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. HeathcoteCentral Laser Facility, CCLRC Rutherford Appleton Laboratory, UK.

C. GregoryDepartment of Physics, University of York, Uk.

K. Krushelnick#Blackett Laboratory, Imperial College, UK

M. H. KeyLawrence 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.StephensGeneral Atomics, San Diego, CA, USA

C. Stoeckl, W. Theobald, M. StormLaboratory of Laser Energetics, University of Rochester, NY, USA

R.R. Freeman, L. Van Workem, R. Weber, K. Highbarger, D. Clark, N. PatelOhio State University, Columbus, Ohio, USA

S. Chen, F. BegUniversity 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 XUVmultilayer mirror

Parabola

2w probe system

X-ray crystal spectrometer

Targets:

CH-Al-CH targets with and without CH 40o flare angel cone

Laser:

300J, 1ps, =1.05m

I=5x1020 Wcm-2 Assuming 30% energy contained in 7m spot.

XUV imaging

Target

Multilayer mirror

28o

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

sourceDetector plane

Centre of crystal

12.5cm12.5cm

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 m Average FWHM – 38 m

No cone Cone

Shadowgraphs of rear surface

CH-Al-CH (4-0.2-4m), no cone, t0+ 400ps CH-Al-CH (4-0.2-4m), CH cone, t0 + 400ps

85m

370m

Shadowgraph of slab without cone geometry shows regular expansion pattern of transverse size 370m. Shadowgraph of slab with cone geometry shows a smaller transverse region of expansion of size 85m 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 60 861 2002

T = 610 eV, n=1024 el/cccone No cone T = 790 eV, n=7x1023 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 582 1241 2003

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 Tc=100 eV and TH=800ev (where 40% of the population was at TH).

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 48 2006 R37).

Simulation conditions

•Fast electron generation consistent with an intensity – 3.5 x 1020 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 ne

Solid line – with enhanced ne

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.