h - formation by scattering of hydrogen atoms/ions on carbonaceous surface y. xiang, h. khemliche,...
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H- Formation by scattering of hydrogen atoms/ions on carbonaceous surface
Y. Xiang, H. Khemliche, A.Momeni, P.
Roncin
Groupe E
L’Institut Science Moléculaire d’Orsay (ISMO)
Université Paris-Sud 11
7 Mars 2011 La journée de l’EDOM7 Mars 2011 La journée de l’EDOM
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Motivation
Charge transfer, neutralization and
formation of ions Plasma-wall interaction
Divertor physics
Negative ion source
ITER (International Thermonuclear
Experimental Reactor) Heat plasma~150 million °C
Maintain kinetic energy
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Why negative ions source?
Neutral beam
D°
D+, D-
Residual Iondump
NeutraliserIon source
Residual Ion deflectionAccelerator
Vacuum cellwith Cryo pumps
ShutterInsulating gate
Vacuum pump
~10-30 m
PlasmaITER
Given or
taken?
http://www.iter.org/sci/plasmaheating
E~1 MeV
Previous generation JET (100 keV capture) H+ -> H°
ITER 1 MeV H- ->H°
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How could make an efficient negative ion source?
Caesiated surface
Too expensive for all the reactor
Poison the plasma- contamination
Metal surface—capture electron
2.1 eVDecrease work function
MetalIsolant
Potentiel image : V ~ -1/(4.R)Potentiel Coulombien : V ~ -1/(R)
MétalHOPG
CBCB
Semi-metal (conductor)
work-function ∼ 5 e
Deep valence band
Low density state at fermi
level
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17 detectors working in coincidence
Faisceau incident pulsé Faisceau direct
Faisceau diffusé
E lectrons
C IB L E electrostaticseparator
Det
ecte
urà
loca
lisat
ion
U nité de détection
4-5 u.a.inc~2 deg. 2 - 3 Å
20 meV < E < 10 eV200 < E0 < 10000 eV )(sin20 EE
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Production of H- on diamond
Diamond CVD (chemical vapor deposition), naturelly hydrogenated
- gap de 5.5 eV
- very deep valence band
- negative electron affinity (-1 eV), depending on H surface coverage
ProjectileE=1 keV
Fraction of H-
(%)
H+ 2.5 ± 0.5
H° 3.0 ± 0.8
H2+ 1.6 ± 0.5
Conclusion : diamond CVD
- resonant neutralization of H+
- formation of H- by capturing electron from moved affinity level
- H- survival thanks to the forbidden band
Resultats of H2+ agree well with the
reference(Wurz P. , Schletti R. and Aellig M.R., Surf. Sci 373, 56, 1997)
BC
5
10
15
20
gap H-
(0.75eV)
BVH° (13.6eV)
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Production of H- on graphite
Graphite HOPG
- semi-metal (conductor)
- work function 4.6 eV
- deep valence band
CB5
10
15
20
H-
(0.75eV)
VBH° (13.6eV)
Energie (keV)
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Fra
ctio
n de
nég
atifs
(%
)
0
2
4
6
8
10
H+ incidentH° incident
H2+ incident
H2° incident
angle plus petit
at the fixed incidence ( 1.5 °), the rate of H- increase with total
energyBoth V⊥ and V// are incresed
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Conclusion
The first results from diamond are disappointing
Uncertainty level of hydrogenation (->temperature
variations)
The trend of H- fraction for graphite is quite different
Results on electron emission to investigate the role
band gap
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Perspective
Extend our work on graphite and possibly on hydrogenated diamond
Exploit energy loss data in coincidence with electron emissionGo to larger incidence anglesInvestigate graphite with H and defects
Investigate other carbon based materials (C60…)
Inelastic Diffraction of neutral H°
Momentum distribution of the quantum stateH° + ExcitonH° + electronReorientation
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Thanks for Thanks for
your attention! your attention!