Application of Thomson Scattering on a high pressure mercury lamp

1
Application of Thomson Scattering on a high
pressure mercury lamp
Nienke de Vries, Xiaoyan Zhu Erik Kieft, Joost
van der Mullen
2
Outlook

• Introduction
• Thomson Scattering on a real lamp
• Thomson Scattering results
• Equilibrium assumptions
• Conclusions

3
Thomson ScatteringIntroduction

• Free electrons oscillate in
  external em-field
• Accelerated electrons in turn
  emit radiation (TS
  light)

TS-spectrum
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Thomson ScatteringIntroduction

• Scattering parameter a
• l ltlt ld a lt 0.1
• Incoherent scattering on random fluctuations in
  ne
• l gtgt ld a gtgt 1.0
• Coherent scattering on correlated ne variations

l Wavelength shift scattered
radiation
ld Debye length
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Thomson Scattering Set-upIntroduction
6
QL-lampIntroduction

• Low pressure gas discharge model lamp
• Stray light prevention
• Brewster windows
• Extension tubes (120 cm)
• Incoherent scattering

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Argon model lampIntroduction

• Model lamp
• Brewster windows
• Extension tubes (60 cm)
• Coherent scattering
• In cooperation with Bochum

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Hg-lampThomson scattering on a real lamp
High pressure mercury lamp

• Electron density
• 1020 lt n lt1022 m-3
• Electron temperature
• Te 6600 K
• Gas pressure
• p 1.5 bar

0.2 lt ? lt 1.2 Coherent Scattering
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Set-up for TS on the Hg-lampThomson scattering
on a real lamp
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Instrumental problemsThomson scattering on a
real lamp

• Stray light reduction
• Broad mask
• Blocking sides of the entrance slit
• Lamp damage due to laser beam
• Low laser power
• Smaller focal length (1m ? 0.25m)
• Laser induced plasma
• Low laser intensity

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Measured spectrum Thomson scattering results
iCCD image of a measured spectrum

• Contributions
• Thomson radiation
• Plasma radiation
• Stray light
• Dark current

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Coherent scattering Thomson scattering on a
real lamp
Shape of TS-spectrum depends on scattering
parameter ?

Hg-lamp 0.2 lt ? lt 1.2 Spectrum is flattened,
width depends on Te
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Coherent scattering Thomson scattering results
Fit of TS-spectrum
TS power
S(k, ?) Spectral distribution
function Salpeter approximation used for S(k, ?).

• Valid for
• Te? Tg
• Maxwellian velocity distribution

Central points blocked by a mask
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Results Thomson scattering results

• Alternating current sine wave
• Radial profiles of ne and Te
• different phases of the current

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Thermal EquilibriumEquilibrium assumptions

• Thermal Equilibrium
• One temperature for all species Te ? Tgas? Tion
• Thermal Equilibrium in the Hg-lamp?
• Te from TS Te 7000 ? 740 K
• Tgas from X-ray Tgas 5200 ? 520 K
• Te ?Tgas

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Chemical Equilibrium Equilibrium assumptions
Saha-Boltzman
Saha balance Hg e- ? Hg 2 e-
n1s

Saha equation
Atomic state distribution function

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Chemical Equilibrium Equilibrium assumptions
ASDF of an ionising plasma
Overpopulation factor b1 n1/n1s n1 Ideal
gas law n1s Saha equation Ionising plasma
b1 gt 10
Overpopulation of n1,
Slope ? Texc ? Te
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Chemical EquilibriumEquilibrium assumptions
Radial profiles for different phases


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Chemical Equilibrium Equilibrium assumptions

• Deviations from Saha-Boltzmann
• Excitation temperature from ASDF Texc 5200 K
• Electron temperature from TS Te 7000 K
• Overpopulation factors b1 gt 10
• Minimum in the centre.
• Increase with increasing filling gas.
• Maximum at zero crossing of the current

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Conclusions

• TS for the first time applied on real lamp
• Indications that the LTE assumption is not
  valid
• Thermal Te ? Tgas
• Chemical Texc ? Te, b1 gt10
• Recommendations
• Model of Hg lamp including molecular processes