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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 10 7 -10 10 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, 052903 (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, 1975. [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 V T V B θ V B α sinθ = V B /V T If V T /V B ≤ 1, no track development If V T sinα < V B , no track development R o R o 2
