FI FSC Progress meeting June 1st, 2005 Todd Ditmire University of Texas at Austin
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Transcript of FI FSC Progress meeting June 1st, 2005 Todd Ditmire University of Texas at Austin
FI FSC Progress meetingJune 1st, 2005
Todd DitmireUniversity of Texas at Austin
Fusion Science Center Research at UT: Hot Fusion Science Center Research at UT: Hot electron and x-ray generation from cone electron and x-ray generation from cone
shaped targets shaped targets
T.E. Cowan, T. Ohkubo, J. Rassuchine, N. Renard-Le Galloudec, Y. Sentoku
Nevada Terawatt Facility, The University of Nevada at Reno
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. , A. FaenovInstitute for High Energy Density, Russian Academy of ScienceE. Foerster, O. WehrhanIOQ, X-ray Optics Group, Jena University
We propose a change to the FSC logo:
The spot from which x-rays originate is found to
be considerably larger than the laser spot
Our contribution to the FI FSC is in the study of Our contribution to the FI FSC is in the study of hot electron generation in cone targetshot electron generation in cone targets
Proposed approach:
Study x-ray source size and brightness from cone targets in silicon with x-ray converter layers
Opening angle < 90°
Small Kα spot
Tip size< 5m
Laser Spot
~ 20m
Many hot electronsHigh energy
conversion efficiency
Bright Bright KKαα
EmissionEmission
Based on modeling at UNR, we have begun a study of Based on modeling at UNR, we have begun a study of hot electron generation in cone targetshot electron generation in cone targets
Contour of Bz/B0
Y. Sentoku, et al., Laser Light and Hot Electron Micro Focusing using a Conical Target; Phys. Plas, 11, 3083 (2004)
3D structure of the magnetic field
x
y
z
Bpola
rizat
ion
polariz
atio
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°=×=
×=−
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/104321
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2180
θcmncmWI
Magnetic field (Bz)
p-po
lp-
pol
Incident & reflective electric fields
Surface current
Magnetic field
Push electrons out to the interior of the cone
x
y
Simualtions indicate that electrons are accelerated Simualtions indicate that electrons are accelerated along the cone surface toward the tip of the conealong the cone surface toward the tip of the cone
Magnetic field (Bz)
Sheath field (En)
Electron angular distribution in p-space Bz & Ey
Electrons are driven back by the sheath field
Forces are balanced
Electrons are trapped on the surface
x
y
z
Bpola
rizat
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polariz
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°=×=
×=−
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θcmncmWI
E
Electron flow
x
y
Y. Sentoku, et al., Laser Light and Hot Electron Micro Focusing using a Conical Target; Phys. Plas, 11, 3083 (2004)
Contour of EM Energy Electron energy density
Hot electrons and laser light are guided inside the Hot electrons and laser light are guided inside the conical target and focused at the tipconical target and focused at the tip
Electron temperature @ tip
A portion of the energy in the tip region is higher than that in the same region of a flat target
The conical target appears to increase the number of electrons accelerated forward by more than one order of magnitude
A portion of the energy in the tip region is higher than that in the same region of a flat target
The conical target appears to increase the number of electrons accelerated forward by more than one order of magnitude
Y. Sentoku, et al., Laser Light and Hot Electron Micro Focusing using a Conical Target; Phys. Plas, 11, 3083 (2004)
