lawrence livermore national laboratory hui chen yuan ping, ronnie shepherd, jim dunn- llnl; dustin...
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Lawrence Livermore National Laboratory
Hui Chen
Yuan Ping, Ronnie Shepherd, Jim Dunn- LLNL; Dustin Offermann, Anthony Link,
Linn Van Woerkom - OSU; James King, Farhat Beg - UCSD; Cliff Chen - MIT;
Lee Elberson, Windell Hill -U. Maryland
Modeling by Andreas Kemp and Scott C. Wilks LLNL
Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344
Angular distribution of fast electrons and
protons in short pulse laser target interaction
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Lawrence Livermore National Laboratory
Angular distribution of electrons and protons measurements are important for understanding the laser target interaction
Highly non-uniform distributions of hot electrons have been found by several groups
The group of Zhang et al. (2000-06) looked at hot electrons at intensities up to mid-1018 W/cm2
Hot electron measurements often have been at single position/angle relative to target
Proton measurements have concentratedon the back of the target where they are mostenergetic and beam-like
Chen et al. APS DPP 2006
We will attempt to answer the following questions with detailed experiments:
1. What is the correlation between the electron dose and its distribution for all angles?
2. What is the correlation between the spatially resolved hot electrons and the protons?
Malka and Miquel
(1996) did first Thot at three angles at upper -1018 W/cm2
€
kThot ~ mec2 1+
Iλ2
2.8×1018−1
⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥
Electrons, 2D PIC, Wilks
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The measurements were performed on the Callisto laser with peak intensity of 3x1018 to 6x1019W/cm2
Chamber center photo
Lasers: E=1 - 10 J, 130 fs, FWHM focus~5um
Diagnostics: Proton ring with radiochromic film (RCF) Two Proton spectrometers using image plates
Electron spectrometers using image plate(4 - 7) Ultra-thin thermoluminescence dosimeters TLDs (20 - 40)
Targets: 1 to 50 um thick foils of two sizes:
A) 2 x 10 mm2 flat foils of CH, Al, Cu, Ag
B) ~ 0.2 x0.2 mm2 foil reduced mass targets of Cu
Mounted flag-style on glass fiber
Exp. setup
P-spec
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0 10000 20000 30000 40000 50000
10000
20000
30000
40000
50000
0
45
90
135
180
225
270
315
shot7-2um Al (0.5 mm Al filter) shot7-2um Al ( 2 mm Al filter)
Full angular coverage of TLDs was complemented by calibrated E-spectrometers at several positions
TLDs: electron dose for E>350 keV (red) electron dose for E>1 MeV (blue)
Especs: spectra for 0.1 - 4.2 MeV at many
TLD: ~6e-7 J for E>350 keV
E-spec: 2e-6 J for E>100 keVQuantitatively consistent
diagnostics
4x10-2
3
2
1
0
0.1 1 10 100
Electron energy (MeV)
Averaged Results_Tanaka
Tanaka et al, 2004
Chen et al, 2007
Laser
E > 350 KeV
E > 1 MeV
Abs Calibration
E-spec #2
Dose
Angle
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Electron distributions and temperatures from two laser intensities for the same target condition
3x1018 W/cm2
3x1019 W/cm2
0.1
2
4
1
2
4
10
2
4
100
40003000200010000Electron energy (keV)
Shot5_espec3_calibed Shot3_espec3_calibed
0.5 MeV
0.7 MeV
Target front
0.1
2
4
1
2
4
10
2
4
100
40003000200010000Electron energy (keV)
Shot5_espec2_calibed Shot4_espec2_calibed Shot3_espec2_calibed
0.5 MeV
0.6 MeV
Back normal
0.1
1
10
100
40003000200010000Electron energy (keV)
Shot5_espec8_calibed Shot3_espec8_calibed
0.8 MeV
~Laser direction
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2D PIC modeling shows hot electron behavior similar to that observed in the experiments
Electrons from back are hotter than Those from front of target
2D PSC modeling by Andreas Kemp
Black:all particles
x (c/0)
Under-dense Over-dense
5e19 W/cm2
Forward direction
Electrons are accelerated mostly In the forward direction
Laser
5e19 W/cm2
Forward direction
Electrons are accelerated mostly In the forward direction
Electron temperatures are higher at the back of the target than at the front of the targets
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7
6
5
4
3
2
x106
2500200015001000500
The proton spatial distribution was recorded by the RCF ring (E>1.3 MeV) and proton spectrometers (0.1 - 4 MeV)
Back NormalTarget edgeTarget edge Front normal
Target back normal
Target front normal
Spectra from 2 proton spectrometers at front and back of target normal positions
LaserTarget
The Ring
Film label
Noise level
Pro
ton
dos
e
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The proton distributions for large and small targets are dramatically different, so are the proton doses
0 3e06 6e06 9e06 1.2e07 1.5e07
3e06
6e06
9e06
1.2e07
1.5e07
0
45
90
135
180
225
270
315
Beam like protons from large foils of metal and CH
10 um Ag, CH
2x10 mm2
5e19 W/cm2
0 3e06 6e06 9e06 1.2e071.5e071.8e07
3e06
6e06
9e06
1.2e07
1.5e07
1.8e07
0
45
90
135
180
225
270
315
Broader proton distributions in reduced mass targets
10 um Cu
0.2 mm2
5e19 W/cm2
1e19 W/cm2
More than a factor of 2 higher conversion from laser to
protons are found in reduced mass target
0 3000 6000 9000 12000 15000
3000
6000
9000
12000
15000
0
45
90
135
180
225
270
315
polarAutoscaleTrace Shot60_reduced mass target, 10 um Cu. 8.8J Shot30_large target, 10 um Cu. 5.3JElectrons: 5e19 W/cm2
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2D PSC modeling by Andreas Kemp
Reduced Mass Targets accelerate protons not only from front and back, but all sides of
target, due to near equal electric field strengths everywhere around the target.
The near isotropic proton acceleration in reduced mass targets can be explained by their unique E-fields
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Conclusions
Measurements were performed on the Callisto laser with a peak
intensity of 3x1018 to 6x1019W/cm2 with full angular coverage using
Multiple charged particle detectors.
We found:
1. The electron angular distributions are highly anisotropic.
2. Electron temperatures in the forward directions are hotter than the backward direction.
3. The proton distributions for reduced mass targets tend to be more isotropic than distributions from large targets. Less difference was observed for hot electrons for these two target types.
4. The 2D collisional PIC simulations of the electric field for reduced mass targets agree with observations