A Joule of Light: Laser-Matter Inter-actions Near the Ablation Threshold

1. 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

2. Regimes of Short-Pulse Laser-Matter Interactions E[V/cm]=27.5 √I [W/cm2]

3. 108 W/cm2:Laser-induced damage to grazing-incidence metal mirrors Goal is 5 J/cm2 normal to the beam for 108 shots l=532 nm

4. 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

5. Experiments are performed at the UCSD laser plasma & laser-matter interactions lab Spectra Physics YAG laser: 2J, 10 ns @1064 nm; 800, 500, 300 mJ @532, 355, 266 nm Injection seeded Peak power density ~1014 W/cm2 1 cm fluence is quoted normal to the beam

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

7. Impurities dominate the damage threshold in Al 6061 & Al 1100 Several shots in Al 6061 at 80˚, 1 J/cm2 1000 shots in Al 1100 at 85˚, 1 J/cm2 MgSi Fe 1000x 1000x Occlusions preferentially absorb light, causing explosive ejection and melting; Fe impurities appear unaffected Exposure of Al 1100 to 1000 shots at 85˚ exhibited no damage up to 18 J/cm2

8. Design window for Al-1100 10000 shots in Al 1100 at 85˚, 20 J/cm2 4000x Exposure of Al 1100 to 104 shots at 85˚ exhibits catastrophic damage at fluence >18 J/cm2 Goal =5 J/cm2 for 108 shots

9. Design window for 99.999% pure Al Estimate of energy required to melt: T - To = (2q”/k) sqrt(att/p) e = q”t/[(1-R) cosq] T-To = 640˚C t = 10 ns, q=85˚e = 143 J/cm2 Design window 180 J/cm2

10. Multipulse damage morphology in pure aluminum, 104 shots IV III II Region I I I. Unaffected zoneII. Slipped zoneIII. Damage haloIV. Catastrophic damage Mechanical damage in pure Al exhibits both slip channels and oriented “ripples”

11. Mesoscopic modeling of surface deformation ~10000 atom diameters ~100 atom diameters (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.

12. Interaction between dislocations and dipolar loops during laser pulses II I 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)"

13. Regimes of Laser-Matter Interactions

14. 1010 W/cm2: 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

15. 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 wpe=wlaser (n=4x1021/cm3 ~150 atm for l=532 nm) • a=(nei/c)(wpe/w)2(1/n) n = no+ik = (1–wp2/w2)1/2 • Self-regulating evaporation during pulse Inverse bremsstrahlung (collisional wave damping) in underdense plasma

16. Estimates of parameters at n=ncr • Temperature estimate from flux limit: fa I ~ F n Te ve • Density in the plume n ~ 1020/cm3, ne ~ 1018/cm3 solid density ~ 6x1022/cm3 ncr = 4x1021/cm3 1 atm (0˚C) ~3x1019/cm3

17. Experimental set-up

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

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

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

21. Plume edge position vs. pressure Fitting Curves: Free expansion: R ~ t Shock expansion: R ~ (Eo/ro)1/5 t2/5 Drag: R = Ro(1–exp-bt) Note: mfp of Al ~1/ns ~3.5mm@150 mTorr

22. Plume velocity is measured using time-of-flight analysis of emission lines Al-I line emission: “Plume splitting” is observed: • slower peak~56 eV (2.0x106 cm/s) • faster peak~600eV (6.6x106 cm/s) (nearly free expansion)

23. Although the ion kinetic energy is up to 1 keV, the temperature in the plume is only a few eV Line ratio measurement: kTe=(E1–E2)/ln(I2l2g1A1/I1l1g2A2)

24. Observations of fast ions were made soon after Q-switching was invented v2max v2avg D. W. Gregg and S. J. Thomas, J. Appl. Phys. 37, 4313 (1966).

25. Comparison of Al-I, Al+ and Al++ time-of-flight spectra suggests presence of electric fields Estimated expansion velocities:Al 2.3x106 cm/s (75 eV)Al+ 4x106 cm/s (224 eV)Al++ 6.6x106 cm/s (610 eV)

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

27. Estimates of electric fields near the wall Ponderomotive force = P•E = (n2-1)/8p 2En2=1–wp2/w2

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

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

30. Initial estimates of plume parameters 109 W/cm2, 10 ns Gaussian pulse, Si target, 1 Torr airat 10 ns, n=1020/cm3 T=6000 K (~0.5 eV) vi = 106 cm/s ne/n ~ 1% r*=2s/(rRTlnS) < 0.1 nm (no barrier to cluster formation)at 100 ns S ~ 20–40 J ~ 1030–1040/m3/s

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

32. Homogeneous nucleation rate depends very strongly on saturation ratio (S=pvap/psat ) 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

33. Modification of homogeneous nucleation rate equation due to small critical radius • Surface of tension is not accurately described by “4pr2 ” • sn=s/(1+d/R)2, where d (~0.1 nm) is the difference between the geometric surface and the “surface of tension” • Js=A e–W*/kT, where A=znNo: z is a barrier shape parameter, No is the gas density and n is the attachment frequency)

34. Ionization in the ablation plume can affect condensation • 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 • Dl~ne • High pressure depresses ionization energy: • ne~0.01 n

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

36. Ionization has a major impact! Cluster birthrate vs. saturation ratio (Si, 109 W/cm2, 1% ionization)

37. Regimes of Laser-Matter Interactions

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

39. 1018 W/cm2Effects of ultra-high field Electrons become relativistic when: eE(l/2p)=mec2=5x105 eV recall E=30I1/2, so l[mm]I1/2= 109 for l=1 mm, I = 1018 W/cm2Effects include: distortion of electron orbits (in vacuum) reduction in plasma frequency (higher me) self-focusing (due to spatial profile of intensity) ponderomotive channeling fast (MeV) ion generation

40. A proposal to generate and study fast ions

41. 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 http://aries.ucsd.edu

42. EXTRAS

43. Crystallization of amorphous coatings 111 001 101 diamond turned surface II I Region I 75 nm Al on superpolished flat: ±2Å roughness, 10Å flatness

44. Dependence of plume behavior on laser intensity 2x1012 W/cm2 1x1011 W/cm2 3x1010 W/cm2 8x109 W/cm2 4x109 W/cm2 2x109 W/cm2 1x109 W/cm2 9x108 W/cm2 4x108 W/cm2

45. Ion energies deviate from a shifted Maxwellian

46. Absorptioncoefficient Z = 1, l=1 mm n = no+ik = sqrt[1–wp2/w2(1+in/w)] a=(nei/c)(wpe/w)2(1/n)

47. Inverse bremsstrahlung absorption