We use the fact that the KOH etching rates in Si are We use the fact that the KOH etching rates in Si are strongly affected by the crystallographic orientationstrongly affected by the crystallographic orientation
CrystallographicOrientation
Etching Rate (m/min)
(100) 0.797 (0.548)
(110) 1.455 (1.000)
(111) 0.005 (0.004)
Anisotropic KOH Etching Rates vs. Orientation
@ 70ºC, 30% KOH
Surface Orientation <100>
<111>
54.74º
Diamond Structure of Silicon
(111) the Close-Packed Surface
Double FCC Slow Chemical Reaction
Surface Orientation <110>
<111>
Immerse in KOH solution
Si2N3 Deposition
Furnace @ 950ºC
Si2N3 Deposition
Furnace @ 950ºC
Optical Lithography
PR coat, UV, Develop
Optical Lithography
PR coat, UV, Develop
Reactive Ion Etching
Vacuum @ 10-5 torr
Reactive Ion Etching
Vacuum @ 10-5 torr
KOH Etching
~ 7 hours @ 60ºC
KOH Etching
~ 7 hours @ 60ºC
30% KOH solution is used to etch the Si 100 plane 30% KOH solution is used to etch the Si 100 plane at a 1at a 1m/min etching ratem/min etching rate
Metal Layer Coating
Low Z material (Ti, Cu)
Metal Layer Coating
Low Z material (Ti, Cu)
The tip of pyramid goes to the reverse side as close as possible
KOH etching done
Deposit low Z material to generate characteristic Kα photons
We then deposit a metal layer with thickness We then deposit a metal layer with thickness optimized for maximum optimized for maximum KK yieldyield
Si2N3 Deposition
Furnace @ 950ºC
Si2N3 Deposition
Furnace @ 950ºC
Optical Lithography
PR coat, UV, Develop
Optical Lithography
PR coat, UV, Develop
Reactive Ion Etching
Vacuum @ 10-5 torr
Reactive Ion Etching
Vacuum @ 10-5 torr
KOH Etching
~ 7 hours @ 60ºC
KOH Etching
~ 7 hours @ 60ºC
Metal Layer Coating
Low Z material (Ti, Cu)
Metal Layer Coating
Low Z material (Ti, Cu)
Titanium or Copper Vapor
Thickness will match about
the length of KK photon
mean free path ~ 30 m
2mm2mm
Arrays of sharp tipped pyramid shaped guides have been fabricated successfully
500500μμmm
500500μμmm
50μm
We have achieved pyramid cones with tips We have achieved pyramid cones with tips smaller than 1 smaller than 1 mm
We have achieved pyramid cones with tips We have achieved pyramid cones with tips smaller than 1 smaller than 1 mm
100100μμmm
< 5 m
We have conducted a series of experiments on the We have conducted a series of experiments on the 20 TW THOR Ti:sapphire laser at UT20 TW THOR Ti:sapphire laser at UT
Characteristics
Pulse Width ~ 40 fs
Energy ~ 750 mJ
Intensity >1019 W/cm2
Repetition Rate 10Hz
Characteristics
Pulse Width ~ 40 fs
Energy ~ 750 mJ
Intensity >1019 W/cm2
Repetition Rate 10Hz
Regenerative amplifier (25 passes)
1.4J @ 10Hz YAG
0.2J @
10Hz YAG
5W
Mil
len
ia
4-pass amplifier
5-pass amplifier
40fs to
Target
1 nJ 20fs
600 ps
2 mJ
20 mJ
1.2 J2
0 f
s T
i:S
ap
ph
ire
O
sc
illa
tor
Single Grating Stretcher
1.4J @ 10Hz YAG
Vacuum Compressor
40 mJ
150 mJ
A 1D imaging spectrometer was employed to measure the 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
Spa
tial
Spectral
K1
K2
Kodak RAR 2492 Film
• Integrate ~50 shots for good signal
X-rays in the vicinity of Ti K and K pass through a filter window and are diffracted and line-focused by a cylindrically bent crystal. Blue lines indicate path of detected x-rays.
Cylindrically Bent PET Crystal
• Bragg reflection, 2nd order
• Oriented to detect 2.21Å - 2.79Å
• Line focus increases intensity of spectrum
K2 & K1 K
A Von Hamos spectrometer measured yield of Ti Kand K
R= 250 mm7th order
A bent mica spectrometer was used to image the Ti K in one dimension
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Clear spatial side bands were observed on flat Ti targets
y
K 1 K 2
-800 -600 -400 -200 0 200 400 600 800
Space dimentions, um
Mean K Max K Blue edge
Side peaks from fountain effect?
