laser plasma and laser-matter interactions laboratory the effect of ionization on condensation in...
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Laser Plasma and Laser-Matter Interactions Laboratory
http://aries.ucsd.edu/LASERLAB
The effect of ionization on condensation in ablation
plumesM. S. Tillack, D. Blair, S. S. Harilal
Center for Energy Research and Mechanical and Aerospace Engineering Department
Jacobs School of Engineering
ARIES Town Meeting on Liquid Wall Chamber DynamicsLivermore, CA5-6 May 2003
Surface absorptionThermal conductionSurface meltingVaporizationMultiphoton ionization Plasma ignitionExplosive phase change
Plasma absorptionSelf-regulating heat transferAdiabatic expansion Collisional accelerationAmbient interpenetration
Adiabatic coolingRapid condensationPlume stagnation
We are investigating late-stage laser ablation plume phenomena at UCSD
2. Modeling and experiments on homogeneous nucleation and growth of clusters
0
8 ns
1000 ns
1. Experimental studies of the expansion dynamics of plumes interpenetrating into ambient gases (with and without magnetic fields)
3. Spinodal decomposition and liquid droplet ejection
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Lasers used in the UCSD Laser Plasma and Laser-Matter Interactions Laboratory
Lambda Physik 420 mJ, 20 ns multi-gas excimer laser (248 nm with KrF)
Spectra Physics 2-J, 8 ns Nd:YAG with harmonics 1064, 532, 355, 266 nm
Similarities and differences in ablation plume parameters
* uncertainties in Ablator ionization
Parameter X-rays from HItarget explosion
Laser simulation(107-1010 W/cm2)
pulse length ~2 ns 8 ns
attenuation length 1–5 μm (P /bFlib )e 10 nm
ablation depth 1–10 μm 1–2 μm (thermal)
initial plume: te .mpdensityZeff
< 30 eV*
< nc ~1021/cm3
??*
1–20 eV~1021/cm3
0–3
plume @1 μs: te .mpplasma density
Zeff
0.5–1.5 eV3 10x 18 cm–3
0–1
backgr ound gasdensity
0–50 mTorr 0–1 atm
backgr ound gastemperature
>1000˚C room temperature
spot size 1000 m2 1 mm2
geometry quasi-1D quasi-1D
Theory
Classical theory of aerosol nucleation and growth
Homogeneous Nucleation (Becker-Doring model)
Condensation Growth
Coagulation
∂n∂t[ ]coag
=12
β V*,V−V *( )n(V*)n(V−V*)dV*0
V
∫ − β V,V*( )n(V)n(V*)dV*0
∞
∫
β V,V*( ) =2π D+D*( ) dp +dp*
( )Fcoagwhere the coagulation kernel is given by
Transport and Rate of Change
∂n
∂t+∇ • nv v ( ) −∇ • D∇n( ) +∇ •
v c n= ∂n
∂t[ ]growth,homo
+ ∂n∂t[ ]growth,
hetero+ ∂n
∂t[ ]coag
Particle Growth Rates
r*=nDn2v∂n/∂t = C Z
Z=DWk/3rkTNk2C=kT
pexp- kTDWke o
J=m/2rkC Tgvcpg- Tf
vepsatf p ps = po exp[Qv/(kTb) – Qv/(kTs)]
Dependence of homogeneous nucleation rate and critical radius on saturation ratio
• High saturation ratios result from rapid cooling due to plume expansion and heat transfer to background gas
• Extremely high nucleation rate and small critical radius result
• Reduction in S due to condensation shuts down HNR quickly; competition between homogeneous and heterogeneous condensation determines final size and density distribution
ΔG =4πr3
3Vm
(μL −μv)+4πσr2
Si, n=1020 cm–3, T=2000 K
Effect of ionization on cluster nucleation rate
ΔG =4π3Vm
(r3 −ra3)(μL −μv)+4πσ (r2 −ra
2)+e2
2(1−ε−1)(r−1 −ra
−1)
• Ion jacketing produces seed sites
• Dielectric constant of vapor reduces free energy
Si, n=1020 cm–3, T=2000 K, Zeff=0.01
Modeling
A 1-D multi-physics scoping tool was developed to help interpret plume condensation results
Laser absorption Thermal response Evaporation flux
Transient gasdynamics Radiation transport Condensation Ionization/recombination
Ioe–x, inverse bremsstrahlung
cond., convection, heat of evaporation
2-fluid Navier-Stokes
Stefan-Boltzmann model
ion-modified Becker-Doring model
high-n Saha, 3-body recombination
j =M2π
Γσ c
pv
RTv
−σ e
psat
RTf
⎛
⎝ ⎜
⎞
⎠ ⎟
Ablation plumes provide a highly dynamic, nonlinear, spatially inhomogeneous environment for condensation, where strong coupling of physics led us to a combined experimental and modeling approach.
