22P156-P
Electrostatic Analysis and Improvement of Voltage Holding Capability of
the 1 MeV Accelerator for ITER NBI
ITER NBI用1MeV加速器の電界解析と耐電圧性能向上
Naotaka Umeda, Masaki Taniguchi, Mieko Kashiwagi, Hiroyuki Tobari, Masayuki Dairaku,
Kazuhiro Watanabe, Haruhiko Yamanaka, Takashi Inoue and Keishi Sakamoto
梅田尚孝, 谷口正樹，柏木美恵子，戸張博之, 大楽正幸, 渡邊和弘, 山中晴彦, 井上多加志,
坂本慶司
Naka Fusion Research Institute, Japan Atomic Energy Agency (JAEA)
801-1, Mukouyama, Naka-shi, Ibaraki-ken, 311-0193, Japan
原子力機構 那珂核融合研究所 〒311-0193 茨城県那珂市向山801－1
In Japan Atomic Energy Agency (JAEA), R&D of the MeV accelerator for the ITER NBI has been
conducted. An issue for stable beam acceleration at 1 MeV was voltage holding in vacuum. The voltage
was limited up to 835 kV for the previous accelerator configuration due to breakdown in vacuum.
Discharge marks were observed on the grid supports. In an electrostatic analysis, it was found that theelectric field concentrated at screen shield and the corner of the grid support. The accelerator was modifiedto lower the local electric filed at these parts. As the results, MeV accelerator was succeeded in sustaining
1 MV in vacuum.

1. Introduction
The neutral beam injector (NBI) for ITER isrequired to produce 16.5 MW of deuterium neutral
beams per one injector at the energy of 1 MeV.
Acceleration of 40 A (200 A/m2) D- ion beams up to1 MeV is required so as to fulfill the requirement. A
multi-aperture multi-grid (MAMuG) accelerator
called “MeV accelerator” has been developed at
Japan Atomic Energy Agency (JAEA) [1]. A goal ofthe MeV accelerator is to demonstrate H- ion beamacceleration up to 1 MeV at the current density of
200 A/m2 for several tens seconds. An issue for therealization is to improve voltage holding capabilityof the accelerator [2]. In the present work,
electrostatic analysis was carried out and theaccelerator was modified to improve the voltage
holding capability.
2. MeV accelerator
Figure 1 shows a cross sectional view of theMeV accelerator. The MeV accelerator is anelectrostatic accelerator composed of fiveacceleration stages which are formed with six
electrodes such as EXG, A1G, A2G, A3G, A4G andGRG. Each stage is insulated with fiber reinforced
plastic (FRP) ring. Inside of the FRP ring is vacuum
and outside of the ring is filled with SF6 gas at 0.6MPa. Voltage of 200 kV is applied to each stage (1MV for five stages). Left side of Fig.1 shows theprevious accelerator tested in 2008. In thisaccelerator, grid and grid support structure of A2G
were removed for simplicity. So voltage of 400 kV

New accelerator(2010)

Previous accelerator(2008)EXG
A1G102
181
78
R15 72

A3G102
100
100 R30
100100Gap between
screen shilds

Curvature of thegrid suport edge

A4G
Gap between
grid surports

GRG
FRP
insulator

Fig.1 Cross section of the MeV accelerator

was applied between A1G and A3G.
Voltage holding test for each stage and full stages
were carried out with the previous accelerator.
Table I shows a result of the test. Voltage of only
one stage between A3G and A4G reached to rated
voltage of 200 kV and the maximum voltages ofother stages were lower than rated voltage. Voltage
holding of the full stage was limited to 835 kV.
After the test, the accelerator was disassembled
and its interior was inspected in detail. Many
discharge marks were observed in theTable I. Result of the voltage holding test
Stage
EXG-A1GA1G-A3GA3G-A4GA4G-GRG
Full stage

Minimum gap
102 mm
181 mm
78 mm
72 mm-Maximum voltage
181 kV334 kV
205 kV184 kV835 kV6.3 kV/mm(a)-1000kV6.7 kV/mm-800kVA3G support

Hole for pumping-400kV-200kV0kV-600kVSF6gas

6.4 kV/mm

6.0 kV/mm[kV/mm]7.1 kV/mm

discharge markFig. 3 Analysis result of the previous accelerator-1000kV(b)

A1G support-800kV-600kV

5.7 kV/mm-400kV-200kV0kV

3.9 kV/mm

discharge markFig. 2 Discharge marks (a) on the A3G support (anodeside) and (b) back side of A1G support (cathode side)

accelerator as shown in Fig. 2. The spotteddischarge marks were left on A3G support (Fig. 2(a), anode side). From the positions of these marks,
it seems as if negatively charged particles wereproduced on the screen shield at -1 MV potential
and pass through the holes opened at A1G supportfor pumping. While the ring shaped discharge markwas left at the back side of A1G support (Fig. 2 (b),cathode side) and seems to be triggered from thecorner of A3G support. From these discharge marks,
it seemed that discharge could be triggered fromboth anode and cathode sides. From the Cranberg’s
clump theory [3], breakdown occurs when thepower density of a clump colliding with theelectrode exceeds a constant value. The power
density is a product of voltage and charge density.
The charge density is in proportion to the local
electric field at the detached electrode surface. Thecharged clump would be released from both anodeand cathode surfaces. So it is necessary to reduce
electric filed at both cathode and anode sides.
3．Electrostatic analysis
A two dimensional electrostatic analysis was
carried out with ANSYS code [4] to clarify electricfield intensity at parts which could originate thedischarges. Figure 3 shows electric field profile
simulated with the previous accelerator when 200
kV is applied to each acceleration stage. Electricfield at the screen shield of -1 MV potential, which
seemed as an origin of the discharge as shown in fig.
2 (a), was 6.7 kV/mm. Electric field at the corner ofSF6gas

4.1 kV/mm[kV/mm]6.0 kV/mm

Fig. 4 Analysis result of the new accelerator

the A3G support, which supposed to be a trigger ofthe discharge as shown in fig.2 (b), was 6.3 kV/mm.
While electric filed at the screen shield of A3G was
6.0 kV/mm, and A3G-A4G stage could sustain
more than 200 kV as shown in Table I.
To reduce the electric field at these high stresspoints, modification of the accelerator wasproposed as follows: 1) increase gap length between
grid supports to 100 mm, 2) increase curvatureradius of the corner of the grid support from 15 to30 mm, 3) increase gap length between screen
shields more than 60 mm. Figure 4 shows theelectric field distribution for the new accelerator.
By the modification, electric field at the screen
shield and at the corner of the grid support wasreduced less than 6.0 kV/mm and 4.1 kV/mm,respectively. These modifications were actually
implemented to the MeV accelerator.
4. Voltage holding with new accelerator
Voltage holding test was carried out with the newaccelerator. After 60 hours of conditioning, voltage
has reached to 1MV without gas feeding andsustained more than 1 hour stably. Thus, it isimportant to reduce local electric field intensity atboth cathode and anode sides.
References[1] T.Inoue et al., Nucl. Fusion 46(6) (2006) S379
[2] M. Kashiwagi et al., Nucl. Fusion 49 (2009) 065008[3] L. Cranberg et al., J.Appl. Phys. 23 (1952) 518

[4] http://www.ansys.com/