Energy transport experiments on VULCAN PW Dr Kate Lancaster Central Laser Facility CCLRC Rutherford Appleton Laboratory

Acknowledgements K. L. Lancaster, P.A.Norreys, J. S. Green#, Gianlucca Gregori, R. Heathcote Central Laser Facility, CCLRC Rutherford Appleton Laboratory, UK. C. Gregory Department of Physics, University of York, Uk. K. Krushelnick #Blackett Laboratory, Imperial College, UK M. H. Key Lawrence Livermore National Laboratory, CA, USA * Also at University of California, Davis M. Nakatsustumi T. Yabuuchi H. Habara, M. Tampo, R. Kodama, Institute of Laser Engineering, Osaka University, Japan R.Stephens General Atomics, San Diego, CA, USA C. Stoeckl, W. Theobald, M. Storm Laboratory of Laser Energetics, University of Rochester, NY, USA R.R. Freeman, L. Van Workem, R. Weber, K. Highbarger, D. Clark, N. Patel Ohio State University, Columbus, Ohio, USA S. Chen, F. Beg University of California, San Diego

Overview Motivation for the work Experimental arrangements and diagnostics XUV imaging data Shadowgraphs Al Spectroscopy data Atomic Kinetic code modelling and results Vlasov-Fokker-Plank modelling and results Conclusions

Purpose of work Hot electrons Cone / Shell Ultra intense laser Hot electrons are generated when an ultra intense laser is focused into the gold cone. Goal is to investigate how energy is transported to the compressed deuterium fuel via the hot electrons and ions.

Experimental setup 2w probe system 256 eV XUV multilayer mirror Parabola 2w probe system X-ray crystal spectrometer Targets: CH-Al-CH targets with and without CH 40 o flare angel cone Laser: 300J, 1ps, =1.05  m I=5x10 20 Wcm -2 Assuming 30% energy contained in 7  m spot.

XUV imaging Target Multilayer mirror 28 o Large area CCD A Spherical multilayer mirror images rear surface emission on to a Princeton Instruments large area 16 bit CCD camera.

Aluminium x-ray spectroscopy Target Hall configuration conical crystal spectrometer CsAP conically curved crystal – range 6.2 – 8.4 A Detector – Fuji-film BAS image plate with Be Filter Crystal centre source Detector plane Centre of crystal 12.5cm Central radius

Transverse optical probe Part of the main beam was frequency doubled laser and used to probe the interaction in the transverse direction. This was split and used as dual probe system to allow probing at 0 and 40 degrees Scattered and collimated light imaged on to 16 bit Andor CCD camera

256eV XUV images Average FWHM – 69  mAverage FWHM – 38  m No coneCone

Shadowgraphs of rear surface CH-Al-CH (  m), no cone, t psCH-Al-CH (  m), CH cone, t ps 85  m 370  m Shadowgraph of slab without cone geometry shows regular expansion pattern of transverse size 370  m. Shadowgraph of slab with cone geometry shows a smaller transverse region of expansion of size 85  m although longitudinal extent is approximately the same. No cone Cone

Discussion of cone geometry Including cone geometry changes the transport pattern somewhat in both shape and lateral extent The extra density of the cone wall that the lateral fast electrons travel through should not effect the rear expansion much There may therefore be fields due to the cone geometry which act to confine the energy at the cone tip Focusing effects were reported by Sentoku et al where quasi- static magnetic and electrostatic sheath fields guide electron flow

Aluminium spectra Ly  He  From the spectra the Lyman a line drops with the addition of a cone This suggests the temperature of the Al layer falls in this situation

Modelling of spectra The synthetic spectra for single temperatures and densities were generated using a code that combines collisional radiative atomic kinetics with spectroscopic quality radiation transport and stark broadening effects* * U. Andiel et al, Europhysics letters T = 610 eV, n=10 24 el/cccone No coneT = 790 eV, n=7x10 23 el/cc Under these conditions the code failed to reproduce the line profiles of the He  and He  lines

Revised atomic model To try to reproduce the He  and He  lines it was necessary to implement new physics in the collisional radiative atomic kinetics code Effects of Li-like Hollow atom states Non-thermal electron distributions Atomic structure and processes calculated using Flexible Atomic Code (FAC)* It is proposed that non-thermal electron distributions in combination with hollow atom states may act as a conduit to enhanced He  and He  lines * M. F. Gu, Astrophysical Journal

Distribution of return current may be non- Maxwellian The best fit to the spectra was produced when a two temperature electron distribution was used with T c =100 eV and T H =800ev (where 40% of the population was at T H ).

KALOS simulations In order to examine the distribution of electrons in the return current modeling was performed with KALOS KALOS was in this case a1D 2P relativistic Vlasov- Fokker-Planck code (for details see A.R.Bell et al PPCF R37). Simulation conditions Fast electron generation consistent with an intensity – 3.5 x Wcm -2 in 700fs Reflective rear boundary Fast electron distribution – relativistic maxwellian Fully ionised slab at 100ev initial temp

KALOS results The buried Al layer is raised to a temperature of 720 eV, in agreement with the experimental result The return current departs from a Spitzer description at the edges of the buried layer This is due to non-Maxwellian component in the return current This may help to explain the enhanced He  and He  emission Dotted line – without enhanced n e Solid line – with enhanced n e

Conclusions Experiments were performed using buried CH-Al-CH slabs with and without CH cone geometry XUV images and Shadowgraphs reveal that the transport pattern changes between the two geometries from a ring structure with no cone to a smaller solid emission region with a cone. This may be due to self generated fields causing the electrons to concentrate at the cone tip Al spectroscopy of the buried layer reveals a slight drop in temperature in going from no-cone geometry (790 eV) to cone geometry (610 eV) Enhanced He  and He  emission suggest that new physics must be considered when modelling PW laser interactions such as non- maxwellian return currents and hollow atom states. A VFP code shows that the buried layer causes a departure from Spitzer behaviour at the layer edges that is due to a non-maxwellian component of the return current.