steering was not observed. The heat load of the grounded grid was scarcely changed up to the displacement distance of 2 mm and then increased from 2.5% to 8% of the total input power when the grid was displaced from 2.5 to 5 mm. Heat loads of the other unsteered grids were not changed. © 2001 American Institute of Physics. [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.

High power and long-pulse negative ion extractor, which is composed of the plasma grid (PG) and the extraction grid (EXG), is newly developed toward the neutral beam injector for heating and current drive of future fusion machines such as ITER, JT-60 Super Advanced and DEMO reactor. The PG is designed to enhance surface production of negative ions efficiently by applying the chamfered aperture. The efficiency of the negative ion production for the discharge power increased by a factor of 1.3 against that of the conventional PG. The EXG is also designed with the thermal analysis to upgrade the cooling capability for the long pulse operation of >1000 s required in ITER. Though the magnetic field for electron suppression is reduced to 0.75 of that in the conventional EXG due to this upgrade, it was experimentally confirmed that the extracted electron current can be suppressed to the allowable level for the long pulse operation. These results show that newly developed extractor has the high potential for the long pulse extraction of the negative ions. [ABSTRACT FROM AUTHOR]

In order to realize neutral beam systems in International Thermonuclear Experimental Reactor whose target is to produce a 1 MeV, 200 A/m² during 3600 s D- ion beam, the electrostatic five-stages negative ion accelerator so-called "MeV accelerator" has been developed at Japan Atomic Energy Agency. To extend pulse length, heat load of the acceleration grids was reduced by controlling the ion beam trajectory. Namely, the beam deflection due to the residual magnetic field of filter magnet was suppressed with the newly developed extractor with a 0.5 mm off-set aperture displacement. The new extractor improved the deflection angle from 6 mrad to 1 mrad, resulting in the reduction of direct interception of negative ions from 23% to 15% of the total acceleration power, respectively. As a result, the pulse length of 130 A/m², 881 keV H- ion beam has been successfully extended from a previous value of 0.4 s to 8.7 s. This is the first long pulse negative ion beam acceleration over 100 MW/m². [ABSTRACT FROM AUTHOR]

The JT-60U negative ion source has been designed to produce high current beams of 22 A through grids of 1080 apertures (five segments with nine rows of 24 apertures). One of the key issues is to steer such a high current beam through the multiaperture grids in order to focus the overall beam envelope because the beamlet-beamlet interaction may deflect the outer beamlets outward due to unbalanced space charge repulsion. To clarify the beam deflection in the JT-60U negative ion source, the beamlet trajectory in a multiaperture ion source was calculated by a three-dimensional simulation code. The measured angles of the outmost beamlets were in agreement with the calculated results where space charge of the beamlets was taken into account. It is noticed that the deflection of the outermost beamlet due to the beamlet-beamlet interaction is saturated at 5.2 mrad outward for beamlets more than ten. [ABSTRACT FROM AUTHOR]

The influence of magnetic filter configuration on the beam uniformity was examined to improve beam uniformity in a large Cs-seeded negative-ion source. By reducing the filter strength of the transverse magnetic field used in a typical negative-ion source, the beam uniformity was largely improved with the improvement of the plasma uniformity while the beam intensity was kept to be nearly constant. However, the coextracted electron current greatly increased. To suppress the coextracted electron current, a tent-shaped magnetic filter was applied together with modifications in the cusp magnets to form a typical multicusp positive-ion source arrangement. The uniformity in longitudinal beam profile was improved with the deviation of local beam intensity within 16% that was nearly equal to the deviation obtained at 50 G cm of the transverse filter strength. In the meantime, the coextracted electron current was kept to be the same as the H- ion current. The present result suggests that the uniformity of H- ion-beam profile is affected by the uniformity of atoms or protons in the source plasma, which are the primary species of negative ions under Cs-seeded surface production. [ABSTRACT FROM AUTHOR]

