A Joule of Light: Laser-Matter Inter- actions Near the Ablation Threshold Mark S. Tillack Mechanical and Aerospace Engineering Department and the Center for Energy Research Jacobs School of Engineering 13 May 2002

Regimes of Short-Pulse Laser-Matter Interactions E[V/cm]=27.5 √I [W/cm 2 ]

10 8 W/cm 2 : Laser-induced damage to grazing-incidence metal mirrors Goal is 5 J/cm 2 normal to the beam for 10 8 shots =532 nm

Surface damage leads to roughening, loss of beam quality and increased absorption  Single Shot Effects:  Laser heating generates defects (or melting)  Coupling between diffusion and elastic fields lead to permanent deformation  Progressive Damage in Multiple Shots:  Thermoelastic stress cycles shear atomic planes relative to one another (slip by dislocations)  Extrusions & intrusions are formed when dislocations emerge to the surface, or by grain boundary sliding.  Operation beyond the normal incidence damage threshold raises new concerns

Experiments are performed at the UCSD laser plasma & laser-matter interactions lab Spectra Physics YAG laser: 2J, 10 nm; 800, 500, , 266 nm Injection seeded Peak power density ~10 14 W/cm 2 1 cm fluence is quoted normal to the beam

Mirrors are fabricated by diamond turning or substrate coating E-Beam Al (<2  m) CVD-SiC (100  m) SiC Foam (3 mm) Composite face (1 mm) SiC Foam (3 mm) MER composite mirror MER composite mirror Diamond-turned Al fabricated at GA micromachining lab

Impurities dominate the damage threshold in Al 6061 & Al 1100 Fe MgSi 1000x Several shots in Al 6061 at 80˚, 1 J/cm shots in Al 1100 at 85˚, 1 J/cm x Exposure of Al 1100 to 1000 shots at 85˚ exhibited no damage up to 18 J/cm 2 Occlusions preferentially absorb light, causing explosive ejection and melting; Fe impurities appear unaffected

Design window for Al-1100 Exposure of Al 1100 to 10 4 shots at 85˚ exhibits catastrophic damage at fluence >18 J/cm shots in Al 1100 at 85˚, 20 J/cm x Goal =5 J/cm 2 for 10 8 shots

Design window for % pure Al Design window Estimate of energy required to melt: T - T o = (2q”/k) sqrt(  t t/  ) e = q”t/[(1-R) cos  ] T-T o = 640˚C t = 10 ns,  =85˚ e = 143 J/cm J/cm 2

Multipulse damage morphology in pure aluminum, 10 4 shots I II III IV Region I I. Unaffected zone II.Slipped zone III.Damage halo IV.Catastrophic damage Mechanical damage in pure Al exhibits both slip channels and oriented “ripples”

Mesoscopic modeling of surface deformation (1)Surface Deforms by Slip Lines (Dislocations) within each grain. (2)Each line is represented by a 3-D space curve that moves and produces its own stress field (like a crack). (3)Slip on atomic planes in one grain results in relative grain rotation and surface misorientation. ~100 atom diameters ~10000 atom diameters

Interaction between dislocations and dipolar loops during laser pulses I II Region I The slip of dislocations pushes the dipolar loops closer to the surface, causing its deformation. This condition is very important in metal fatigue by laser pulses, and is known as "Persistent Slip Bands (PSB's)"

Regimes of Laser-Matter Interactions

10 10 W/cm 2 : Ablation plume dynamics Applications:  Process improvements for laser micromachining, cluster production, thin film deposition  Ion source for laser-IFE blast simulations Physical processes:  Laser absorption  Thermal response  Evaporation  Transient gasdynamics  Radiation transport  Condensation  Ionization/recombination

Laser absorption processes Initial absorption creates high-pressure vapor Large E-field (30√ I V/cm) ionizes the vapor Electron density cascades as high as  pe =  laser (n=4x10 21 /cm 3 ~150 atm for =532 nm)  =( ei /c)(  pe /  ) 2 (1/n) n = n o +ik = (1–  p 2 /  2 ) 1/2 Self-regulating evaporation during pulse Inverse bremsstrahlung (collisional wave damping) in underdense plasma

Estimates of parameters Temperature estimate from flux limit: f a I ~ F n T e v e Density in the plume n ~ /cm 3, n e ~ /cm 3 solid density ~ 6x10 22 /cm 3 n cr = 4x10 21 /cm 3 1 atm (0˚C) ~3x10 19 /cm 3 at n=n cr

Experimental set-up

Ablation plume evolution strongly depends on background pressure Above ~10 Torr, the plume stalls and is “slammed” back into the target 100 Torr Visible emission measured by 2-ns gated iCCD camera Al target, 0.6 mm spot

Ablation plume behavior in the low pressure regime 10 –6 Torr Below ~1 mTorr, the plume expands freely

0.15 Torr In the intermediate pressure regime, the plume detaches but continues to interact with the background gas Ablation plume behavior in the intermediate pressure regime

Plume edge position vs. pressure Fitting Curves: Free expansion: R ~ t Shock expansion: R ~ (E o /  o ) 1/5 t 2/5 Drag: R = R o (1–exp -bt ) Note: mfp of Al ~1/n  mTorr

