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Application of Thomson Scattering on a high pressure mercury lamp

Application of Thomson Scattering on a high pressure mercury lamp. Nienke de Vries, Xiaoyan Zhu Erik Kieft, Joost van der Mullen. Outlook. Introduction Thomson Scattering on a real lamp Thomson Scattering results Equilibrium assumptions Conclusions. h  i. e -. Area n e

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Application of Thomson Scattering on a high pressure mercury lamp

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  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. hi e- Area ne Width Te Te ne Dl l 0 Thomson ScatteringIntroduction • Free electrons oscillate in external em-field • Accelerated electrons in turn emit radiation (TS light) TS-spectrum

  4. Thomson ScatteringIntroduction • Scattering parameter a • l << lda < 0.1 • Incoherent scattering on random fluctuations in ne • l >> lda >> 1.0 • Coherent scattering on correlated ne variations l: Wavelength shift scattered radiation ld: Debye length

  5. Thomson Scattering Set-upIntroduction

  6. QL-lampIntroduction Low pressure gas discharge model lamp • Stray light prevention: • Brewster windows • Extension tubes (120 cm) • Incoherent scattering

  7. Argon model lampIntroduction • Model lamp • Brewster windows • Extension tubes (60 cm) • Coherent scattering • In cooperation with Bochum

  8. Hg-lampThomson scattering on a real lamp • Electron density: • 1020 < n <1022 m-3 • Electron temperature: • Te» 6600 K • Gas pressure: • p » 1.5 bar High pressure mercury lamp 0.2 <  < 1.2 Coherent Scattering

  9. Set-up for TS on the Hg-lampThomson scattering on a real lamp

  10. 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

  11. Measured spectrum Thomson scattering results Contributions • Thomson radiation • Plasma radiation • Stray light • Dark current iCCD image of a measured spectrum

  12. Coherent scattering Thomson scattering on a real lamp Shape of TS-spectrum depends on scattering parameter  • Hg-lamp: 0.2 <  < 1.2 • Spectrum is flattened, width depends on Te

  13. Coherent scattering Thomson scattering results Fit of TS-spectrum TS power • S(k, ): Spectral distribution function • Salpeter approximation used forS(k, ). • Valid for • Te Tg • Maxwellian velocity distribution Central points blocked by a mask

  14. Results Thomson scattering results • Alternating current: sine wave • Radial profiles of ne and Te • different phases of the current

  15. 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

  16. Electrical properties ne Saha Te-1  Ip Chemical Equilibrium Equilibrium assumptions Saha-Boltzman Saha balance : Hg + e-  Hg+ + 2 e- n1s Saha equation: Atomic state distribution function

  17. n1 Texc-1 n1s Saha Te-1  Ip Chemical Equilibrium Equilibrium assumptions ASDF of an ionising plasma • Overpopulation factor: b1 = n1/n1s • n1 : Ideal gas law • n1s : Saha equation • Ionising plasma : b1 > 10 Overpopulation of n1, Slope  Texc  Te

  18. Chemical EquilibriumEquilibrium assumptions Radial profiles for different phases

  19. 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 > 10 • Minimum in the centre. • Increase with increasing filling gas. • Maximum at zero crossing of the current

  20. 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 >10 • Recommendations • Model of Hg lamp including molecular processes

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