1. UNCLASSIFIED Heat Transport Measurements in Foil Targets Irradiated with Picosecond Timescale Laser Pulses D. J. Hoarty 1, S F James 1, C R D Brown 1, R Shepherd 2, J. Dunn 2, M. Schneider 2, G. Brown 2, P. Beiersdorfer 2, H. Chen 2, A J Comley 1, J E Andrew 1, D. Chapman 1, N. Sircombe 1, R W Lee 3, H. K.Chung 4 1 AWE Plasma Physics Department, Building E1.1, Reading, Berkshire RG7 4PR, UK 2 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, USA 4 Department of Mechanical and Aerospace Engineering, University of California, San Diego, CA

2. UNCLASSIFIED 2 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.

3. UNCLASSIFIED 3 1.5µm CH 2µm CH 0.1µm mid Z/ low Z mixture. Laser X-ray spectrometer and ultra fast streak camera Two crystal, time-integrating spectrometer X-ray pinhole camera (Electron Spectrometer) Generic experimental diagnostic and target setup Time resolved and time-integrated diagnostics were fielded. Target mounting

4. UNCLASSIFIED 4 1.6 1.8 2.0 2.2 X-ray energy (keV) Intensity (arbitrary units) (a) Spectra with green light (b) Spectra with infrared light He-like n=2-1 H-like n=2-1 He-like n=3-1 H-like n=3-1 He-like n=2-1 H-like n=2-1 He-like n=3-1 H-like n=3-1 Electron heating of the target not effective in the presence of pre-pulse.

5. UNCLASSIFIED 5 2  conversion and plasma mirrors were used to mitigate pre- pulse 0.5 -16 µm CH 2µm CH 0.15µm Al 1  or 2  Plasma mirrors were used in 1.06µm wavelength experiments. Pre-pulse mitigation was important for effective heating

6. UNCLASSIFIED 6 IR +plasma mirror produces similar plasma conditions to using green light in experiments with aluminium buried layers IR+plasma mirrorGreen light Broad 1-3 line emission indicates densities ~ 1-2g/cc. Ly b /He b ratio indicates high Te ~500eV Plasma mirror and 2  results compared.

7. UNCLASSIFIED 7 Heat penetration was measured using aluminium layers buried in parylene N plastic foils. Al layer Depth, microns Depth, microns Temperature, eV 10 19 W/cm 2 P polarisation 2  10 19 W/cm 2 S polarisation 2  6x10 17 W/cm 2 P polarisation 2  6x10 17 W/cm 2 S polarisation 2  5x10 18 W/cm 2 P polarisation 1  plasma mirror Penetration with 0.5ps pulses Penetration with 2ps pulses 1.6x10 17 W/cm 2 S polarisation 2  1.6x10 17 W/cm 2 P polarisation 2  5x10 18 W/cm 2 S polarisation 2  5x10 18 W/cm 2 P polarisation 1  plasma mirror Detection threshold Detection threshold

8. UNCLASSIFIED 8 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 substrate 4, 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 Temperature, keV 0 1 2 3 4 5 6 7 Depth, microns Thermal conduction predicts a rapid fall off of temperature with depth Predicted heat front position v time assuming thermal diffusion of electrons into the target 0 10 20 30 40 50 Time, picoseconds 12 10 8 6 4 2 0 Heat front position (microns)

9. UNCLASSIFIED 9 Temperatures inferred from analysis of the Silicon emission are in reasonable agreement with those inferred from aluminium spectra. Si Ly α Si He α Al Ly  Si Ly α Si He α Al Ly  Si Ly α Si He α Al Ly  Si He α S-pol; 10 19 W/cm 2 ; 4  m separation P-pol; 10 19 W/cm 2 ; 15  m separation P-pol; 10 16 W/cm 2 ; 4  m separation P-pol; 5x10 17 W/cm 2 ; 10  m separation Electron transport experiments with multilayer targets

10. UNCLASSIFIED 10 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 target conductivity . jhjh jcjc Hot electron current “cold” electron return current Laser beam  Thor II electron transport model includes return current heating. Thor II predictions of target heating v experiment Dashed lines experiment; solid lines prediction

11. UNCLASSIFIED 11 The effect of conductivity change was studied using chlorinated plastic layer targets. Target 2µm PyN/1µm PyD/ 2µm PyN PyD –parylene D 50% by weight chlorine 41J, 0.5ps 2x10 19 W/cm 2.

12. UNCLASSIFIED 12 Chlorine 1s3p data indicate higher temperatures than Al, Si. 6µm PyN /1µm PyD/ 2µm PyN 20J 0.5ps ~1x10 19 W/cm 2 2µm PyN /1µm PyD/ 2µm PyN 41J 0.5ps ~2x10 19 W/cm 2 Data is normalised – not an absolute flux comparison. Intensity a.u.

13. UNCLASSIFIED 13 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. Intensity a.u.

14. UNCLASSIFIED 14 HELEN experiments have demonstrated the technique of long pulse shock compression and short pulse heating. Shock velocity ~3x10 6 cm/s Ablation rate ~6x10 5 cm/s Ablation pressure ~12Mb CPA 2w 0.5ps 1ns square Pulse 2w ~5x10 14 W/cm 2 14  m 10  m SP 30J 0.5ps LP 300J 1ns XRSS 1 XRSS 2 West beam Experimental setup schematic Density (g/cc) Time (nanoseconds) Temperature (keV) Predicted density and temperature histories for shock compression and short pulse heating.

15. UNCLASSIFIED 15 Shocked aluminium experiment streak data – Line broadening used to diagnose shock compression of an aluminium layer. X-ray energy Shock arrival time -100ps0 compression decompression (shock breakout) +50ps +150ps Pre-shock Energy eV Intensity a.u. Fits to peak compression indicate density 3g/cc and peak temperature 600 ±50eV. Ly  He 

16. UNCLASSIFIED 16 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 Emissivity J/keV/cm 2 /sr/s x10 12 Energy (eV) Experiment FLYCHK Non-LTE FLYCHK LTE ionization Temperature (eV)

17. UNCLASSIFIED 17 Experiments to measure the electron distribution Aluminium emission Thermal emission K α fluorescence 2PN/ 0.15Al/ 8PN / 15Sc/1PN frequency Film data Scandium  K  laser Emergent electron spectra for targets with different density scale-lengths Electron spectrometer Electron distribution inferred from fluorescence Electron spectrometer measurements

18. UNCLASSIFIED 18 FLYCHK calculations including background hot electrons show these generally have little effect on the spectrum. T hot ~200keV based on Beg scaling. HELEN K α measurements show conversion efficiency 10 -4 and imply f hot 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)

19. UNCLASSIFIED 19 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.

20. UNCLASSIFIED 20 Future work  Experiments proposed for the TITAN laser will, if approved, continue this work in the next year. In the longer term studies will continue on ORION.  It is proposed to better characterise the electron distribution using electron spectrometers and K  fluorescence and possibly He  emission.  It is proposed to investigate further the role of conductivity and to sample deeper buried layers using absorption spectroscopy.