hui chen yuan ping, ronnie shepherd, jim dunn- llnl ; dustin offermann, anthony link,

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

2Option:UCRL# Option:Additional Information


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



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



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



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


Shot5_espec2_calibed Shot4_espec2_calibed Shot3_espec2_calibed

0.5 MeV

0.6 MeV

Back normal

0.1

1

10

100


Shot5_espec8_calibed Shot3_espec8_calibed

0.8 MeV

~Laser direction



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



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



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



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



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

hui chen yuan ping, ronnie shepherd, jim dunn- llnl ; dustin offermann, anthony link,

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