Plume velocity is measured using time- of-flight analysis of emission lines “Plume splitting” is observed: slower peak~56 eV (2.0x10 6 cm/s) faster peak~600eV (6.6x10 6 cm/s) (nearly free expansion) Al-I line emission:

Although the ion kinetic energy is up to 1 keV, the temperature in the plume is only a few eV Line ratio measurement: kT e =(E 1 –E 2 )/ln(I 2 2 g 1 A 1 /I 1 1 g 2 A 2 )

Observations of fast ions were made soon after Q-switching was invented D. W. Gregg and S. J. Thomas, J. Appl. Phys. 37, 4313 (1966). v 2 max v 2 avg

Comparison of Al-I, Al + and Al ++ time-of-flight spectra suggests presence of electric fields Estimated expansion velocities: Al2.3x10 6 cm/s (75 eV) Al+4x10 6 cm/s (224 eV) Al++6.6x10 6 cm/s (610 eV)

Estimates of electric fields near the wall “Double layer” (or sheath) potential is ~3/2 kT

Estimates of electric fields near the wall Ponderomotive force = P  E = (n 2 -1)/8   2 E n 2 =1–  p 2 /  2

Condensation of aerosol creates problems in several laser (and IFE) applications

A 1D multi-physics model is being developed to explore process improvements for laser micromachining Physical processes:  Laser absorption  Thermal response  Evaporation  Transient gasdynamics  Radiation transport  Condensation  Ionization/recombination Simple absorption coefficient, I=I o e – x 1D conduction&convection 1D, 2-fluid Navier Stokes fluid equations (with Knudsen layer jump conditions) TBD See below... Modified Saha, 3-body recombination

Initial estimates of plume parameters 10 9 W/cm 2, 10 ns Gaussian pulse, Si target, 1 Torr air at 10 ns, n=10 20 /cm 3 T=6000 K (~0.5 eV) v i = 10 6 cm/s n e /n ~ 1% r*=2  /(  RTlnS) < 0.1 nm (no barrier to cluster formation) at 100 ns S ~ 20–40 J ~ –10 40 /m 3 /s

Classical aerosol generation and transport Homogeneous Nucleation (Becker-Doring model) Condensation Growth Coagulation where the coagulation kernel is given by Convective Diffusion and Transport Particle Growth Rates

Homogeneous nucleation rate depends very strongly on saturation ratio (S= p vap /p sat ) Formation Rate and Size of Pb droplets in an IFE System High saturation ratios result from rapid cooling from adiabatic plume expansion Extremely small critical radius results Competition between homogeneous and heterogeneous condensation determines final size and density distribution; Reduction in S due to condensation shuts down HNR quickly

Modification of homogeneous nucleation rate equation due to small critical radius Surface of tension is not accurately described by “4  r 2 ”  n =  /(1+  /R) 2, where  (~0.1 nm) is the difference between the geometric surface and the “surface of tension” J s =A e –W*/kT, where A=z N o : z is a barrier shape parameter, N o is the gas density and is the attachment frequency)

Stark broadening is the dominant broadening mechanisms for many laser- produced plasmas Electric microfields produced by nearby charged particles modify the excitation energy of emitters  ~n e High pressure depresses ionization energy: n e ~0.01 n Ionization in the ablation plume can affect condensation

Mechanisms of enhanced cluster formation Gibbs free energy Cluster radius Ion jacketing results in an offset in free energy (toward larger r * ) Dielectric constant of liquid reduces free energy

Ionization has a major impact! Cluster birthrate vs. saturation ratio (Si, 10 9 W/cm 2, 1% ionization)

Regimes of Laser-Matter Interactions

CPA enables table-top ultra-high (TW) intensity research at a “modest” price CPA = Chirped pulse amplification

10 18 W/cm 2 Effects of ultra-high field Electrons become relativistic when: eE(  =m e c 2 =5x10 5 eV recall E=30I 1/2, so [  m]I 1/2  =   for =1  m, I = W/cm 2 Effects include: distortion of electron orbits (in vacuum) reduction in plasma frequency (higher m e ) self-focusing (due to spatial profile of intensity) ponderomotive channeling fast (MeV) ion generation

A proposal to generate and study fast ions

Laser-matter interactions touch upon many fields of engineering science, and offer numerous opportunities for student research Mechanics of materials Ablation plume dynamics Laser plasmas Cluster formation Laser propagation Relativistic plasma physics

EXTRAS

Crystallization of amorphous coatings I II Region I 75 nm Al on superpolished flat: ±2Å roughness, 10Å flatness diamond turned surface

Dependence of plume behavior on laser intensity 2x10 12 W/cm 2 1x10 11 W/cm 2 3x10 10 W/cm 2 8x10 9 W/cm 2 4x10 9 W/cm 2 2x10 9 W/cm 2 1x10 9 W/cm 2 9x10 8 W/cm 2 4x10 8 W/cm 2

Ion energies deviate from a shifted Maxwellian

Absorption coefficient  =( ei /c)(  pe /  ) 2 (1/n) n = n o +ik = sqrt[1–  p 2 /  2 (1+i /  )] Z = 1, =1  m

Inverse bremsstrahlung absorption