Improving Laser/Plasma Coupling with Rough Surfaces K. Highbarger 1, R. Stephens 2, E. Giraldez 2, J. Jaquez 2, L. VanWoerkom 1, R. Freeman 1 1 The Ohio State University, 2 General Atomics Abstract  Photons from a laser are converted into electrons at a solid surface with 30% efficiency.  Recent experiments using sub-picosecond laser pulses have shown this can be doubled with a rough surface [1].  We can produce such a surface with particle track etching.  Using this surface in Fast Ignition requires much longer pulses.  We want to look at rough surface reflectivity as a function of pulse length. Conical Pit Formation  Material along track dissolves faster than bulk.  Etch rate along track is dependent on ion charge, mass, energy and material.  Ratio between track etch rate and bulk etch rate, called track-etch ratio, is an important parameter affecting dimensions of conical pits.  Plastics have a high track-etch ratio allowing for conical pits with a large peak to diameter ratio. Flat Foil Target Fabrication  Initial targets will be 1mm 2 flat foils.  Ion energies need to be large enough for range and small enough for track production.  We require ion energies in the range of ~2.0 MeV/nucleon, mass in the range of Si-Zn and an ion fluence of ions/cm 2.  Ions of these sorts can be achieved at a Tandem Van de Graff accelerator facility (Argonne, Brookhaven).  1”x9” piece of polycarbonate will be fed through accelerator ion beam.  Irradiated polycarbonate will be etched in an NaOH solution.  Etched pieces will be sputter coated with a few microns of Al.  Completely dissolve polycarbonate leaving an Al replica of etched tracks.  Targets will be shot on Titan at LLNL with pulses ranging from 1 – 30ps. Material Sensitivity to Track Formation  Particle tracks can only be formed in dielectrics.  Plastic is the most sensitive material to track formation.  Slower moving ions cause greater damage (ionization) than faster moving ions.  Range of particle increases and ionization decreases with increased ion energy.  There is a critical ionization level for polycarbonate below which no tracks will form. Rough Surfaces as Selective Absorbers  As wavelength becomes small compared to structure, reflectance undergoes an abrupt to gradual transition.  Reflectance is mainly dependent on surface depth, D, and particle radius, a, of structure [2].  For relatively deep fine structures there is a window of low reflectance. Particle Tracks  Particle tracks are damage regions caused by ionization created along the path of charged particles in a material.  Particle tracks can be revealed through etching. References [1] P. P. Rajeev, S. Banerjee, A. S. Sandhu, R. C. Issac, L. C. Tribedi, and G. R. Kumar, Phys. Rev. A 65, (2002). [2] R. B. Stephens, G. D. Cody, Solar Energy Materials 1, 397 (1979). [3] R. L. Fleischer, P.B.Price, R. M. Walker, Nuclear Tracks in Solids: Principles and applications. University of California, Berkeley, [4] R. L. Fleischer, P.B.Price, R. M. Walker, Phys. Rev. A 133, 1443 (1964). Acknowledgements This work was supported by the U.S. Department of Energy under contract No. DE-FG02-05ER5484. Cone Targets  With particle track etching a rough surface can be easily put onto the inside tip of a cone by irradiating the plastic cone mandrel.  Etch dynamics can limit roughness to tip of cone. Figure 4. Etched Tracks of 139-MeV S 32 Ions in Lexan Average track length ~53 microns Courtesy R. L. Fleischer 4 Figure 5. Rough Surface Target Track length.25 – 6 microns Figure 2. Tracks in Cloud Chamber Figure 3. Damage vs. Velocity Courtesy of R. L. Fleischer 3 Figure 1. Reflectance Curve for Inhomogenous Surfaces Courtesy R. B. Stephens 2 Figure 7. Cone Tip Figure 7. Fast Ignition Cone Target Figure 3. Track Development Geometry surface after etching initial surface ion entrance end point of ion VTVT VBVB θ VBVB α sinθ = V B /V T If V T /V B ≤ 1, no track development If V T sin α < V B, no track development RoRo Ro2Ro2