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

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

Thomson Scattering Introduction Free electrons oscillate in external em-field Free electrons oscillate in external em-field Accelerated electrons in turn emit radiation (TS light) Accelerated electrons in turn emit radiation (TS light) e-e- h i   nene TeTe Area n e Width T e TS-spectrum

Thomson Scattering Introduction Scattering parameter  Scattering parameter  << d  < 0.1 << d  < 0.1 Incoherent scattering on random fluctuations in n e Incoherent scattering on random fluctuations in n e >> d  >> 1.0 >> d  >> 1.0 Coherent scattering on correlated n e variations Coherent scattering on correlated n e variations : Wavelength shift scattered radiation : Wavelength shift scattered radiation d : Debye length d : Debye length

Thomson Scattering Set-up Introduction

Low pressure gas discharge model lamp Stray light prevention:Stray light prevention: Brewster windows Brewster windows Extension tubes (120 cm) Extension tubes (120 cm) Incoherent scatteringIncoherent scattering QL-lamp Introduction

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

Electron density: < n <10 22 m -3 Electron temperature: T e  6600 K Gas pressure : p  1.5 bar High pressure mercury lamp Hg-lamp Thomson scattering on a real lamp Hg-lamp Thomson scattering on a real lamp 0.2 <  < 1.2 Coherent Scattering

Set-up for TS on the Hg-lamp Thomson scattering on a real lamp

Stray light reduction Stray light reduction Broad maskBroad mask Blocking sides of the entrance slitBlocking sides of the entrance slit Lamp damage due to laser beam Lamp damage due to laser beam Low laser powerLow laser power Smaller focal length (1m  0.25m)Smaller focal length (1m  0.25m) Laser induced plasma Laser induced plasma Low laser intensityLow laser intensity Instrumental problems Thomson scattering on a real lamp

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

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

Coherent scattering Thomson scattering results TS power S(k,  ): Spectral distribution function Salpeter approximation used for S(k,  ). Valid for –T e  T g –Maxwellian velocity distribution Fit of TS-spectrum Central points blocked by a mask

Alternating current: sine wave Alternating current: sine wave Radial profiles of n e and T e Radial profiles of n e and T e different phases of the currentdifferent phases of the current Results Thomson scattering results

Thermal Equilibrium Thermal Equilibrium One temperature for all species: T e  T gas  T ionOne temperature for all species: T e  T gas  T ion Thermal Equilibrium in the Hg-lamp? Thermal Equilibrium in the Hg-lamp? T e from TS: T e = 7000  740 KT e from TS: T e = 7000  740 K T gas from X-ray: T gas = 5200  520 KT gas from X-ray: T gas = 5200  520 K T e T gasT e T gas Thermal Equilibrium Equilibrium assumptions

Chemical Equilibrium Equilibrium assumptions n1sn1s  Ip Ip nene Saha T e -1 Electrical properties Saha-BoltzmanSaha balance : Hg + e -  Hg e - Saha equation: Atomic state distribution function

Chemical Equilibrium Equilibrium assumptions Saha T e -1 n1sn1s  Ip Ip n1n1 T exc -1 ASDF of an ionising plasma Overpopulation factor: b 1 = n 1 /n 1 s n 1 : Ideal gas law n 1 s : Saha equation Ionising plasma : b 1 > 10 Overpopulation of n 1, Slope  T exc  T e

Chemical Equilibrium Equilibrium assumptions Radial profiles for different phases

Deviations from Saha-Boltzmann Deviations from Saha-Boltzmann Excitation temperature from ASDF: T exc = 5200 KExcitation temperature from ASDF: T exc = 5200 K Electron temperature from TS: T e = 7000 KElectron temperature from TS: T e = 7000 K Overpopulation factors: b 1 > 10 Overpopulation factors: b 1 > 10 Minimum in the centre.Minimum in the centre. Increase with increasing filling gas.Increase with increasing filling gas. Maximum at zero crossing of the currentMaximum at zero crossing of the current Chemical Equilibrium Equilibrium assumptions

Conclusions TS for the first time applied on real lamp TS for the first time applied on real lamp Indications that the LTE assumption is not valid Indications that the LTE assumption is not valid Thermal: T e  T gasThermal: T e  T gas Chemical: T exc  T e, b 1 >10Chemical: T exc  T e, b 1 >10 Recommendations Recommendations Model of Hg lamp including molecular processesModel of Hg lamp including molecular processes