Lawrence Livermore National Laboratory Pravesh Patel 10th Intl. Workshop on Fast Ignition of Fusion Targets June 9-13, 2008, Hersonissos, Crete Experimental

Lawrence Livermore National Laboratory

Pravesh Patel10th Intl. Workshop on Fast Ignition of Fusion Targets

June 9-13, 2008, Hersonissos, Crete

Experimental measurements of electron energy spectra at FI-relevant intensities

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

2Lawrence Livermore National Laboratory

Acknowledgements

T. Bartal, T. Ma, J. King, M.S. Wei, F. Beg

K. Akli, R. Stephens

T. Link, L. Van Woerkom, R.R. Freeman,D. Offermann, V. Ovchinnikov

C. Chen, M. Porkolab, MIT, Cambridge, MA

Y.Y. Tsui, University of Alberta, Canada

D. Hey, A.J. Mackinnon, A. MacPhee, S. Le Pape, H. Chen,A.J. Kemp, M.H. Key, M. Tabak, R. Town, E. Storm


The mean energy of electrons produced by the ignitor laser is a critical parameter in fast ignition

We typically use ponderomotive scaling for the mean electron energy

Recent work (Sentoku, Kemp, Chrisman) indicates that steepening of the plasma density gradient will reduce Thot by a factor (nc/np)

Degree of steepening dependent on a combination of laser intensity, pulselength, and pre-plasma

We have performed a study of Thot as a function of intensity and pre-pulse

€

Thot = 1+Iλ2

1.37 ×1018−1

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟× 0.511 MeV

A. Kemp

100fs 200fs 300fs

Thot

421 MeV


Experimental setup on the TITAN laser

150J in 0.6ps, 5µm FWHM focal spot Intrinsic pre-pulse measured at ~10mJ (104 energy, 108 intensity contrast)

Laser

10µm Al 25µm Cu 1000µm Al

Electron spectrometer

Bremsstrahlung spectrometer

Absolute K-alpha yield

Intensity scan by varying laser energy (constant pulselength, spot size) Pre-pulse scan by adding a 3ns long-pulse with varying energy

16˚S-pol


Focal spot is measured at low power OPCPA at target chamber center

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15%

50%

150J, 0.6ps: 15% energy in 5µm FWHM spot 2x1020 W/cm2

150J, 0.6ps: 50% energy in 15µm diameter 7x1019 W/cm2

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Electron spectrum measured with vacuum electron spectrometer (1-100 MeV)

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Electron energy (MeV)

121 J shot Multi-temperature

distribution


Electron spectrum measured with vacuum electron spectrometer

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1.1 MeV

13 MeV

121 J shot Multi-temperature

distribution

Peak intensity of 1.5x1020 W/cm2 Thot~5 MeV


Escaping electron spectrum shows strong dependence on pre-pulse level

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10mJ (intrinsic)41 mJ110 mJ


High energy component (>5 MeV) increases with pre-pulse, consistent with acceleration from under-dense plasma [NOT ponderomotive acc.]

Low energy component (1-5 MeV) is relatively insensitive to pre-pulse (may be consistent with ponderomotive acc.) [caveat: only small frac. of electrons escape target –spectrum will be modified by target potentials]


Electron spectrum inside target can be inferred from bremsstrahlung measurements




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Monte Carlo code ITS is used to compute response of target and detector to trial electron source function

Targetresponse

Detectorresponse

10keV 500keV

0.001 0.01 0.1 1 10 100

Photon energy (MeV)


Bremsstrahlung data is generally well reproduced by a 1-Temp exponential distribution

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10

100

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Data1.0 MeV1.4 MeV1.7 MeV

Layer

Least squares fit gives unique solution for 1-T fits

Error on Thot ~ 0.2 MeV

Caveat: 2-T fits with cold and hot components can also reproduce the data


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Intensity (W/cm2)

Thot data as a function of laser intensity



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Intensity (W/cm2)

Ponderomotive scaling



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1018 1019 1020 1021

Intensity (W/cm2)



Focal spot is measured at low power OPCPA at target chamber center

0.0

0.1

0.2

0.3

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0.5

0.6

0.7

0.8

0.9

1.0

-25 -20 -15 -10 -5 0 5 10 15 20 25Radius (microns)

15%

50%

150J, 0.6ps: 15% energy in 5µm FWHM spot 2x1020 W/cm2

150J, 0.6ps: 50% energy in 15µm diameter 7x1019 W/cm2




Thot data as a function of laser intensity using mean laser intensity for 50% encircled energy

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Intensity (W/cm2)



What intensity should we use?

We should start with the full energy on-shot focal intensity distribution (full energy spot may be larger than measurement due to thermal distortions in amplifiers and non-linear effects)

Focal spot at TCCEquivalent plane measurement


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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Power fraction

Full energy focal spot intensity distribution

Low power

Full energyQuickTime™ and aTIFF (Uncompressed) decompressor




OPCPA

Full shot

Focal spot does not degrade at full energy (with adaptive optic)


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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Power fraction

Full energy focal spot intensity distribution

Low power

Full energyQuickTime™ and aTIFF (Uncompressed) decompressor




OPCPA

Full shot

Focal spot does not degrade at full energy (with adaptive optic)

Distribution of intensities distribution of Thot s

1 MeV

4 MeV

2 MeV

.5 MeV

3 MeV

6 MeV

Thot


Electron spectrum calculated from measured focal spot using ponderomotive scaling

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We can’t describe the electron energy distribution as a single temperature, or by a single mean energy

Actual electron spectrum may be better described as a ‘sum of exponentials’

kT~1 MeV

kT~2 MeV

kT~4 MeV

Sum of electronspectra


We can perform forward calculation to test ponderomotive scaling theory against exp data

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Exp data


Layer



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Focal spot

Power distribution

Synthesized electron spectrum

ITS brems calculation

Experimental spectrum is cooler, or lower temperature, than that calculated from pondoromotive scaling we may be observing some degree of density steepening

Caveat: Monte Carlo modeling of electrons in target neglects field effects


Summary

Escaping electron spectra exhibit multi-temp distributions with a very hot component, kT>10 MeV, consistent with under-dense instabilities (NOT ponderomotive), and a lower temp component, kT~1 MeV; since escaping electrons are affected by potentials it’s questionable how to interpret this

Electron temperature, Thot, calculated from bremsstrahlung ranges from 0.1–1.2 MeV, many times lower than ponderomotive would predict using standard definition of focal spot intensity

Using the full focal intensity distribution demonstrates that there are effectively multiple hot electron temperatures, not a single Thot

Forward calculation suggests that we still observe slightly lower temperatures than standard ponderomotive scaling: may mean that we have some degree of density profile steepening occurring

Caveat: We still have some model dependency, primarily through using ITS, which neglects field effects – we will repeat with hybrid modeling

Documents

Lawrence Livermore National Laboratory Pravesh Patel 10th Intl. Workshop on Fast Ignition of Fusion Targets June 9-13, 2008, Hersonissos, Crete Experimental