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FTP/1-2
クライオポンプ
Acceleration of 1 MeV H- ion beams
at ITER NB relevant high current density
負イオン源
Takashi INOUE
M. Taniguchi, M. Kashiwagi, N. Umeda, H. Tobari, M. Dairaku, J. Takemoto,
K. Tsuchida, K. Watanabe, T. Yamanaka, A. Kojima, M. Hanada and K. Sakamoto
Japan Atomic Energy Agency
H-イオンビーム
24th IAEA Fusion Energy Conference
San Diego, CA, USA
100 mm
October 8th, 2012
NBTF/ITER NB procurement by JA
1 MV insulating trans.
HV bushing
DC generators
-200kV
•Feedthrough for bus bars
and water pipes
•Bulkhead for SF6 and
vacuum.
•Double layers:
− f1.56 m ceramic rings
− Large FRP rings
• Five rings in series for 1 MV insulation.
ITER NB Injector
16.5 MW D0 at 1 MeV for 1 hour
-400kV
HVD2
-600kV
HVD1
(EU)
-800kV
-1MV
1 MV dc power supply
MAMuG accelerator
Procurement arrangement for NBTF
signed in Feb. 2012.
•Large electrostatic
accelerator
•Multiaperture
•Multigrid
•1 MeV, 40 A D- ion beams
at 200 A/m2
MeV accelerator for ITER R&D
The MeV accelerator has been developed at JAEA since 1994.
Due to RIC in ITER, vacuum insulation for 1 MV high voltage
MeV accelerator
Metal flange
Large
stress
ring
Triple
junction
FRP insulator ring
The stress rings suppress
surface flashover on FRP.
Progress in 2010 - 2012
 Improvement of voltage holding by mitigation of local electric field,
 Compensation of beamlet deflection using a 3D trajectory analysis,
 Analysis of gird heat load reduction for long pulse operations.
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 Voltage holding improvement
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Insulation of high voltage (-200 kV) in long vacuum gap (50-100 mm)
Vacuum insulation in MeV accelerator is “low average electric field”,
different parameter ranges than positive ion sources .
(a) Before
(b) After
6.3
5.8
Accelerators show lower voltage
holding than equal field.
6.4
2.8
4.2
3.7
4.2
3.9
(a) Discharge marks at the opposite sides of
edges/steps, where local stress was higher
than several kV/mm. Breakdowns triggered
by local high stress in long vacuum gap.
(b) The gaps were extended to mitigate the
local field.
Gaps extension to mitigate local stress, 1 MV for more than 1 hour,
Beam energy increased: 880 keV
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 Beamlet deflections
Extractor
Measured beam footprint
500 keV
S
N
S
N
S
N
Beamlets in each row deflected by
magnetic field in alternative directions
Outermost beamlets deflected by magnetic
deflection + space charge repulsion
The outermost beamlets are deflected with largerer angle,
intercepted at grids giving high heat load,
Beam current reduced and target temperature rise are lower.
3D beam trajectory analysis
Extractor
A1G

① Magnetic filter and electron suppression
magnets
② Current reduction by stripping loss

A2G
A4G
GRG
H- beam
A3G
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
③ Aperture offset at ESG
for magnetic deflection:
0.8 mm
④ Kerb at ESG backside for
space charge repulsion:
1 mmt
⑤ Grid support structure


OPERA-3D code (Vector Fields software)
Aperture offset and kerb
Aperture offset
against magnetic deflection
Footprint obtained
by 3D analysis
Before compensation
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Kerb against
space charge repulsion
Magnetic deflection:
4.7 mrad
Magnetic deflection + space
charge repulsion: 9.5 mrad
For compensation of the beamlet deflection due to:
• Magnetic field, Aperture offset δ = 0.8 mm
• Space charge repulsion, Kerb with 1 mm in thickness at dk = 16 mm.
Compensation of beamlet deflections
Measured at 0.98 MeV
• Footprint envelope closer
to square shape
• Smaller distance between
beamlets
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Calculated at 1 MeV
Aperture offset
Kerb
are effective for compensation
• Intensity is high even in outermost beamlets,
• Less interception of beams at grids, and hence, low grid heat load
Achievement of 0.98 MeV H- ion beam
Measured grid heat load
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Summary of beam progress
Compensation resulted in reduction of grid
heat load (17% -> 10%).
Compensation of beam deflection led to improvement of voltage holding
Achievement of 0.98 MeV 185 A/m2 H- ion beam.
World first achievement of ITER-relevant negative ion beams.
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EAMCC: secondary particle analysis
Further reduction in heat load , further improvement in voltage
holding, and so, Long pulse 1 MeV beam.
Another source of grid heat load is accelerated secondary particles.
MeV accelerator in 2007 geometry
PG
EXG
Stripped electrons
generated in and
around extractor
A1G
Stripped electrons
deflected by EXG
magnet
A2G
Secondary electron
emission by
collision of H0 on
grid side wall
A3G
Direct
interception of
H- ions
Grid heat loads are mainly caused by:
1) Electrons stripped from H- ions in extractor,
2) Direct interception of H- ions at grid aperture,
3) Secondary electrons emitted at grid aperture.
A4G
GRG
He-
H0
H+
Suppression of electron acceleration
Modification of grid geometry:
MeV accelerator in 2010 geometry
Geometry
2007
2010
Thickness
20 mm
10 mm
Aperture dia.
f16 mm
A1-A2G: f14
A3G-GRG: f16
A test of MeV accelerator showed:
•Higher heat load in A1G,
•Similar tendencies of heat load
distribution among grids.
• By reduced grid thickness, area for ion
collision is reduced inside aperture
− heat loads by secondary electrons decreased in all grids,
− direct interception of H- ions decreased in A4G, GRG.
• By reduced aperture dia. stripped electrons were intercepted in A1G and A2G, and
subsequently heat loads on A3G- GRG were decreased.
• Total heat load was decreased from 49 kW to 35 kW.
Grid heat load in ITER can be reduced further for long pulses.
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Conclusion
• Voltage holding improvement, and compensation of beam deflections
have been applied to the MeV accelerator.
• Reduction of beam direct interception at grids has brought further
improvement in voltage holding during beam acceleration.
0.98 MeV, 185 A/m2 H- ion beam acceleration achieved.
This almost satisfies the ITER NB requirements (1 MeV, 200 A/m2 D-).
• For further reduction of grid heat load, EAMCC analysis has run:
− Smaller aperture (f14 mm) in upstream grids (A1G and A2G) to stop
electrons before acceleration to high energy,
− Thinner grid to suppress secondary electron generation.
• Preliminary results of the MeV accelerator and EAMCC show possible
further reduction of grid heat load to an acceptable level.
• Long pulse test of the MeV accelerator will be restarted in 2013 after
recovery of the facility from damages of the 3.11.
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