heat transport measurements in foil targets irradiated with picosecond timescale laser pulses
Post on 18-Mar-2016
64 Views
Preview:
DESCRIPTION
TRANSCRIPT
UNCLASSIFIED
UNCLASSIFIED
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
2UNCLASSIFIED
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.
3UNCLASSIFIED
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
4UNCLASSIFIED
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
(arb
itrar
y un
its)
(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.
5UNCLASSIFIED
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
6UNCLASSIFIED
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.
7UNCLASSIFIED
700
600
500
400
300
200
100
0151050
700
600
500
400
300
200
100
01614121086420
Heat penetration was measured using aluminium layers buried in parylene N plastic foils.
Al layer Depth, microns Depth, microns
Tem
pera
ture
, eV
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
8UNCLASSIFIED
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 7Depth, 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 (m
icro
ns)
9UNCLASSIFIED
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
10UNCLASSIFIED
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
11UNCLASSIFIED
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.
12UNCLASSIFIED
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
.
13UNCLASSIFIED
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
.
14UNCLASSIFIED
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
Den
sity
(g/c
c)
Time (nanoseconds)
Time (nanoseconds)
Tem
pera
ture
(keV
)
Predicted density and temperature histories for shock compression and short pulse heating.
15UNCLASSIFIED
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
16UNCLASSIFIED
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 /s
r/s x
1012
Energy (eV)
ExperimentFLYCHK Non-LTEFLYCHK LTE
ioni
zatio
n
Temperature (eV)
17UNCLASSIFIED
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
18UNCLASSIFIED
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)
19UNCLASSIFIED
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.
20UNCLASSIFIED
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.
top related