In the JT-60U negative-ion-based neutral beam injection system, the effect of negative ion and electron deflection by the plasma grid (PG) magnetic filter was studied. After a long-pulse operation of up to 19 s, a local melting was observed on the beamline near the ion source, facing the electron drift side of the PG magnetic field. It is confirmed that the experimental deflection of the negative-ion beam agrees well with the three-dimensional beam simulation result by taking account of the measured magnetic field. By using the code, it is found that some stripped electrons produced in the first acceleration gap pass through the down pitch of the multiple apertures in the next stage of acceleration grids, and then collide on the beamline around the melted location. [ABSTRACT FROM AUTHOR]

The H- ion production and transport processes are numerically simulated to clarify the origin of H- beam nonuniformity. The three-dimensional transport code using the Monte Carlo method has been applied to H0 atoms and H- ions in the large “JAERI 10A negative ion source” under the Cs-seeded condition, in which negative ions are dominantly produced by the surface production process. The results show that a large fraction of hydrogen atoms is produced in the region with high electron temperature. This leads to a spatial nonuniformity of H0 atom flux to the plasma grid and the resultant H- ion surface production. In addition, most surface-produced H- ions are extracted even through the high Te region without destruction. These results indicate a correlation between the production process of the H- ion and the spatial nonuniformity of the H- ion beam. [ABSTRACT FROM AUTHOR]

To clarify the physical mechanism of the plasma spatial nonuniformity observed in tandem-type negative-ion sources, the primary electron-transport process has been analyzed by a three-dimensional Monte Carlo simulation code. In the model, equations of motion for electrons are numerically solved. The realistic geometry and magnetic-field configuration are taken into account. Various collision processes with neutral particles are also included in the model. The simulation results show that (1) the primary electrons have been lost from the source region to the extraction region due to the magnetic drift in the filter magnetic region and (2) the magnetic drift velocities have opposite directions near the sidewalls. The effect of the magnetic drift is one of the possible causes to explain the spatial nonuniformity observed in the experiment. [ABSTRACT FROM AUTHOR]

A spatial uniformity of negative ion production has been studied experimentally in a tandem-type volume negative ion source, where the plasma is divided into two regions by a magnetic filter, i.e., a driver region and an extraction region near the plasma grid (PG). A plasma density profile and the negative ion beam profile were measured. By the Langmuir probe measurement, a strong nonuniformity of the plasma density was observed near the plasma grid when the plasma grid is biased electrically with respect to the anode. By changing the polarity of the bias voltage, the deviation of the plasma density profile changed to the direction of E x B. In the pure hydrogen operation, the extracted beam profile corresponded to the plasma density profile in the extraction region. In the Cs-seeded operation, on the other hand, the uniformity of the beam current density became uniform despite the nonuniformity of the plasma density profile in the extraction region. The uniformity of the negative ions corresponded to the plasma uniformity in the driver region. These results indicate that the negative ions are not produced by positive ions in the extraction region in the Cs-seeded operation. We conclude that the negative ions are produced mainly by neutral atoms produced in the driver region via the surface production on the PG. [ABSTRACT FROM AUTHOR]

Describes the grid power loading in a multistage negative ion accelerator. Suppression of the electrons in an exactor; Occurrence of the leakage of extracted electrons from the plasma to the grids; Dominance of the electron acceleration over the power loadings of the acceleration grids.

A large semicylindrical negative ion source, whose dimensions are 34 cm in diameter and 104 em in length, has been developed. By optimizing a chamber depth and a filter strength, a H[sup -] current of 650 mA (7.7 mA/cm²) was obtained at an arc condition of 1.3 Pa, 70 V, and 1200 A. In order to increase the H[sup -] current and to reduce an operating pressure, a small amount of Cs was injected. As a result, the H[sup -] current was increased from 550 mA (6.5 mA/cm²) to 850 mA (10 mA/cm²) under an are power of 70 V×800 A. The most significant feature of the cesium effect was the reduction of the operating pressure. With maintaining the sufficient current of 670 mA (8 mA/cm²), the operating pressure could be lowered to 0.03 Pa. At this pressure, the highest gas efficiency of 20% was achieved. [ABSTRACT FROM AUTHOR]

