plasma diagnostics using spectroscopic techniques
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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Plasma diagnostics using spectroscopic techniquesTimo Gans
York Plasma Institute
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YPI – Low temperature plasma activities
• 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
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Plasmas & other disciplines
• Optics• Atomic & Molecular Physics• Laser Physics• Surface Science• Electro Dynamics• Statistics• Numerical Simulations
• Electrical Engineering• Chemistry• Bio-medical Sciences
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What is a plasma? ionised gas with variety of particles electrons positive and negative ions neutral particles
(atoms, molecules, radicals) excited species dust particles
What do we like to measure? densities distribution functions (temperatures) electric and magnetic fields
electrons
ionsradicals
neutralsbulkplasma
sheathchemistry
synergisms
physics
power
electrons
ionsradicals
neutralsbulkplasma
sheathchemistry
synergisms
physics
electrons
ionsradicals
neutralsbulkplasma
electrons
ionsradicals
neutralsbulkplasma
sheathchemistry
synergisms
physics
power
power
Plasma – Complex Multi-Particle System
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Multiphase interfaces:
Plasma – gas – liquid – surface (solid)
Multispecies:
Electrons, pos. ions, neg. ions, neutrals, radicals, excited species, 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
Challenges & opportunities
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Electrical diagnostics charged particles and fields
external current and voltage measurements+ simple+ non-intrusive– indirect– model based– global information only
probe measurements+ simple+ local information+ direct– model based– reactive environment (gases)– intrusive
How do we measure plasma quantities?
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Mass spectrometry neutral particles and ions energy distribution functions
+ non-intrusive+ direct
How do we measure plasma quantities?
– complicated in detail– external measurement– reactive gases
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Optical diagnostics
in principle all plasma parameters+ non-intrusive+ high temporal and spatial resolution
Plasma physics
Atomic & molecular physics
Optical diagnostics
How do we measure plasma quantities?
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Emission spectroscopy+ passive+ simple+ robust– indirect– model based– data needed
Laser spectroscopy+ direct+ highly reliable– active– involving– expensive
Combination of passive and active methods
Optical Diagnostics
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Typical OES set-up
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line emission which emission lines (qualitative)
® species absolute intensities (calibration difficult)
® density of excited species line ratios
® robust model based analysis (this lecture!) line shapes (high experimental requirements)
® temperatures, fields, densities temporal variations
® plasma dynamicscontinuum radiation
spectral distributions absolute intensities
Optical Emission Spectroscopy (OES)
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Complete thermodynamic equilibrium (CTE) homogeneity unique temperature (Te = Ti = Tgas) black body radiation
Maxwell – Boltzmann distribution
1
2 :Planck3
2
Tk
hBec
hI
Plasma concepts - CTE
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Maxwell – Boltzmann distribution population distributions
TkEE
gg
nn
B
jk
i
k
j
k exp
:Boltzmann
® Spectroscopy: line intensities and ratios velocity distribution
Tk
mv
mTk
vndvvdn
BB2
exp2
4)(:Maxwell
2
23
2
® Spectroscopy: line shape, e.g. Doppler effect
Plasma concepts - CTE
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Main constraints and limitations inhomogeneities Planck
® Local thermodynamic equilibrium?
Plasma concepts - CTE
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Local thermodynamic equilibrium (LTE) local parameters collision dominated
® equilibrium of collisions® no equilibrium of radiation
requirement
example (hydrogen arc)
ne = 1016 cm-3, Te 104 K
® (Ek - Ei)LTE 4 eV
® Partial LTE
312106.1 ikee EETn
Plasma concepts - LTE
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Partial local thermodynamic equilibrium (PLTE) over population of the ground state LTE for excited states constraints and limitations
low electron densities® Corona model® collisional radiative models
Plasma concepts - PLTE
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Corona model model for plasmas with "low" electron densities
(ne < 1013 cm-3) applicable to most technological plasmas far from thermodynamic equilibrium most particles are in the ground state
electron impact excitation=
relaxation by radiation
(spontaneous emission)
Plasma concepts - Corona
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iiki,Ph nAn
ni : population density of state i
Aik : spontaneous emission rate
nPh,i : photons per unit volume and time
Plasma concepts - Corona
Electron impactexcitation
Ground state n0
i
kik
Corona model
electron impact excitation=
relaxation by radiation
(spontaneous emission)
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k
ikieeii AnTnEndtdn ,0
n0 : ground state density
Ei : electron impact excitation rate of state i,
(depending on ne and Te)
ikikA
1
i : radiative lifetime
Plasma concepts - Corona
dEEfmEEnvnEe
ieiei2
0
i : electron impact excitation cross-section of state i
f(E): normalised EEDF
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kik
ikik
i0iki0
kik
ikiiki,Ph
AAa
EnaEnAAnAn
aik : branching ratio
RF - discharges
iRFT
1
(later) OES Resolved Phase 0 dtdni
steady state of excited states
®
kik
ii
i
AEnn
dtdn 0 0
Plasma concepts - Corona
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Additional excitation and de-excitation processes applicable to most technological plasmas
cascades from higher electronic states
one dominating or effective cascade state
ccii
ii0
i nAnEndtdn
Aci : transition rate from the cascade state c
nc : population densities of the cascade state c
nc = ?
Corona: cascade transitions
Electron impactexcitation
Ground state n0
i
kik
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neglecting second order cascades:
ccii0iii
i
icc0cii0
i
cc0cc
c
cc0
c
EaEnn0dtdn
nEnAEndtdn
Enn0dtdn
nEndtdn
Determination of Ec is difficult(reabsorption!)
Corona: cascade transitions
Electron impactexcitation
Ground state n0
i
kik
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excitation out of metastable states
?
,0
m
mimi
iccii
i
n
EnnnAEndtdn
long lifetimes of metastable states transport problem plasma wall interaction
complex® avoid through proper choice of state i with small cross-
sections for excitation out of metastable states (small Ei,m)
® turn into diagnostics of metastable states by comparing with states excited out of metastable levels
Corona: stepwise excitation
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collisional de-excitation (quenching)
A* + Q ® ?
especially important at high pressures!
qik
ikimimcciii knnAnEnnAEndtdn ,0
qk
iki nkAA
kq : quenching coefficient with species Q
nq : density of species Q
Corona: collisional de-excitation
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q : quenching cross-section, Tgas indepedent
Tgas : gas temperature
<v> : mean velocity
: reduced mass
gasBqqgasq
TkvTk
8
® Importance of Tgas
® Which quenching partners are present?® What are the densities?
Corona: collisional de-excitation
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selection rules for emission lines good quality of electron impact excitation cross-sections
(same source!) negligible excitation out of metastables small cascade contribution short lifetimes
competition with quenching high intensities high temporal resolution
known quenching coefficient (better small) no excitation transfer with other species no spectral overlap with other emission
Which emission line should I analyse?
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Actinometry & Limitations
Direct excitation:
Dissociative excitation:
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Influence of the EEDF
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Time & space dependence of the EEDF
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N Knake, et al., APL, 93 (2008) 131503
Comparison with laser spectroscopy
K NIEMI, et al., Appl. Phys. Lett. 95 (2009) 151504
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Thank you!
York Plasma Institute