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
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
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°
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
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
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)
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
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
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
Thanks for Thanks for
your attention! your attention!
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