Masaki Kamada

Compensation of beamlet repulsion in a large negative ion source with a multi aperture accelerator

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 ...

Beamlet deflection due to beamlet-beamlet interaction in a large-area multiaperture negative ion source for JT-60U

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.

Recent R&D Activities of Negative-Ion-Based Ion Source for JT-60SA

The JT-60 Super Advanced (JT-60SA) tokamak aims to perform the ITER support and to demonstrate steady-state high-beta plasma project with the collaboration between Japan and EU. To attain these objectives, the negative-ion-based NBI (N-NBI) system is required to inject 10 MW for 100 s at the beam energy of 500 keV. On JT-60U, the present N-NBI ion source has injected 3.2 MW for 21 s at 320 keV; however, three key issues should be solved for the JT-60SA N-NBI ion source. One is to improve the voltage holding capability of the large negative ion source, where the available acceleration voltage has been limited to less than ~400 kV due to breakdowns. The accelerator of the JT-60U ion source is composed of large three-stage grids and three fiberglass reinforced plastic (FRP) insulators. Recent R&D tests suggested that the FRP insulators were not the main factor to trigger the breakdowns at the early conditioning stage. The accelerator with a large area of grids and their supporting structure may need a high margin in the design of electric field and a long time for conditioning. The second issue is to reduce the power loading of the acceleration grids. It was found that some beamlets were strongly deflected due to beamlet-beamlet interaction and strike on the grounded grid in the accelerator. Moreover, the electrons generated in the accelerator caused the grid loading and the overheating of the beamline components. The acceleration grids for JT-60SA are to be designed by taking account of the beamlet-beamlet interaction and the applied magnetic field in 3-D simulation. Third is to maintain the D production for 100 s. Although a constant D- beam power was confirmed on JT-60U for 21 s, an active cooling system is required to keep the temperature of the plasma grid (PG) under optimum condition during 100-s operation. A simple cooling structure is proposed for the active cooled PG, where a key is the temperature gradient on the PG for uniform D- production. In the present schedule, design work, reflecting the latest R&D progress, will continue until ~2011. The modified N-NBI ion source will start on JT-60SA in 2015.

Steering of Multiple Beamlets in the JT‐60 U Negative Ion Source

The direct interception of D− ion beam by the acceleration grid was reduced by modifying field shaping plates (FSPs) in JT‐60 negative ion sources to inject powerful neutral beams for long pulse duration of >10u2009s. The modified FSPs are designed by a 3D simulation code to properly steer the outermost beamlets that are deflected outward by space charge of the inward beamlets. The measured steering angles were −1.2u2009mrad and −6.4/−4.5u2009mrad in the vertical and horizontal direction, which were in agreement with the designed value calculated by the 3D simulation code. The proper steering of the outermost beamlets allowed a significant reduction of the power loading of the grounded grid. The power loading was successfully reduced to an allowable level of 5% with respect to the acceleration beam power.

Correlation between voltage holding capability and light emission in a 500 keV electrostatic accelerator utilized for fusion application

Voltage holding capability in a 500 keV electrostatic accelerator with large FRP insulators was examined without the beam acceleration. When high voltage was applied, the light with a broad peak at a wavelength of 420 nm was mostly emitted inside the accelerator even though breakdown did not occur. The voltage holding capability has a strong correlation with the light intensity. Stable voltage holding was realized under the conditions in which light emission was well suppressed.

Electrons in the negative‐ion‐based NBI on JT‐60 U

The stripped electron trajectories in a large negative ion accelerator with multi‐apertures and three acceleration stages, where non‐uniform stray magnetic field is horizontally created, are calculated in the JT‐60 negative ion source by a 3‐D numerical code. The horizontal non‐uniform stray field results in a significant power loss of the stripped electrons in the outmost acceleration channel on the grounded grid (GRG). The power loss in the outmost acceleration channel is more than twice higher than that in the central channel due to the weaker stray field although the total power loading on the GRG is by 25% larger than that by assuming a uniform stray field.

Long pulse production of high current D− ion beams in the JT-60 negative ion sourcea)

The first long pulse production of high power D(-) ion beams has been demonstrated in the JT-60 U negative ion sources, each of which was designed to produce 22 A, 500 keV D(-) ion beams. Voltage holding capability and the grid power loading were examined for long pulse production of high power D(-) ion beams. From the correlation between voltage holding and the light intensity of cathodoluminescence from the Fiber Reinforced Plastic insulators, the acceleration voltage for stable voltage holding capability was found to be less than 320-340 kV where the light was sufficiently suppressed. By tuning the extraction voltage, the grid power loadings in the ion sources were decreased to the allowable levels for long pulse injection without a significant reduction of the beam power. After tuning the acceleration and extraction voltages, D(-) ion beams of 12.5 and 9.8 A were produced at 340 keV with cesium seeding at a rate of approximately 14 microg/s into the ion sources. The pulse duration of these D(-) ion beams was extended step by step, and then was successfully extended up to 18 s without degradation of the negative ion production. The D(-) ion beams were neutralized to yield 3.6 MW D(0) beams by a gas cell, and then injected into the JT-60 U plasma. Further, a slight reduction of D(-) ion beam power allowed the longer injection duration of 21 s at a D(0) beam power of 3.2 MW. The success in the long pulse production of a high power D(-) ion beam shows that negative ion beams can be produced during a few tens of seconds without degradations of negative ion production and the voltage holding in a large Cs-seeded negative ion source.

Energy Spectra of Bremsstrahlung X-Rays Emitted From an FRP Insulator

Energy spectra of X-rays emitted from the surface of a fiberglass-reinforced plastic (FRP) insulator were measured at three different positions and compared with those of the vacuum gap between electrodes. Near the anode, the X-ray spectrum was dominated by monoenergetic electrons. Near the cathode, the spectrum peak shifted to low energy as compared with that near the anode. This result showed that a large amount of low-energy electrons was generated on the surface of the FRP insulator near the cathode.

Power Loading of Electrons Ejected From the JT-60 Negative Ion Source

The power loading of electrons ejected from the negative ion accelerator to the beamline was first measured in the negative-ion-based neutral beam injector on JT-60U. At 0.3 Pa of the operating pressure in the arc chamber, the heat flux and the total power load for the single segment were ~8 W/cm2 and 27 kW for the D ion beam of 300 keV and 3.4 A, respectively. The normalized total power loading on the electron dump was no more than 2.6% of the electric power in the acceleration power supply. About 70% of the total power is originated by the electrons stripped from D ions due to collisions with residual gas molecules in the accelerator. The calculation of the stripped electron trajectories shows that the electrons stripped in the second acceleration gap are the main origin of the power loading in the beamline.

Conditioning characteristics of DC500kV large electrostatic accelerator in negative-ion based NBI on JT-60U

Voltage holding capability of a 500 kV, 22 A three-stage electrostatic accelerator, where large-area grids of 0.28 m2 and large FRP insulators of 1.8 m in diameter are used, was examined. High voltage was independently applied to each acceleration stage. The voltage holding capabilities of 130 kV were obtained for each acceleration stage. Namely, there is no particular weak stage for voltage holding capability. To identify the breakdown location in each stage, the grids were removed. Breakdown voltages without grids, i.e., for the FRP insulator itself reached 170 kV, that is the design value for each stage. These results show that the breakdown voltage of the accelerator was mainly determined by the vacuum gap between the large-area grids. In this paper, the influence of non-uniform electric field and multi-stage grids on the voltage holding capabilities was also discussed.