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