X-ray Imaging Spectroscopy of Ti Foils and Pyramidal Targets

1. Surface Orientation <100> <111> 54.74º X-ray yield from 25µm foil for flat, s-wedge, and p-wedge Targets 0.15 Flat, 0º S -wedge 0.10 P -wedge 0.05 Photons/µm2/shot on film 0 -0.05 2.73 2.74 2.75 2.76 2.77 Wavelength (Å) Gilliss Dyer, Byoung-Ick Cho, Stefan Kneip, Daniel Symes and Todd Ditmire Texas Center for High Intensity Laser Science, The University of Texas at Austin S.A. Pikuz, Jr. Institute for High Energy Density, Russian Academy of Science E. Foerster, O. Wehrhan IOQ, X-ray Optics Group, Jena University T.E. Cowan, T. Ohkubo, J. Rassuchine, N. Renard-Le Galloudec, Y. Sentoku Nevada Terawatt Facility, The University of Nevada at Reno A 1D imaging spectrometer was employed to measure spatial extent of titanium K A well-imaged K line from flat targets showed well-defined side peaks Anisotropic Etching of Si Is Used to Produce Sharp Tipped Pyramids X-ray Imaging Spectroscopy of Ti Foils and Pyramidal Targets Is there an electron “fountain” effect? • Electrons leaving front and back surfaces are pulled back by space charge but could also be influenced by an azimuthal magnetic field • L≈ 125m  H~20-30kG • Electrons responsible for Ti K-shell ionization are largely in the 10-100keV range Spherically Bent Mica Crystal • Bragg reflection, 7th order • Oriented to reflect K1 and K2 • Spatially images horizontal direction • Demagnification of 1.15 Run 19 (false color) Ti K brightness [photons/µm2/shot @ film] magnets target Energy selection for Ti Ka production2 Kodak RAR 2492 Film • Directly exposed by Ti Kx-rays • High (5m) resolution1 • Requires ~30 shots integrated ? 1m  Hot electrons Spectral K1 spatial of flats: With best imaging conditions and 35 integrated shots, clear side peaks become visible for 11m Ti foils shot at 0º. Using anisotropic etching of Si square-based cones, or “pyramids”, with mirror flat walls and sub-micron sharp tips, can be cheaply produced in large quantities. Cone angle is always 71º This geometry of silicon is exploited by the Nanomechanics group at UNR, to produce free-standing gold pyramids X-rays in the vicinity of K1 and K2 are focused by an off-axis spherically bent crystal which is fine-tuned to give spatial resolution in 1 dimension Spatial K1 K2 The spatial extent of Ka from cone targets showed no side peaks or plateau PIC simulations explore the absorption properties of pyramids for our conditions A second spectrometer with a cylindrical crystal measured Ti Kthrough K Far-flung electrons stopped by Si 2D pyramids for S and P polarization Free standing Au vs. flat foil Cylindrically Bent PET Crystal • Bragg reflection, 2nd order • Oriented to detect 2.21Å - 2.79Å • Line focus increases intensity of spectrum Ti K brightness [photons/µm2/shot @ film] Laser in Laser in Kodak RAR 2492 Film • Same film as above • Integrate ~50 shots for good signal Si Ti Energy density at 60fs for s (left) and p (right) polarizations. Surface compression heats the surface in the s-case, but very hot electrons escape normal to the surface and from the tip in the p- case. In the simulations, no significant surface guiding occurs at this cone angle. Energy density at 60fs for free standing gold cone shot with p polarization as compared to flat foil shot at 0º. A free standing pyramid better contains hot electrons, leading to higher energy densities by the surface. Ti K1 spatial from flats, p-wedges, and cones (offset): Cones and wedges do not show a plateau or side peaks. Fountain screening: In cones, electrons which “fountain” >50m from center on the front side will be separated from Ti by enough Si to stop 100keV electrons3. This could account for the lack of side peaks & plateau for cone and wedge targets X-rays in the vicinity of Ti Ka and Kb pass through a filter window and are diffracted and line-focused by a cylindrically bent crystal. Blue lines indicate path of detected x-rays. K2 & K1 K Our investigations are ongoing This experimental run was performed on the THOR laser at the University of Texas, Austin S-oriented wedges showed higher K yield than p-oriented wedges, but less than flat foils Future/current projects • Further experiments and modeling of side peaks in spatial distribution of Ti K for flat targets • Seek laser parameters for which significant electron channeling can occur (e.g. higher intensity) • Spectroscopy of Si (better sensitivity from crystals) • Study higher Z Ka source materials (e.g. gold) for which hot electrons >100keV are more relevant (crystal spectrometers do not operate at Au Ka energies) • Investigate narrowing of cone angle (e.g. “half pyramids”, laser machining, etc.) • Analyze and model K1 & 2 as per results from recent experiment at COMET4 • THOR: 20TW CPA TiSapp laser system • • 400mJ on target for this run • • 40fs pulse duration • • 10m focal spot size from f#3 parabola • ~1x1019 W/cm2 peak intensity External diagnostic: 3Scintillator+PMT hard x-ray detectors p s Flat, 0º Ti Si Ti point line 100µm A wedge is a “2-D” cone “p” and “s” refer to wedge orientation relative to laser polarization Possible explanations • Imperfect coupling between wedge and foil • Mid-temperature (~10keV) electrons stopped by Si bulk material; • Minimal surface guiding of electrons towards tip • P polarized produced more hot electrons (>100 keV, which interact less with Ti) than s-polarized wedges References B.L. Henke, et. al. Low-energy x-ray response of photographic films. II. Experimental characterization J. Opt. Soc. Am B 1, 6 (1984) E. Casnati, et. al. An empirical approach to K-shell ionisation cross section by electrons J. Phys. B. 15, 1 (1982) NIST estar database program http://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html S. Hansen, et. al. Temperature determination using Kα spectra from M-shell Ti ions at solid densityIn submission, Phys. Rev. E. rapid comm. Data taken with Von Hamos spectrometer; 50 shots integrated for each target type Contact: gilliss@physics.utexas.edu