Kodak RAR 2492 Film Same film as above Integrate ~50 shots for good signal X-ray Imaging Spectroscopy of Ti Foils and Pyramidal Targets 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 T.E. Cowan, T. Ohkubo, J. Rassuchine, N. Renard-Le Galloudec, Y. Sentoku Nevada Terawatt Facility, The University of Nevada at Reno S.A. Pikuz, Jr. Institute for High Energy Density, Russian Academy of Science Anisotropic Etching of Si Is Used to Produce Sharp Tipped Pyramids A 1D imaging spectrometer was employed to measure spatial extent of titanium K  magnets Spherically Bent Mica Crystal Bragg reflection, 7th order Oriented to reflect K  1 and K  2 Spatially images horizontal direction Demagnification of 1.15 target Kodak RAR 2492 Film Directly exposed by Ti K  x-rays High (5  m) resolution 1 Requires ~30 shots integrated 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 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. Spatial Spectral K1K1K2K2 Contact: A second spectrometer with a cylindrical crystal measured Ti K   through K  Cylindrically Bent PET Crystal Bragg reflection, 2nd order Oriented to detect 2.21Å Å Line focus increases intensity of spectrum Surface Orientation 54.74º 1m1m K  2 & K  1 KK Wavelength (Å) Photons/µm 2 /shot on film Flat, 0º S -wedge P -wedge X-ray yield from 25µm foil for flat, s- wedge, and p-wedge Targets S-oriented wedges showed higher K  yield than p-oriented wedges, but less than flat foils Data taken with Von Hamos spectrometer; 50 shots integrated for each target type point  line 100µm A wedge is a “2-D” cone psFlat, 0º “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 The spatial extent of K  from cone targets showed no side peaks or plateau A well-imaged K  line from flat targets showed well-defined side peaks 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 PIC simulations explore the absorption properties of pyramids for our conditions 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. 2D pyramids for S and P polarizationFree standing Au vs. flat foil 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. This experimental run was performed on the THOR laser at the University of Texas, Austin THOR: 20TW CPA TiSapp laser system 400mJ on target for this run 40fs pulse duration 10  m focal spot size from f#3 parabola  ~1x10 19 W/cm 2 peak intensity External diagnostic: 3 Scintillator+PMT hard x-ray detectors 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. 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 keV range Energy selection for Ti Ka production 2 ? Ti K  brightness [photons/µm 2 film] Hot electrons  Ti K  brightness [photons/µm 2 film] K  1 spatial from flats, p-wedges, and cones (offset): Cones and wedges do not show a plateau or side peaks. Laser in 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 electrons 3. This could account for the lack of side peaks & plateau for cone and wedge targets Far-flung electrons stopped by Si References 1.B.L. Henke, et. al. Low-energy x-ray response of photographic films. II. Experimental characterization J. Opt. Soc. Am B 1, 6 (1984) 2.E. Casnati, et. al. An empirical approach to K-shell ionisation cross section by electrons J. Phys. B. 15, 1 (1982) 3.NIST estar database program 4.S. Hansen, et. al. Temperature determination using K α spectra from M-shell Ti ions at solid density In submission, Phys. Rev. E. rapid comm. SiTi Si Ti Our investigations are ongoing 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 COMET 4 E. Foerster, O. Wehrhan IOQ, X-ray Optics Group, Jena University Run 19 (false color)