The effect of ionization on condensation in ablation plumes

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The effect of ionization on condensation in
ablation plumes
M. 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
Dynamics Livermore, CA 5-6 May 2003
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We are investigating late-stage laser ablation
plume phenomena at UCSD
0
8 ns
1000 ns
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Lasers used in the UCSD Laser Plasma and
Laser-Matter Interactions Laboratory
Spectra Physics 2-J, 8 ns NdYAG with harmonics
1064, 532, 355, 266 nm
Lambda Physik 420 mJ, 20 ns multi-gas excimer
laser (248 nm with KrF)
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Similarities and differences in ablation plume
parameters
uncertainties in Ablator ionization
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Theory
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Classical theory of aerosol nucleation and growth
Homogeneous Nucleation (Becker-Doring model)
?n/?t C b Z
Condensation Growth
ps po expQv/(kTb) Qv/(kTs)
Coagulation
where the coagulation kernel is given by
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Dependence of homogeneous nucleation rate and
critical radius on saturation ratio
Si, n1020 cm3, T2000 K

• 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

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Effect of ionization on cluster nucleation rate

• Ion jacketing produces seed sites
• Dielectric constant of vapor reduces free energy

Si, n1020 cm3, T2000 K, Zeff0.01
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Modeling
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A 1-D multi-physics scoping tool was developed to
help interpret plume condensation results
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.

• Laser absorption
• Thermal response
• Evaporation flux
• Transient gasdynamics
• Radiation transport
• Condensation
• Ionization/recombination

Ioeax, 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
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Model prediction of expansion dynamics
Target Si Laser Intensity 5x109 W cm-2
(peak of Gaussian) Ambient 500 mTorr He

• High ambient pressure prevents interpenetration
• (note, the 2-fluid model lacks single-particle
  effects)

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The plume front is accelerated to hypersonic
velocities
Thermal energy is converted into kinetic energy
collisions also appear to transfer energy from
the bulk of the plume to the plume front
62 eV
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Model prediction of cluster birth and growth
Clusters are born at the contact surface and
grow behind it Nucleation shuts down rapidly as
the plume expands
ms
Spatial distribution of nucleation () and growth
(o) rates at 500 ns
Time-dependence of growth rate/birth rate
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Experiments
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Experimental setup for studies of ablation plume
dynamics
Target Al, Si Laser Intensity 1075x109
W/cm2 Ambient 10-8 Torr 100 Torr air
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Expansion of interpenetrating plumes depends
strongly on the background pressure
0.01 Torr
Free expansion (collisionless) Weakly
collisional transition flow Collisional
transition flow Fully collisional
plume Confined plume
0.1 Torr
1 Torr
10 Torr
100 Torr
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Example plume behavior in weakly collisional
transition regime (150 mTorr)
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Plume behavior in weakly collisional transition
regime (150 mTorr)

• 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

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Plasma parameters are measured using
spectroscopic techniques
Electron Density Measured using Stark broadening
Initial 1019cm-3 Falls very rapidly
within 200 ns Follows 1/t Adiabatic
Temperature Measured from line intensity
ratios Initial 8 eV falls very rapidly

(Experiment Parameters 5 GW cm-2, 150 mTorr air)
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Besides spectroscopy, witness plates served as a
primary diagnostic
Witness plate preparation technique

• Start with single crystal Si
• HF acid dip to strip native oxide
• Spin, rinse, dry
• Controlled thermal oxide growth at 1350 K to
  1mm, 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
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Measurement of final condensate size
500 mTorr He
5x109 W/cm2
5x108 W/cm2
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Cluster size distribution comparison of theory
experiment

• Good correlation between laser intensity and
  cluster size is observed.
• Is it due to increasing saturation ratio or
  charge state?

note the discrepancy at low irradiance is
believed to be caused by anomolously high charge
state induced by free electrons
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Saturation ratio and charge state derived from
experimental measurements
Saturation ratio is inversely related to laser
intensity!
Maximum ionization state derived from
spectroscopy, assuming LTE
Saturation ratio derived from spectroscopy,
assuming LTE
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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