The effect of ionization on condensation in ablation plumes

1. 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

2. We are investigating late-stage laser ablation plume phenomena at UCSD Experimental studies of the expansion dynamics of plumes interpenetrating into ambient gases (with and without magnetic fields) Modeling and experiments on homogeneous nucleation and growth of clusters Spinodal decomposition and liquid droplet ejection 0 8 ns 1000 ns

3. Lasers used in the UCSD Laser Plasma and Laser-Matter Interactions Laboratory Spectra Physics 2-J, 8 ns Nd:YAG with harmonics 1064, 532, 355, 266 nm Lambda Physik 420 mJ, 20 ns multi-gas excimer laser (248 nm with KrF)

4. Similarities and differences in ablation plume parameters * uncertainties in Ablator ionization

5. Theory

6. Classical theory of aerosol nucleation and growth Transport and Rate of Change Particle Growth Rates Homogeneous Nucleation (Becker-Doring model) ∂n/∂t = C b Z Condensation Growth ps = po exp[Qv/(kTb) – Qv/(kTs)] Coagulation where the coagulation kernel is given by

7. Dependence of homogeneous nucleation rate and critical radius on saturation ratio Si, n=1020 cm–3, T=2000 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

8. Effect of ionization on cluster nucleation rate • Ion jacketing produces seed sites • Dielectric constant of vapor reduces free energy Si, n=1020 cm–3, T=2000 K, Zeff=0.01

9. Modeling

10. 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 Ioe–ax, 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

11. 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)

12. 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

13. 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

14. Experiments

15. Experimental setup for studies of ablation plume dynamics Target : Al, Si Laser Intensity : 107–5x109 W/cm2 Ambient : 10-8 Torr – 100 Torr air

16. 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

17. Example: plume behavior in weakly collisional transition regime (150 mTorr)

18. 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

19. 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)

20. 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

21. Measurement of final condensate size 500 mTorr He 5x108 W/cm2 5x109 W/cm2

22. 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

23. Saturation ratio and charge state derived from experimental measurements • Saturation ratio is inversely related to laser intensity! Saturation ratio derived from spectroscopy, assuming LTE Maximum ionization state derived from spectroscopy, assuming LTE

24. 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