Model prediction of expansion dynamics
High ambient pressure prevents interpenetration(note, the 2-fluid model lacks single-particle effects)
Target : SiLaser Intensity : 5x109 W cm-2 (peak of Gaussian)Ambient : 500 mTorr He
The plume front is accelerated to hypersonic velocities
~62 eV
Thermal energy is converted into kinetic energy; collisions also appear to transfer energy from the bulk of the plume to the plume
front
Model prediction of cluster birth and growth
Spatial distribution of nucleation (*) and growth (o) rates at 500 ns
Time-dependence of growth rate/birth rate
• Clusters are born at the contact surface and grow behind it
• Nucleation shuts down rapidly as the plume expands
μs
Experiments
Experimental setup for studies of ablation plume dynamics
Target : Al, SiLaser Intensity : 107–5x109 W/cm2
Ambient : 10-8 Torr – 100 Torr air
Expansion of interpenetrating plumes depends strongly on the background pressure
0.01 Torr
1 Torr
0.1 Torr
10 Torr
100 Torr
Free expansion (collisionless)
Weakly collisional transition flow
Collisional transition flow
Fully collisional plume
Confined plume
Example: plume behavior in weakly collisional transition regime (150 mTorr)
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• Strong interpenetration of the laser plasma and the ambient low density gas
• Plume splitting and sharpening observed
• This pressure range falls in the region of transition from collisionless to collisional interaction of the plume species with the gas
• Enhanced emission from all species
Plume behavior in weakly collisional transition regime (150 mTorr)
Plasma parameters are measured using spectroscopic techniques
Electron Density:Measured using Stark broadening
Initial ~ 1019cm-3
Falls very rapidly within 200 nsFollows ~1/t – Adiabatic
Temperature: Measured from line intensity ratios
Initial ~8 eVfalls very rapidly
(Experiment Parameters: 5 GW cm-2, 150 mTorr air)
Besides spectroscopy, witness plates served as a primary diagnostic
• Start with single crystal Si• HF acid dip to strip native
oxide• Spin, rinse, dry• Controlled thermal oxide
growth at 1350 K to ~1μm, 4 Å roughness
• Ta/Au sputter coat for SEM• Locate witness plate near
plume stagnation point
Witness plate prior to exposure, showing a single defect in the native crystal structure
Witness plate preparation technique:
Measurement of final condensate size
500 mTorr He5x109 W/cm25x108 W/cm2
Cluster size distribution – comparison of theory & experiment
note: the discrepancy at low irradiance is believed to be caused by anomolously high charge state induced by free electrons
• Good correlation between laser intensity and cluster size is observed.
• Is it due to increasing saturation ratio or charge state?
Saturation ratio and charge state derived from experimental measurements
Maximum ionization state derived from spectroscopy, assuming LTE
Saturation ratio derived from spectroscopy, assuming LTE
•Saturation ratio is inversely related to laser intensity!
Summary
• We have obtained a better understanding of the mechanisms which form particulate in laser plasma, through both modeling and experiments
• We have shown that ionization has a dominant effect on cluster formation in laser ablation plumes, even at low laser intensity
• The cluster sizes obtained are very small – of the order of 10 nm
• Model improvements are needed: 2-D, kinetic treatment, ...
• In-situ particle measurements (scattering, cluster spectroscopy) would be very useful to further validate the mechanisms
• IFE relevance of experiments would be improved greatly with control of the background gas temperature
• Other applications include nanocluster formation, laser micromachining quality, thin film deposition by PLD