K1
~ 520 um
Spatial sidebands are seen throughout the K line
30x10-3
25
20
15
10
5
0
-5
-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]
Side peakSide peak
Plateau
Spatial Extent of K1 11 0From µm Ti Foil Shot at º
18Run 19Run
30x10-3
25
20
15
10
5
0
-5
-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]
Side peakSide peak
Plateau
Spatial Extent of K1 11 0From µm Ti Foil Shot at º
18Run 19Run
A well-imaged K line from flat targets showed well-defined side peaks
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
?Ti K
b
righ
tne
ss [
ph
oto
ns/
µm
2 /sh
ot @
film
]
Hot electrons
800760720680640600560520480440400 800760720680640600560520480440400
Run 19 (false color)
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03.02.82.62.42.22.01.81.61.41.21.00.80.60.40.20.0 mm
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740720700680660640620600580560540520500
30x10-3
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Ti K1
[brightness K
1
/photons µm
2
/ ]shot on film
-2mm -1 0 1 2 ( ) [ ]Spatial extent in horizontal polarization direction mm
,11Flat Ti µm71 º square cones25 µm Ti on back
.0 Direct brightness comparison of cones vs º flats
Cone targets exhibit lower yield but substantially smaller source size when compared to flat Ti targets
Flat Ti target
Si cone with Ti fluor layer target
60x10-3
50
40
30
20
10
0
-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]
Spatial Extend of K1 From 11µm flat Ti foil and 71º 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
20
10
0
-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]
Spatial Extend of K1 From 11µm flat Ti foil and 71º 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
The spatial extent of Kfrom cone targets showed no side peaks or plateau
Ti K
b
righ
tne
ss [
ph
oto
ns/
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 electrons. This could account for the lack of side peaks & plateau for cone and wedge targets
Far-flung electrons stopped by Si
Si Ti
Ti
We investigated the role of polarization by elongating the pyramid and comparing P and S polarization
Investigation of polarization effectsInvestigation of polarization effects
Various lengths of grooves will be necessaryVarious lengths of grooves will be necessary
Investigation of polarization effectsInvestigation of polarization effects
Various lengths of grooves will be necessaryVarious lengths of grooves will be necessary
Sharp end in polarization plane Wide end in polarization plane
Anisotropic etching of Si is used to produce shaped targets
point line 100µm
A wedge is a “1-D” cone
p s Flat, 0º
“p” and “s” refer to wedge orientation relative to laser polarization
Si TiTi
S-oriented wedges showed higher K yield than p-oriented wedges, but less than flat foils
2.73 2.74 2.75 2.76 2.77Wavelength (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
50 shots integrated for each target type
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
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
B
Photomultipliertube
NaI scintillatorcrystal
Al, Cu and filters
Pb shielding
tube
Pb
THOR laser
f/2.8
Filter Cutoff energy
32 mm Pb 600 keV
48 mm Pb 800 keV
95 mm Cu 1200 keV
NaI detectors
We measured hot electron production with NaI hard x-ray detectors
Hard x-ray detectors show greater hot e- production for P-pol shots than S-pol or flat targets
~50 shots averaged for each target type
Hard X-ray detector1 2 3
0
100
200
300
400S
ign
al @
sco
pe
[nV
s] Flat, 0º
S -wedge
P -wedge
Hard X-ray yield from 25µm foil for flat, s-wedge, and p-wedge Targets
>0.8MeV4.8cm Pb
>1.2MeV9.5cm Cu
>0.6MeV3.2cm Pb
Explanation:•Hard X-ray yields suggest, that p-polarized wedges are able to create more hot electrons than s -polarized wedges.•Hotter electrons in case of p-wedges will lead to lower Kalpha production since they pass the thin Titanium slab unaffected.
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 COMET
Near term plan will study sidebands (and lack of) and hot electron temperature from cones