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Plasma diagnostics using spectroscopic techniques

Plasma diagnostics using spectroscopic techniques. Timo Gans. York Plasma Institute. YPI – Low temperature plasma activities. Plasma dynamics & chemical kinetics Advanced plasma diagnostics Special emphasis on optical diagnostics & laser spectroscopy.

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Plasma diagnostics using spectroscopic techniques

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  1. Plasma diagnostics using spectroscopic techniques Timo Gans York Plasma Institute

  2. YPI – Low temperatureplasmaactivities • Plasma dynamics & chemical kinetics • Advanced plasma diagnostics • Special emphasis on optical diagnostics & laser spectroscopy • Modelling& numerical simulations • Technological exploitations • Special emphasis on plasma medicine, plasma etching, plasma deposition

  3. Plasmas & other disciplines • Optics • Atomic & MolecularPhysics • Laser Physics • Surface Science • Electro Dynamics • Statistics • NumericalSimulations • Electrical Engineering • Chemistry • Bio-medicalSciences

  4. Plasma – Complex Multi-Particle System Whatis a plasma? • ionised gas withvarietyofparticles • electrons • positive and negative ions • neutral particles(atoms, molecules, radicals) • excitedspecies • dustparticles What do weliketomeasure? • densities • distributionfunctions (temperatures) • electricandmagneticfields

  5. Challenges & opportunities • Multiphase interfaces: • Plasma – gas – liquid – surface (solid) • Multispecies: • Electrons, pos. ions, neg. ions, neutrals, radicals, excitedspecies, photons • Multiscale problem – time: • Electron dynamics: ps – ns • Ion dynamics: 100 ns – μs • Plasma chemistry: 100 μs – ms • Surface chemistry: s – min • Multiscale problem – space: • Surface structures: nm – μm • Charged particle gradients: μm – m • Neutral particle gradients: 10 μm – m

  6. How do we measure plasma quantities? Electricaldiagnostics • chargedparticlesandfields • externalcurrentandvoltagemeasurements • simple • non-intrusive • indirect • modelbased • global informationonly • probe measurements • simple • localinformation • direct • modelbased • reactiveenvironment (gases) • intrusive

  7. How do we measure plasma quantities? Massspectrometry • neutral particlesandions • energydistributionfunctions • non-intrusive • direct • complicated in detail • externalmeasurement • reactivegases

  8. Plasma physics Optical diagnostics Atomic & molecular physics How do we measure plasma quantities? Optical diagnostics • in principle all plasmaparameters • non-intrusive • high temporal andspatialresolution

  9. Optical Diagnostics • Emission spectroscopy • passive • simple • robust • indirect • modelbased • dataneeded • Laser spectroscopy • direct • highlyreliable • active • involving • expensive  Combinationof passive andactivemethods

  10. Typical OES set-up

  11. Optical Emission Spectroscopy (OES) lineemission • whichemissionlines (qualitative) • species • absolute intensities (calibrationdifficult) • densityofexcitedspecies • lineratios • robust modelbasedanalysis (thislecture!) • lineshapes (high experimental requirements) • temperatures, fields, densities • temporal variations • plasmadynamics continuumradiation • spectraldistributions • absolute intensities

  12. Plasma concepts - CTE Completethermodynamicequilibrium (CTE) • homogeneity • uniquetemperature (Te = Ti = Tgas) • blackbodyradiation • Maxwell – Boltzmann distribution

  13. Plasma concepts - CTE Maxwell – Boltzmann distribution • populationdistributions • Spectroscopy: lineintensitiesandratios • velocitydistribution • Spectroscopy: lineshape, e.g. Doppler effect

  14. Plasma concepts - CTE Main constraintsandlimitations • inhomogeneities • Planck • Localthermodynamicequilibrium?

  15. Plasma concepts - LTE Localthermodynamicequilibrium (LTE) • localparameters • collisiondominated • equilibriumofcollisions • noequilibriumofradiation • requirement • example (hydrogen arc) ne = 1016 cm-3, Te 104 K • (Ek - Ei)LTE 4 eV • Partial LTE

  16. Plasma concepts - PLTE Partial localthermodynamicequilibrium (PLTE) • overpopulationofthegroundstate • LTE forexcitedstates • constraintsandlimitations • lowelectrondensities • Corona model • collisional radiative models

  17. Plasma concepts - Corona Corona model modelforplasmaswith "low" electrondensities(ne < 1013 cm-3) applicabletomosttechnologicalplasmas farfromthermodynamicequilibrium mostparticlesare in thegroundstate electronimpactexcitation=relaxationbyradiation (spontaneousemission)

  18. Plasma concepts - Corona Corona model electronimpactexcitation=relaxationbyradiation (spontaneousemission) ni : populationdensityofstate i Aik: spontaneousemission rate nPh,i: photons per unitvolumeand time

  19. Plasma concepts - Corona n0 : groundstatedensity Ei : electronimpactexcitation rate ofstate i, (depending on neandTe) i : electronimpactexcitationcross-sectionofstate i f(E): normalised EEDF i : radiativelifetime

  20. Plasma concepts - Corona steadystateofexcitedstates aik : branchingratio RF - discharges

  21. Corona: cascade transitions Additional excitation and de-excitation processes • applicable to most technological plasmas • cascades from higher electronic states • one dominating or effective cascade state nc= ? Aci: transition rate fromthecascadestate c nc : populationdensitiesofthecascadestate c

  22. Corona: cascade transitions neglectingsecondordercascades: Determination of Ecisdifficult(reabsorption!)

  23. Corona: stepwise excitation excitation out ofmetastablestates • long lifetimes of metastable states • transport problem • plasma wall interaction • complex • avoidthrough proper choiceofstate i withsmallcross-sectionsforexcitation out ofmetastablestates (smallEi,m) • turn intodiagnosticsofmetastablestatesbycomparingwithstatesexcited out ofmetastablelevels

  24. Corona: collisional de-excitation collisional de-excitation (quenching) A* + Q  ? • especiallyimportantat high pressures! kq: quenchingcoefficientwithspecies Q nq : densityofspecies Q

  25. Corona: collisional de-excitation q : quenchingcross-section, Tgasindepedent Tgas: gas temperature <v> : meanvelocity  : reducedmass • Importance of Tgas • Which quenching partners are present? • What are the densities?

  26. Which emission line should I analyse? selectionrulesforemissionlines • goodqualityofelectronimpactexcitationcross-sections (same source!) • negligibleexcitation out ofmetastables • smallcascadecontribution • shortlifetimes • competitionwithquenching • high intensities • high temporal resolution • knownquenchingcoefficient (bettersmall) • noexcitationtransferwithotherspecies • nospectraloverlapwithotheremission

  27. Actinometry & Limitations Direct excitation: Dissociative excitation:

  28. Influence of theEEDF

  29. Time & space dependence of the EEDF

  30. Comparison with laser spectroscopy N Knake, et al., APL, 93 (2008) 131503 K NIEMI, et al., Appl. Phys. Lett. 95 (2009) 151504

  31. Thank you! York Plasma Institute

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