Transcript Document

Fusion Science Center Research at UT: Hot
electron and x-ray generation from cone
shaped targets
FI FSC Progress meeting
June 1st, 2005
Todd Ditmire
University of Texas at Austin
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. , A. Faenov
Institute for High Energy Density, Russian Academy of Science
E. Foerster, O. Wehrhan
IOQ, X-ray Optics Group, Jena University
We propose a change to the FSC logo:
Our contribution to the FI FSC is in the study of
hot electron generation in cone targets
The spot from which xrays originate is found to
be considerably larger
than the laser spot
Many hot electrons
High energy
conversion efficiency
Small Kα
spot
Proposed approach:
Study x-ray source size and
brightness from cone
targets in silicon with x-ray
converter layers
Laser
Spot
~ 20mm
Tip size
< 5mm
Opening angle
< 90°
Bright Kα
Emission
Based on modeling at UNR, we have begun a study of
hot electron generation in cone targets
z
I 0  4 10 W / cm
n0  5.5 1021 cm3
  26
18
2
y
Incident & reflective electric fields
x
B
Surface current
Magnetic field
Push electrons out to the interior of the cone
y
Magnetic field (Bz)
x
Contour of Bz/B0
3D structure of the magnetic field
Y. Sentoku, et al., Laser Light and Hot Electron Micro Focusing using a Conical Target; Phys. Plas, 11, 3083 (2004)
Simualtions indicate that electrons are accelerated
along the cone surface toward the tip of the cone
z
I 0  4 10 W / cm
n0  5.5 1021 cm3
  26
18
2
y
Electrons are driven back by the sheath field
x
B
Forces are balanced
Electrons are trapped on the surface
E
Electron flow
Sheath field (En)
y
Magnetic field (Bz)
x
Electron angular
distribution in p-space
Bz & Ey
Y. Sentoku, et al., Laser Light and Hot Electron Micro Focusing using a Conical Target; Phys. Plas, 11, 3083 (2004)
Hot electrons and laser light are guided inside the
conical target and focused at the tip
Contour of EM Energy
Electron energy density
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
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
strongly affected by the crystallographic orientation
(111) the ClosePacked Surface
Diamond Structure
of Silicon
Double FCC
Anisotropic KOH Etching
Rates vs. Orientation
Crystallographic
Orientation
Etching Rate
(mm/min)
(100)
0.797 (0.548)
(110)
1.455 (1.000)
(111)
0.005 (0.004)
Slow Chemical Reaction
@ 70ºC, 30% KOH
Surface Orientation
<100>
Surface Orientation
<110>
<111>
<111>
54.74º
30% KOH solution is used to etch the Si 100 plane
at a 1mm/min etching rate
The tip of pyramid goes to the
reverse side as close as possible
Si2N3 Deposition
Furnace @ 950ºC
Immerse in KOH solution
Optical Lithography
PR coat, UV, Develop
Reactive Ion Etching
KOH etching done
Vacuum @ 10-5 torr
KOH Etching
~ 7 hours @ 60ºC
Metal Layer Coating
Low Z material (Ti, Cu)
We then deposit a metal layer with thickness
optimized for maximum Ka yield
Deposit low Z material to generate
characteristic Kα photons
Si2N3 Deposition
Furnace @ 950ºC
Optical Lithography
PR coat, UV, Develop
Titanium or Copper Vapor
Reactive Ion Etching
Vacuum @ 10-5 torr
Thickness will match about
the length of Ka photon
mean free path ~ 30 mm
KOH Etching
~ 7 hours @ 60ºC
Metal Layer Coating
Low Z material (Ti, Cu)
Arrays of sharp tipped pyramid shaped guides
have been fabricated successfully
< 5 mm
500μm
50μm
2mm
500μm
We have achieved pyramid cones with tips
smaller than 1 mm
100μm
We have conducted a series of experiments on the
20 TW THOR Ti:sapphire laser at UT
5W Millenia
20 fs Ti:Sapphire
Oscillator
Single Grating
Stretcher
1 nJ
20fs
Regenerative amplifier (25 passes)
600 ps
2 mJ
4-pass amplifier
20 mJ
1.4J @ 10Hz YAG
40 mJ
150 mJ
0.2J
@
10Hz
YAG
5-pass amplifier
1.2 J
1.4J @ 10Hz YAG
40fs
to
Target
Characteristics
Pulse Width ~ 40 fs
Energy ~ 750 mJ
Intensity >1019 W/cm2
Repetition Rate 10Hz
Vacuum
Compressor
A 1D imaging spectrometer was employed to
measure the spatial extent of titanium Ka
Spherically Bent Mica Crystal
• Bragg reflection, 7th order
• Oriented to reflect Ka1 and Ka2
• Spatially images horizontal direction
magnets
• Demagnification of 1.15
target
Kodak RAR 2492 Film
• Directly exposed by Ti Ka x-rays
• High (5mm) resolution1
• Requires ~30 shots integrated
X-rays in the vicinity of Ka1 and Ka2 are focused by an offaxis spherically bent crystal which is fine-tuned to give spatial
resolution in 1 dimension
Spatial
Spectral
Ka
1
Ka2
A Von Hamos spectrometer measured yield of
Ti Kaand Kb
Cylindrically Bent PET Crystal
• Bragg reflection, 2nd order
• Oriented to detect 2.21Å - 2.79Å
• Line focus increases intensity of spectrum
Kodak RAR 2492 Film
• Integrate ~50 shots for good signal
Ka2 & Ka1
Kb
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.