To understand the physical mechanism of the beam halo formation in negative ion beams, a two-dimensional particle-in-cell code for simulating the trajectories of negative ions created via surface production has been developed. The simulation code reproduces a beam halo observed in an actual negative ion beam. The negative ions extracted from the periphery of the plasma meniscus (an electro-static lens in a source plasma) are over-focused in the extractor due to large curvature of the meniscus. [ABSTRACT FROM AUTHOR]

Abstract: In the negative ion based neutral beam injector (N-NBI) for JT-60U, an extension of the pulse duration up to 30s has been intended to study quasi-steady state plasmas, where the NBI pulse duration is significantly longer than the plasma current diffusion time. Achieving long pulse injection requires a reduction of the heat load on the grids of the ion source. The ion source was modified to reduce the pressure within the grid structure by increasing the vacuum conductance of the acceleration grids. The acceleration efficiency was improved from 0.74 to 0.79. The heat load of the grounded grid, which was the main heat load of the acceleration grids, decreased from 9.2 to 7.0% at a constant arc chamber pressure of 0.3Pa. These improvements seemed to be caused by the reduction of beam ion stripping loss. Up to now, a long pulse injection of 17s at the power of 1.6MW with beam energy of 366keV was obtained by using one ion source. The temperature rise of the cooling water from the grounded grid saturated at less than 35°C during operation, so the heat load of the grid is significantly reduced for steady state operation at this power level. [Copyright &y& Elsevier]

The ITER NB System consists of two injectors each delivering 16.7 MW to the plasma in the form of neutral deuterium atoms accelerated at 1 MeV. Each injector has a single negative ion source connected to a 1 MV electrostatic accelerator. The accelerated negative ions pass through a neutraliser, then an electrostatic deflection system, where residual ions, exiting after neutralisation, are dumped. The neutral part of the beam continues to the torus through a duct connecting to the torus vessel. The 1 MV bushing is an insulated cylindrical, feedthrough (about 2 m in diameter and 2.5 m in height), which forms the physical boundary between the gas-insulated transmission line and the torus primary vacuum. An innovative design has been developed fulfilling all the requirements, including 1 MV class insulation in vacuum for a large number of services, vacuum compatibility, operation under neutrons and gamma radiation, safety requirements for pressure containment. In particular, the design allows a nitrogen pressurised guard volume that prevents any leakage of the external insulation gas (sulphur hexaflouride, potentially damaging for platinum/palladium catalysts of the tritium recovery system) inside the torus primary vacuum, and relieves the alumina insulator from substantial structural functions. The article describes the design, the selection of the materials and the mechanical analyses. To validate the design a reduced scale prototype, including all the relevant mechanical and electrical features of the bushing insulator, has been successfully manufactured and tested. The article reports an overview of this development. [Copyright &y& Elsevier]

In order to reduce the tritium inventory in a fuel circulation system in fusion plants, a possibility of deuterium-tritium (DT) mixed neutral beam injectors with DT negative ion beams has been investigated by analyzing the beam optics of the DT-mixed negative ion beams for the first time. In a two-dimensional beam analysis, DT-mixed ion beams could be accelerated regardless of the DT current mixing ratio (T/D) from the identical acceleration geometry with the identical applied voltages by adjusting total space charge for the D and T components. However, in a three-dimensional beam analysis including magnetic fields, the DT-mixed ion beams were separated due to a mass dependence of the beam deflection by orthogonal magnetic fields of an electron suppression and transverse filter fields for negative ion production. In the case of the beamline for ITER, which requires beam deflection angle within ±1 mrad, the operational range for the DT-mixed ion beams was found to be the DT current mixing ratio of 0.7-1.5 and beam energy of 0.9-1.0 MeV. Although the operational range was not wide, the DT-mixed beam was found to be feasible in terms of the beam acceleration. These analyses also contribute to the study of the acceleration of other mixed ion beams such as impurity or heavy ions, which are useful techniques for diagnostic beams. [ABSTRACT FROM AUTHOR]