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!