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Heat Transport Measurements in Foil Targets Irradiated with Picosecond Timescale Laser Pulses

D. J. Hoarty1, S F James1, C R D Brown1, R Shepherd2, J. Dunn2, M. Schneider2, G. Brown2, P. Beiersdorfer2, H. Chen2, A J Comley1, J E Andrew1, D. Chapman1, N. Sircombe1, R W Lee3, H. K.Chung4

1 AWE Plasma Physics Department, Building E1.1, Reading, Berkshire RG7 4PR, UK2 Lawrence Livermore National Laboratory, P. O. Box 808, L-399, Livermore, CA 94550 USA 3 Department of Applied Science, University of California, Davis, Davis, California 95616, USA4 Department of Mechanical and Aerospace Engineering, University of California, San Diego, CA

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Background

• High temperature and high density opacity experiments have been performed on the HELEN CPA laser using short pulse driven electron transport to heat buried layers in plastic foils but the details of the heating mechanism are not understood.

• A number of measurements using X-ray spectroscopy and electron spectrometers have been carried out to try to better understand the heating mechanisms and to benchmark electron transport codes under development at AWE.

• The time delay of heating at different depths in plastic foils has been investigated using X-ray spectroscopy and an ultra-fast streak camera.

• The effect of target resistivity and refluxing of electrons has been investigated.

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1.5µm CH2µm CH

0.1µm mid Z/ low Zmixture.

Laser

X-ray spectrometerand ultra fast streak camera

Two crystal,time-integratingspectrometer

X-ray pinhole camera

(ElectronSpectrometer)

Generic experimental diagnostic and target setup

Time resolved and time-integrateddiagnostics were fielded.

Target mounting

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1.0

0.8

0.6

0.4

0.2

0.02200200018001600

0.30

0.25

0.20

0.15

0.10

0.05

0.002.22.01.81.6

1.6 1.8 2.0 2.21.6 1.8 2.0 2.2

X-ray energy (keV) X-ray energy (keV)

Inte

nsity

(ar

bitr

ary

units

)

(a) Spectra with green light (b) Spectra with infrared light

He-liken=2-1

H-liken=2-1

He-liken=3-1

H-liken=3-1

He-liken=2-1

H-liken=2-1

He-liken=3-1 H-like

n=3-1

Electron heating of the target not effective in the presence of pre-pulse.

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2 conversion and plasma mirrors were used to mitigate pre-pulse

0.5 -16 µm CH2µm CH

0.15µm Al

1 or 2

•Plasma mirrors were used in 1.06µm wavelength experiments.

Pre-pulse mitigation was important for effective heating

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2.0

1.5

1.0

0.5

0.02.01.81.61.4

1.0

0.8

0.6

0.4

0.2

0.02200200018001600

IR +plasma mirror produces similar plasma conditions to using green light in experiments with aluminium buried layers

IR+plasma mirror Green light

Broad 1-3 line emission indicates densities ~ 1-2g/cc.Lyb/Heb ratio indicates high Te ~500eV

Plasma mirror and 2 results compared.

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700

600

500

400

300

200

100

0

151050

700

600

500

400

300

200

100

0

1614121086420

Heat penetration was measured using aluminium layers buried in parylene N plastic foils.

Al layer Depth, microns Depth, microns

Te

mp

era

ture

, e

V

1019W/cm2 P polarisation 2 1019W/cm2 S polarisation 26x1017W/cm2 P polarisation 26x1017W/cm2 S polarisation 2 5x1018W/cm2 P polarisation 1plasma mirror

Penetration with 0.5ps pulses Penetration with 2ps pulses

1.6x1017 W/cm2 S polarisation 2 1.6x1017 W/cm2 P polarisation 2 5x1018 W/cm2 S polarisation 2 5x1018W/cm2 P polarisation 1plasma mirror

Detection thresholdDetection threshold

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Time delay in the emission of the two layers expected for

• (a) Thermal heat conduction

• (b) Coronal radiative heating enhancing the thermal conduction

• Alternative/ additional heating mechanisms

• (c) Return currents

• (d) electron refluxing

Polypropylene substrate4, 10, 15 µm

Buried layer A

Buried layer B

laser

Heating studies using multilayer targets

CH 1 µm

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Tem

pera

ture

, keV

0 1 2 3 4 5 6 7

Depth, microns

Thermal conduction predicts a rapid fall offof temperature with depth

Predicted heat front position v time assuming thermal diffusion of electrons into the target

0 10 20 30 40 50Time, picoseconds

12

10

8

6

4

2

0

Hea

t fro

nt p

ositi

on (

mic

rons

)

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Temperatures inferred from analysis of theSilicon emission are in reasonable agreement with those inferred from aluminium spectra.

Si Lyα

Si Heα

Al LyAl Ly

Si Lyα

Si Heα

Al Ly

Si Lyα

Si Heα

Al Ly

Si Heα

S-pol; 1019 W/cm2 ; 4m separation P-pol; 1019 W/cm2 ; 15m separation

P-pol; 1016 W/cm2 ; 4m separationP-pol; 5x1017 W/cm2 ; 10m separation

Electron transport experiments with multilayer targets

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Electron transport models are being developed and will be incorporated into an AWE radiation-hydrodynamics code.