A bent mica spectrometer was used to image the
Ti Ka in one dimension
R= 250 mm
7th order
Clear spatial side bands were observed on flat
Ti targets
Ka 1
Ka 2
y
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
l
Spatial sidebands are seen throughout the Ka line
Ka
1
l
~ 520 um
Side peaks from
fountain effect?
Mean Ka
Max Ka
Blue edge
-800 -600 -400 -200 0
200 400 600 800
Space dimentions, um
A well-imaged Ka line from flat targets showed
well-defined side peaks
480
520
560
600
640
680
720
760
800
240
280
320
360
400
440
Run 19
(false
color)
200
Ti Ka brightness [photons/µm2/shot @ film]
400
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≈ 125mm  H~20-30kG
• Electrons responsible for Ti K-shell ionization are
largely in the 10-100keV range
?

Ka1 spatial of flats: With best imaging conditions and
35 integrated shots, clear side peaks become visible for
11mm Ti foils shot at 0º.
Hot electrons
Cone targets exhibit lower yield but substantially
smaller source size when compared to flat Ti targets
0.2
0.4
0.6
0.8
1.0
1.2
1.4
mm
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
0
0.0
30
20
10
Direct brightness comparison of cones vs. 0º flats
-3
120
/shot on film]
110
100
90
80
70
60
50
40
30x10
20
520
540
560
580
600
620
640
660
680
700
720
740
210
220
230
240
250
260
270
280
290
300
310
320
330
500
Si cone with Ti fluor
layer target
Ti Ka1 brightness [ K a1 photons/µm
2
Flat Ti target
25
Flat Ti, 11µm
71º square cones
25µm Ti on back
15
10
5
0
-5
-2mm
-1
0
1
2
Spatial extent in horizontal (polarization) direction [mm]
The spatial extent of Ka from cone targets showed
no side peaks or plateau
Ti Ka brightness [photons/µm2/shot @ film]
Far-flung electrons
stopped by Si
Laser in
Laser in
Si
Ti
Ti
Ka1 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” >50mm 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
We investigated the role of polarization by elongating
the pyramid and comparing P and S polarization
Sharp end in polarization plane
Wide end in polarization plane
Investigation of polarization effects
Various lengths of grooves will be necessary
Anisotropic etching of Si is used to produce
shaped targets
point line
100µm
A wedge is a “1-D” cone
p s
Si
Ti
Flat, 0º
“p” and “s” refer to wedge orientation
relative to laser polarization
Ti
S-oriented wedges showed higher Ka yield than
p-oriented wedges, but less than flat foils
Photons/µm2/shot on film
X-ray yield from 25µm foil for flat, s-wedge, and p-wedge
Targets
Flat, 0º
S -wedge
0.15
P -wedge
0.10
0.05
50 shots
integrated
for each
target type
0
-0.05
2.73
2.74
2.75
2.76
2.77
Wavelength (Å)
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 spolarized wedges
We measured hot electron production with NaI
hard x-ray detectors
NaI detectors
THOR laser
B
f/2.8
Pb shielding
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Al, Cu and Pb
filters
NaI scintillator
crystal
Photomultiplier
tube tube
Filter
Cutoff
energy
32 mm Pb
600 keV
48 mm Pb
800 keV
95 mm Cu
1200 keV
Hard x-ray detectors show greater hot e- production
for P-pol shots than S-pol or flat targets
Hard X-ray yield from 25µm foil for flat, s-wedge,
and p-wedge Targets
Signal @ scope [nVs]
400
Flat, 0º
S -wedge
300
P -wedge
200
~50 shots
averaged for
each target
type
100
0
1
>0.6MeV
3.2cm Pb
2
Hard X-ray detector
3
>0.8MeV
4.8cm Pb
>1.2MeV
9.5cm Cu
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.
Near term plan will study sidebands (and lack of)
and hot electron temperature from cones
Future/current projects
• Further experiments and modeling of side peaks in spatial distribution of Ti
Ka 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 Ka1 & 2 as per results from recent experiment at COMET