ReferencesINSTITUTE OF PHYSICS PUBLISHING PLASMA PHYSICS AND CONTROLLED FUSIONPlasma Phys. Control. Fusion 48 (2006) L11–L22 doi:10.1088/0741-3335/48/2/L01LETTER TO THE EDITORObservation of annular electron beam transport inmulti-TeraWatt laser-solid interactionsP A Norreys1,JSGreen1,2,JRDavies3, M Tatarakis4, E L Clark5,F N Beg6, A E Dangor2, K L Lancaster1,MSWei2, M Zepf7andK Krushelnick21Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK2Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BZ, UK3GoLP, Instituto Superior T´ecnico, Lisbon, Portugal4Technological Educational Institute of Crete, Department of Electronics, 73133 Chania, Crete,Greece5Plasma Physics Division, AWE plc., Aldermaston RG7 4PR, UK6Department of Mechanical and Aerospace Engineering, University of California San Diego,San Diego, 9500 Gilman Drive #0411, La Jolla, CA 92093-0411, USA7Department of Pure & Applied Physics, Queens University Belfast, Belfast BT7 1NN, UKReceived 19 July 2005, in final form 8 November 2005Published 4 January 2006Online atstacks.iop.org/PPCF/48/L11AbstractElectron energy transport experiments conducted on the Vulcan 100 TW laserfacility with large area foil targets are described. For plastic targets it is shown,by the plasma expansion observed in shadowgrams taken after the interaction,that there is a transition between the collimated electron flow previouslyreported at the 10 TW power level to an annular electron flow pattern witha20◦divergence angle for peak powers of 68 TW. Intermediate powers showthat both the central collimated flow pattern and the surrounding annular-shapedheated region can co-exist. The measurements are consistent with the Daviesrigid beam model for fast electron flow (Davies 2003 Phys. Rev. E 68 056404)and LSP modelling provides additional insight into the observed results.(Some figures in this article are in colour only in the electronic version)The study of fast electron energy transport with current densities in the range 1012Acm−2is a fascinating subject in its own right and one that also has important implications forapplications of ultra-high intensity lasers, for example, in the optimization of proton andheavy ion acceleration processes [1], hot spark formation in fast ignition [2] and in denseplasma radiography for inertial confinement fusion [3].This field of study has grown rapidly since the first measurements of energy transport ofelectron beams generated by multi-TW laser pulses showing collimated electron flow patternsin both deuterated plastic targets [4] and large area glass slabs [5]. Gremillet et al, for example,have used glass slab targets and observed both filamented jets moving into the target with avelocity close to c followed by a slower moving hemispherical ionization front (at c/2) [6].0741-3335/06/020011+12$30.00 © 2006 IOP Publishing Ltd Printed in the UK L11L12 Letter to the EditorIt was concluded that the energy within the jet was quite small, otherwise the transverse probeused to observe the effect would be inaccessible to the ionized region around it, and that the bulkof the energy was within the ionized ‘cloud’ region. Since then, various degrees of electronbeam divergence have been reported in the literature. Optical images of the rear surface ofaluminium targets have shown a divergence pattern of 34◦from optical transition radiation and25◦from Planckian thermal radiation using the LULI and GEKKO XII 100TW laser facilities,respectively [7, 8].Koch et al measured the background electron temperature by K-shell resonance x-rayspectroscopy from buried Al layers using the NOVA PW laser facility and inferred a rapidexpansion of the heated region to ∼100 µm diameter within 30 µm inside a plastic target [9].X-ray pinhole camera images taken at the same time also revealed an annular distributionwith an azimuthal structure in the heating pattern. Evans et al also measured the backgroundtemperature of buried Al layers in plastic targets using K-shell emission spectroscopy on theVulcan PW laser facility under somewhat higher irradiation conditions [10]. When resistivitymismatching between the Al and plastic layers was included in hybrid computer code simula-tions, it was concluded that there was a barrier on the front surface of the target that preventedfast electron penetration into the target and resulted in reduced background electron heating.In experiments at the Vulcan 100 TW facility, a spherically bent Bragg crystalmonochromatic two-dimensional (2D) x-ray imaging technique was used to record the originof Kαphotons created in a 20 µm thick buried Ti or Cu fluor layer in a planar Al or CHtarget. The photons were emitted following K-shell ionization by fast electrons. The x-rayswere sufficiently energetic to escape through the surrounding target material, permitting theobservation of the transverse dimension of the laser-generated electron beam with a resolutionat the target of about 12 µm and depths inside the target up to several hundred microns. Theresults indicated that the Kαsource size was always larger than the focal spot dimensions andthat the electron beam diverged with an angular spread of 40◦[11].We report here electron energy transport experiments conducted on the Vulcan 100 TWlaser facility with large area and thick foil targets that provide new insight into electrontransport in low Z targets. For plastic targets it is shown, by the plasma expansion measuredin shadowgrams taken after the interaction pulse, that there is a transition between previouslyreported collimated electron flows at 10 TW to an annular electron flow pattern with a 20◦divergence angle for peak powers of ∼70 TW. Intermediate powers on target suggest that boththe central collimated flow pattern and the annular structure co-exist.The experiments described here were conducted on the Vulcan 100 TW laser facility [12].This laser delivered up to 120 J in a pulse of 0.9–1.1 ps duration to target in a focal spot diameterof 10 µm by a 60 cm focal length, F/4.2 off-axis parabola. 35% of the laser energy wascontained within the focal spot and corresponded to a peak intensity of up to 5 × 1019Wcm−2.The intensity contrast ratio was 10−7. The laser wavelength was 1054 nm. The p-polarizedlaser radiation was incident onto the target at a 45◦angle of incidence. The targets were largearea Mylar foils.The profile of the heated target was obtained from shadowgrams using an optical