The target heating depends on return currents and the targetconductivity .

jh

jc

Hot electroncurrent

“cold” electronreturn current

Ejj hc .

Laserbeam

Thor II electron transport model includes return current heating.

Thor II predictions of target heating v experimentDashed lines experiment; solid lines prediction

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4.0

3.0

2.0

1.0

0.0

3800360034003200300028002600Energy (eV)

He

Ly

Ly

He

1.6

1.2

0.8

0.4

0.0

3800370036003500340033003200Energy (eV)

He Ly

HeHe

The effect of conductivity change was studied using chlorinated plastic layer targets.

Target 2µm PyN/1µm PyD/ 2µm PyNPyD –parylene D 50% by weight chlorine41J, 0.5ps 2x1019W/cm2.

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Chlorine 1s3p data indicate higher temperatures than Al, Si.

6µm PyN /1µm PyD/ 2µm PyN

20J 0.5ps ~1x1019W/cm2

2µm PyN /1µm PyD/ 2µm PyN

41J 0.5ps ~2x1019W/cm2

1.0

0.8

0.6

0.4

0.2

0.0

3800370036003500340033003200Photon Energy (eV)

Shot 5800 FLY NLTE 1keV 1g/cc

1.0

0.8

0.6

0.4

0.2

0.0

3800370036003500340033003200Photon energy (eV)

Shot 5799 FLY NLTE SS 1keV 1g/cc

Data is normalised – not an absolute flux comparison.

Inte

nsity

a.u

.

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Low and high spectral resolution data agree.

Although resolution is low (E/dE~300), the temperature inferred from the CsAP crystal spectrum agrees with that from the PET crystal.

0.3

0.2

0.1

0.0

-0.1

360035003400330032003100Photon Energy (eV)

CsAP Data 0.9keV 1g/cc

0.3

0.2

0.1

0.0

-0.1

360035003400330032003100Photon energy (eV)

CsAP Data 1keV 1g/cc

Inte

nsity

a.u

.

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HELEN experiments have demonstrated the technique of long pulse shock compression and short pulse heating.

Shock velocity ~3x106 cm/sAblation rate ~6x105 cm/s Ablation pressure ~12Mb

CPA 2w 0.5ps

1ns squarePulse 2w~5x1014W/cm2

14m10m

SP 30J 0.5psLP 300J 1ns

XRSS 1

XRSS 2

Westbeam

Experimental setup schematic

De

nsi

ty (

g/c

c)

Time (nanoseconds)

Time (nanoseconds)

Te

mp

era

ture

(ke

V)

Predicted density and temperature histories for shock compression and short pulse heating.

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Shocked aluminium experiment streak data – Line broadening used to diagnose shock compression of an aluminium layer.

X-r

ay e

nerg

y

Shock arrival

time-100ps 0

compressiondecompression(shock breakout)

+50ps +150ps

Pre-shock

2000

1500

1000

500

02200210020001900

Energy eV

Inte

nsity

a.u

.

Fits to peak compression indicate density 3g/ccand peak temperature 600 ±50eV.

Ly

He

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Comparison of LTE and non-LTE opacity code predictions to the shocked germanium data.

The shocked germanium samples are nearer LTE but gradients are an issue.

Gradients from radiation-hydrodynamics 450-650eV, 4g/cc

Ge 4g/cc

LTE

nonLTE

- LTE- Non-LTE

Em

issi

vity

J/k

eV/c

m2 /

sr/s

x10

12

Energy (eV)

ExperimentFLYCHK Non-LTEFLYCHK LTE

ioni

zatio

n

Temperature (eV)

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Experiments to measure the electron distribution

Aluminium emission

Thermalemission

Kα fluorescence

2PN/ 0.15Al/ 8PN / 15Sc/1PN

frequency

Film dataScandiumK

laserEmergent electron spectra for targetswith different density scale-lengths

Electron spectrometer

Electron distribution inferred from fluorescence Electron spectrometer measurements

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FLYCHK calculations including background hot electrons show these generally have little effect on the spectrum.

e

hothot n

nf

• Thot ~200keV based on Beg scaling.

• HELEN Kα measurements show conversion efficiency 10-4 and imply fhot closer to 1% than 5%.

• Possible effect on Ti spectrum. Ly/He ratio not a good temperature diagnostic.

• Al Ly/He ratio not affected.

Energy (eV) Energy (eV)

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Summary/conclusions

Electron transport experiments using buried layer targets have shown that target heating using ultra-short pulse lasers is not due to thermal conduction.

Near instantaneous heating through up to 15µm of plastic is consistent with the Thor2 model of hot electron collisional heating and Ohmic heating via a thermal return current, with Ohmic heating the dominant mechanism.

Initial experiments changing the target conductivity show an increased heating for insulator rather than metal buried layers.

Experimental measurements have begun to better characterise the electron distribution in the target.

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Future work

Experiments proposed for the TITAN laser will, if approved, continuethis work in the next year. In the longer term studies will continue on ORION.

It is proposed to better characterise the electron distribution using electronspectrometers and K fluorescence and possibly Heemission.

It is proposed to investigate further the role of conductivity and to sample deeper buried layers using absorption spectroscopy.