22D01
Development of 1 MeV Accelerator for ITER;
Achievement of 0.98 MeV, 185 A/m2 Negative Ion Beam Acceleration
ITER 1 MeV加速器開発；0.98 MeV, 185 A/m2負イオンビームの達成
Mieko Kashiwagi, Masaki Taniguchi, Naotaka Umeda, Masayuki Dairaku,
Jyunpei Takemoto, Hiroyuki Tobari, Kazuki Tsuchida, Haruhiko Yamanaka,
Kazuhiro Watanabe, Keishi Sakamoto and Takashi Inoue
柏木美恵子, 谷口正樹, 梅田尚孝, 大楽正幸, 武本純平, 土田一輝, 戸張博之,山中晴彦, 渡邊和弘, 坂本慶司, 井上多加志
Japan Atomic Energy Agency (JAEA),
801-1-Mukouyama, Naka, Ibaraki 311-0193, Japan（独）日本原子力研究開発機構 〒311-0193

茨城県那珂市向山801-1

In a MeV accelerator test toward 1 MeV, 200A/m2 H- ion beam acceleration for the ITER neutral beam
injector (NBI), the beam energy was limited up to 0.8 MeV due to insufficient voltage holding capability
and direct interception of deflected beamlets at acceleration grids. To improve the voltage holding, local
electric field concentrations were lowered by expanding gaps and increasing radii of corners of gridsupports based on voltage holding tests and electric field analysis. To compensate the beamlet deflectionsdue to magnetic fields for electron suppression and space charge repulsion between the beamlets,compensation methods were designed in a three dimensional beam analysis, so as to utilize aperture offset
in the electron suppression grid (ESG) and a metal plate attached beneath the ESG, so-called a fieldshaping plate. As the results, beam parameters increased to 0.98 MeV, 185 A/m2, which are almost
achieving the target values.

1. Introduction
In a negative ion accelerator for the ITER
neutral beam injector (NBI), it is required togenerate 1 MeV, 40 A (200 A/m2) deuterium
negative ion (D-) beams for 3600 s. Toward therequirement, beam acceleration tests areprogressed in the MeV test facility (MTF) andJT-60U NBI at Japan Atomic Energy Agency(JAEA) [1, 2]. At the MTF, a five stage
multi-aperture multi-grid (MAMuG) accelerator,
so-called “MeV accelerator”, is tested todemonstrate 1 MeV, 200 A/m2 (0.5 A in total) Hion beam accelerations. However, the beamenergy was limited to 0.8 MeV till 2008 [3]. Thisis because of insufficient voltage holding
capability and direct interception of the beams atacceleration grids, which could cause breakdowns[4].
To increase the beam parameters, the MeV
accelerator was modified according to results of a
voltage holding test. Moreover, compensation
methods of the beamlet deflections were applied
based on a three dimensional (3D) beam analysis.
After these modifications, beam acceleration tests
have been performed toward 1 MeV beams.

shown in Fig.1. The negative ions are extracted
from apertures (14 mm in diameter) drilled in a
lattice pattern of 5 (R1-R5) × 3 (C1-C3) in theextractor. In the extraction grid (EXG), permanent
magnets are embedded between each aperture row
for electron suppression. The beam is accelerated to200 keV in each acceleration stage, and as aconsequence, 1 MeV at the exit of the fifth stage.
At 2.5 m downstream of the accelerator exit, atarget made of one-dimensional carbon-fibercomposite (CFC) and a water-cooled copper
calorimeter are installed to measure beam footprints

2. MeV Accelerator
A schematic view of the MeV accelerator isFig.1. MeV accelerator

and beam current, respectively.
3. Modification Based on Voltage Holding Test
In the voltage holding tests and electric field
analysis [4, 5], it was found that local electric fields
at steps/edges of grid supporting structure should belowered to suppress breakdowns. For this purpose,
the accelerator was modified as shown in Fig.1. The
minimum gap length was extended from 72 mm in
the original to 100 mm so as to hold 200 kV stably(1 MV for five stages) including the margin of
20 %. A radius of the corner of the grid support wasincreased from 15 mm to 30 mm. After thesemodifications, voltage holding capability wasdrastically improved and achieved stable voltage
holding of 1 MV without gas feeding (basepressure; 2 x 10-4 Pa) for 4000 s. As a result, thebeam parameters increased to 0.86 MeV, 160 A/m2.
4. Compensations of the Beamlet Deflections
To study the beamlet deflections and thecompensations, a 3D beam analysis has been
carried out using OPERA-3d code [6]. The
magnetic field generated by the permanent magnets
was taken into account. The stripping loss ofnegative ions in the course of acceleration was also
included to provide a practical accuracy in the spacecharge repulsion. The detailed grid support
structure was included in the model to study
influence of electric field distortions formed by thesteps/edges of the grid supports on the beamlet
deflection.
Figure 2 (a) and (b) show the calculated
footprints of 1 MeV beams on the CFC target
position before and after the compensations of thebeamlet deflections, respectively. The beamlets are
deflected alternatively in the horizontal direction ineach aperture row due to i) the magnetic field
formed by the permanent magnets in the EXG.
Hence, the beam footprint shows a zigzag patternon the CFC target position. Moreover, theperipheral beamlets are deflected outwards due toii) the space charge repulsion between beamlets.
The beamlet deflection angles are 4.7 mrad in theFig.2. Calculated beam footprints on the target position.
(a) Before and (b) after the compensations.

Fig.3. Progress of beam accelerations

center beamlet (R3C2) due to i) and 9.5 mrad in theperipheral beamlet (R3C3) due to superposition of i)
and ii). As a compensation method, the aperture
offset [7] of 0.8 mm in the electron suppressiongrids (ESG) was designed for the beamlet
deflection due to i). In addition, the field shaping
plate (FSP) [8], which is a metal plate of 1 mm
thick attached beneath the ESG, was designed forthe peripheral beamlet deflected due tosuperposition of i) and ii). After applying thesemethods, the calculated beamlet deflections arecompensated as shown in Fig.2 (b). In theexperiment, the measured deflection angles of thebeamlets were in good agreement with thecalculated ones. Finally, the beam accelerations
were demonstrated at around 1 MeV.
5. Summary
As a result of improvement of the voltage
holding and the compensations of the beamlet
deflections, the beam parameters increased from 0.8
MeV to 0.98 MeV, 185 A/m2 and 0.94 MeV, 190
A/m2 as shown in Fig.3. These are almost achieving
the target values. The modifications are applied tothe ITER accelerator design.
References[1] T. Inoue et al., Nucl. Fusion 46(6) (2006) S379.
[2] A. Kojima et al., Nucl. Fusion 51 (2011) 083049.
[3] M. Kashiwagi et al., Nucl. Fusion 49 (2009) 065008.
[4] M. Taniguchi, et al., AIP Conf. Proc. 1390 (2011)449.
[5] N. Umeda et al., 22P156-P in this conference.
[6] M. Kashiwagi, et al., AIP Conf. Proc. 1390 (2011)457.
[7] T. Inoue, JAERI-Tech.2000-051 (2000).
[8] M. Kamada et al., Rev. Sci. Instrum. 79 (2008)
02C114.