Application of Thomson Scattering on Application of Thomson Scattering on a high pressure mercury lampa high pressure mercury lamp

Nienke de Vries, Xiaoyan Zhu

Erik Kieft, Joost van der Mullen

OutlookOutlook

IntroductionIntroduction Thomson Scattering on a real lamp Thomson Scattering on a real lamp

Thomson Scattering resultsThomson Scattering results Equilibrium assumptionsEquilibrium assumptions ConclusionsConclusions

Thomson ScatteringThomson ScatteringIntroductionIntroduction

Free electrons oscillate in external Free electrons oscillate in external em-fieldem-field

Accelerated electrons in turn emit Accelerated electrons in turn emit radiation (TS light)radiation (TS light)

e-

hi

ne

TeArea ne

Width Te

TS-spectrum

Thomson ScatteringThomson ScatteringIntroductionIntroduction

Scattering parameter Scattering parameter • << << dd < 0.1 < 0.1

Incoherent scattering on random fluctuations in nIncoherent scattering on random fluctuations in nee

• >> >> dd >> 1.0 >> 1.0 Coherent scattering on correlated nCoherent scattering on correlated nee variations variations

d

2

1

24 en

Tk

e

eBd

: : Wavelength shift Wavelength shift scattered radiation scattered radiation dd: : Debye lengthDebye length

Thomson Scattering Set-upThomson Scattering Set-upIntroductionIntroduction

Nd:YAG 532nm

dichroicmirrors

plasma

lenses

TGS

beamdump

iCCD

d

Low pressure gas dischargeLow pressure gas discharge model model lamp lamp • Stray light prevention:Stray light prevention:

Brewster windows Brewster windows Extension tubes (120 cm)Extension tubes (120 cm)

• Incoherent scatteringIncoherent scattering

QL-lampQL-lampIntroductionIntroduction

RF-Coil

11 cm 60 cm60 cm

Brewsterwindow

• Model lampModel lamp Brewster windowsBrewster windows Extension tubes Extension tubes

(60 cm)(60 cm)

• Coherent Coherent scatteringscattering

• In cooperation In cooperation with Bochumwith Bochum

Argon model lampArgon model lampIntroductionIntroduction

• Electron density:Electron density: 10102020 < n <10 < n <1022 22 mm-3-3

• Electron temperature:Electron temperature: TTee 6600 K 6600 K

• Gas pressureGas pressure:: p p 1.5 bar 1.5 bar

High pressure mercury lamp

Hg-lampHg-lampThomson scattering on a real lampThomson scattering on a real lamp

0.2 < < 1.2

Coherent Scattering

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

Nd:YAG 532nm

mirror

Hg-lamp

imagerotator

grating 1

mask grating 2

intermediateslit

grating 3

polariser

iCCD

d

entranceslit

diaphragm

beamsplitter

beam dump

Triple Grating Spectrograph

lenses

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 beamLamp damage due to laser beam• Low laser powerLow laser power• Smaller focal length (1m Smaller focal length (1m 0.25m) 0.25m)

Laser induced plasmaLaser induced plasma• Low laser intensityLow laser intensity

Instrumental problemsInstrumental problemsThomson scattering on a real lampThomson scattering on a real lamp

ContributionsContributions Thomson radiationThomson radiation

Plasma radiationPlasma radiation

Stray lightStray light

Dark current Dark current

iCCD image of a measured spectrum

Measured spectrum Measured spectrum Thomson scattering resultsThomson scattering results

Coherent scattering Coherent scattering Thomson scattering on a real lampThomson scattering on a real lamp

Hg-lamp: 0.2 < < 1.2

Spectrum is flattened, width depends on Te

Shape of TS-spectrum depends on scattering parameter

Coherent scattering Coherent scattering Thomson scattering resultsThomson scattering results

TS power

S(k, ): Spectral distribution function

Salpeter approximation used for S(k, ).

Valid for

–Te Tg

–Maxwellian velocity distribution

Fit of TS-spectrum

Central points blocked by a mask

Alternating current: sine wave Alternating current: sine wave Radial profiles of nRadial profiles of nee and T and Tee

• different phases of the currentdifferent phases of the current

Results Results Thomson scattering resultsThomson scattering results

Thermal Equilibrium Thermal Equilibrium • One temperature for all species: TOne temperature for all species: Tee T Tgasgas T Tionion

Thermal Equilibrium in the Hg-lamp?Thermal Equilibrium in the Hg-lamp?• TTee from TS: T from TS: Tee = 7000 = 7000 740 K 740 K

• TTgasgas from X-ray: T from X-ray: Tgasgas = 5200 = 5200 520 K 520 K

• TTe e TTgasgas

Thermal EquilibriumThermal EquilibriumEquilibrium assumptionsEquilibrium assumptions

Chemical Equilibrium Chemical Equilibrium Equilibrium assumptionsEquilibrium assumptions

n1s

Ip

neSahaTe

-1

Electricalproperties

Saha-Boltzman Saha balance :

Hg + e- Hg+ + 2 e-

eee

es

kT

I

kTm

hnn 1

32

1 exp24

Saha equation:

Atomic state distribution function

Chemical Equilibrium Chemical Equilibrium Equilibrium assumptionsEquilibrium assumptions

SahaTe

-1

n1s

Ip

n1

Texc-1

ASDF of an ionising plasma

Overpopulation factor: b1 = n1/n1

s

n1 : Ideal gas law

n1s : Saha equation

Ionising plasma : b1 > 10

Overpopulation of n1,

Slope Texc Te

Chemical EquilibriumChemical EquilibriumEquilibrium assumptionsEquilibrium assumptions

Radial profiles for different phases

Deviations from Saha-BoltzmannDeviations from Saha-Boltzmann• Excitation temperature from ASDF: TExcitation temperature from ASDF: Texcexc= 5200 K= 5200 K

• Electron temperature from TS: TElectron temperature from TS: Tee = 7000 K = 7000 K

Overpopulation factors: bOverpopulation factors: b11 > 10 > 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 current Maximum at zero crossing of the current

Chemical Equilibrium Chemical Equilibrium Equilibrium assumptionsEquilibrium assumptions

ConclusionsConclusions

TS for the first time applied on real lampTS for the first time applied on real lamp Indications that the LTE assumption is not Indications that the LTE assumption is not

valid valid • Thermal: TThermal: Tee T Tgasgas

• Chemical: TChemical: Texcexc T Tee, b, b11 >10 >10

RecommendationsRecommendations• Model of Hg lamp including molecular processes Model of Hg lamp including molecular processes