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Surface Orientation . . 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/µm 2 /shot on film. 0. -0.05. 2.73. 2.74. 2.75. 2.76. 2.77. Wavelength (Å). - PowerPoint PPT Presentation
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60x10-3
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-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]
Spatial Extend of K From 11µm flat Ti foil and 71¼ P-wedges and cones with 25µm Ti on Back Surface
11µm Ti foil shot at 0º
p-oriented Si wedge with 25µm Ti
Si pyramid with 25µm Ti
60x10-3
50
40
30
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-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]
Spatial Extend of K From 11µm flat Ti foil and 71¼ P-wedges and cones with 25µm Ti on Back Surface
11µm Ti foil shot at 0º
p-oriented Si wedge with 25µm Ti
Si pyramid with 25µm Ti
30x10-3
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-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]
Side peakSide peak
Plateau
Spatial Extent of K1 From 11µm Ti Foil Shot at 0º
Run 18 Run 19
30x10-3
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-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]
Side peakSide peak
Plateau
Spatial Extent of K1 From 11µm Ti Foil Shot at 0º
Run 18 Run 19
Kodak RAR 2492 Film
• Same film as above
• Integrate ~50 shots for good signal
X-ray Imaging Spectroscopy of Ti Foils and Pyramidal TargetsGilliss 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
magnetsmagnets
Spherically Bent Mica Crystal
• Bragg reflection, 7th order
• Oriented to reflect K1 and K2
• Spatially images horizontal direction
• Demagnification of 1.15
targettarget
Kodak RAR 2492 Film
• Directly exposed by Ti K x-rays
• High (5m) resolution1
• Requires ~30 shots integrated
X-rays in the vicinity of K1 and K2 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.
Spa
tial
Spectral
K1 K2
Contact: [email protected]
A second spectrometer with a cylindrical crystal measured Ti Kthrough K
Cylindrically Bent PET Crystal
• Bragg reflection, 2nd order
• Oriented to detect 2.21Å - 2.79Å
• Line focus increases intensity of spectrum
Surface Orientation <100>
<111>
54.74º
1m
K2 & K1 K
2.73 2.74 2.75 2.76 2.77
Wavelength (Å)
-0.05
0
0.05
0.10
0.15
Pho
tons
/µm
2 /sh
ot o
n fi
lm
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
p s Flat, 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 polarization Free 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
• 10m focal spot size from f#3 parabola
~1x1019 W/cm2 peak intensity
External diagnostic: 3 Scintillator+PMT hard x-ray detectors
K1 spatial of flats: With best imaging conditions and 35 integrated shots, clear side peaks become visible for 11m 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≈ 125m H~20-30kG
• Electrons responsible for Ti K-shell ionization are largely in the 10-100keV range
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Ka p
roduct
ion c
ross
sect
ion (
arb
. unit
s)
0.001 0.01 0.1 1
Electron Energy (MeV)
Energy selection for Ti Ka production2
?Ti K
b
right
ness
[pho
tons
/µm
2 /sh
ot @
film
]
Hot electrons
Ti K
b
right
ness
[pho
tons
/µm
2 /sh
ot @
film
]
K1 spatial from flats, p-wedges, and cones (offset): Cones and wedges do not show a plateau or side peaks.
Laser in Laser in
Fountain screening: In cones, electrons which “fountain” >50m 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
Far-flung electrons stopped by Si
References1. 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 http://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html4. S. Hansen, et. al. Temperature determination using Kα spectra from M-shell Ti ions at solid density In submission, Phys. Rev. E. rapid comm.
Si Ti
Ti
Si TiTi
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 K1 & 2 as per results from recent experiment at COMET4
E. Foerster, O. WehrhanIOQ, X-ray Optics Group, Jena
University
Run 19 (false color)
