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GREPHE/LAPLACE
University of Toulouse (Paul Sabatier)
31062 Toulouse, [email protected]
G. Fubiani, L. Garrigues and J.P. Boeuf
Physics of negative ion extraction in high brightness plasma sources
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High brightness negative ion sources
Image taken from Stockli et al., Journal of Physics: Conference Series 399, 012001 (2012)
Spallation Neutron source, ORNL, Oak Ridge, TN, USA
Wunderlich et al., Plasmas Source Science and Technology 23, 015008 (2014)
ITER BATMAN prototype negative ion source, IPP Garching, Germany
Filter magnets
Driver
Plasma electrode
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Calculated plasma potential profile in the ITER prototype source (BATMAN, IPP)
3D PIC-MCC simulation of the whole ion source volume
PE bias
Beginning of magnetic filter
X
24 cm
16 cm
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Similar behavior is observed in filamented ion sources with a magnetic filter field
Image taken from Leung et al., Review of Scientific Instruments 55, 338 (1984)
Experimental measurements
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Experimental setup
Image taken from Leung et al., Review of Scientific Instruments 55, 338 (1984)
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Plasma potential profile in the extraction area (3D)
• Model ITER BATMAN prototype ion source (RF driven)• Plasma electrode is floating (X=0)• 600 A/m2 negative ion current density emitted on the electrode surface
• Formation of a virtual cathode (depth Δφ≈ -0.9V)
virtual cathode
Plasma electrode
Periodic boundary conditions
Ext
ract
ion
elec
trode
(9kV
)
Bext
Bf
Pla
sma
Acc
eler
ator
Plasma electrode
Periodic boundary conditions
Ext
ract
ion
elec
trode
(9kV
)
Bext
Bf
Pla
sma
Acc
eler
ator
20 mm
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Ion/electron density profiles in the extraction area
Location were measurements are performed
Unknown experimentally
n+≈ 3×1017 m-3 and nH-≈ 5×1016 m-3 about 20 mm for the PE in the experiments
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Principles for producing and extracting negative ions in high brightness plasma sources
• High brightness negative ion beams are produced on the plasma electrode surface
• Cesium is added to lower the metal work function (typically Cs/Mo converters)
• This method for producing negative ions may induce aberrations in the extracted beam
• Aberrations may have a devastating effect on the accelerator parts
• High power deposition on the grids
• Generation of secondary particles
H
H-
Hx+
Cs/Mo plasma electrode
few kVs
Pla
sma
Extraction electrode
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Numerical model: Particle-in-Cell method with Monte-Carlo Collisions (PIC-MCC)
1) Model in 3D the whole ion source volume (low resolution)
Mag
netic
filte
r Bf
ICP discharge
Permanent magnets 2) Duplicate plasma near one aperture (high resolution)
Advantages:
• Full knowledge of particle distribution functions
• Exact fit of the plasma potential profile near an aperture
Plasma electrode
Periodic boundary conditions
Ext
ract
ion
elec
trode
(9kV
)
Bext
Bf
Pla
sma
Acc
eler
ator
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Numerical model (Cont’d)
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Transverse beam profile (x=50 mm)
Negative ion beamlet extraction from an ITER prototype ion source (2D)
• Thickness of aperture: 1 mm
• H- (extracted) current density ~230 A/m2
• Extraction probability ~38%
• Beam divergence (RMS) ~4.5 mrad (1 MeV)
Negative ion flux (axial) x 1021 m2/s
Meniscus
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Conclusion
• High brightness negative ion sources are used in numerous applications
• Storage rings, spallation neutron source, ITER neutral beam injector, etc
• Negative ions are produced on the PE surface near the aperture(s)
• Aberrations may occur and must be characterized
• Extraction of negative ions from an ITER prototype source was modeled
• Properties of the negative ion beam comparable to experiments
• Current density, divergence, halo, ion density in front of the PE
• Negative ion beam profile & shape of meniscus similar to positive ion beams
• Simpler ray-tracing models may be used as an alternative
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EXTRAS
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JAEA flat plasma electrode surface
• 14 mm aperture, 2 mm thick• 600 A/m2 emitted from the plasma electrode surface • Extraction probability ~40%• Beam divergence (RMS) ~7.2 mrad (1 MeV)
X (mm)
y (m
m)
Negative ion flux (axial) x 1021 m2/s
