present address: Department of Physics, Imperial College, London SW7 2AZ, United Kingdom

1. Si re-entrant pyramid or wedge 11-25um Titanium substrate 500 mu Silicon substrate Spheres monolayer (0,1-2.9um) 40fs, 600mJ, 800nm, 1019 W/cm2 1mu Laser 100fs, 12mJ, 400nm,2x1017 W/cm2 Laser bremsstrahlung emission (>100keV) Bright Si K-alpha emission (7.2 A) Hot electrons, high energy conversion efficiency Bright Ti K-alpha emission (2.75 A) bremsstrahlung emission (>10keV) Pyramid WEDGE (P-polarized) WEDGE (S-polarized) Hot electron and X-ray yield optimization with micro-shaped targets Y. Sentoku, N. Renard-Le Galloudec, T.E. Cowan Nevada Terawatt Facility, University of Nevada at Reno S. Kneip1,*, B.I. Cho, I.V. Churina, A.V. Belolipetski, A. Henig, G.M. Dyer, D.R. Symes2, T.D. Donnelly3, A.C. Bernstein and T. Ditmire Texas Center for High Intensity Laser Science, University of Texas, Austin E. Förster, O. Wehrhan IOQ, X-ray Optics Group, University of Jena A. Karmakar, A. Pukhov Institut für theoretische Physik, University of Düsseldorf Abstract X-ray and hot electron production of targets with micro structured features irradiated by multi TW laser pulses has been studied. Furthermore, X-ray production from re-entrant targets etched into silicon has been investigated. Ka and hard X-ray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though Ka is maximized with wedge targets and parallel polarization. These results are explained with particle-in-cell simulations. These studies illustrate that, with correct tailoring of the target surface, field enhancement can be used to optimize X-ray and hot electron production. Examination of Ka (hard X-ray) emission from Si targets coated with micron scale polystyrene spheres indicated that the emission is enhanced by a factor of 10 (100) over emission from planar targets. The X-ray yield is found to depend critically on the sphere size with maximum emission for spheres with diameter close to half the laser wavelength. These findings are explained by Mie Enhancement of the laser field at the sphere surface and particle-in-cell simulations. Sphere coated targets also emit He-like Si radiation indicating the presence of a hot dense plasma beneath the microspheres. The line ratio analysis of the Li-like satellite emission indicates plasma temperatures on the order of 250eV and plasma densities of 1.9x1022cm-3. Sphere Targets Target Design Pyramid and Wedge Targets The re-entrant targets are produced by standard semiconductor processing techniques. A square mask leads to pyramids and a rectangular mask leads to wedges. Titanium foils are adhered to the back to generate K-shell fluorescence. 2 dimensional arrays of polystyrene spheres are deposited on silicon wafers, usually forming hexagonal-packed single layers. Targets were coated with single size spheres ranging from 0.1um to 2.9um, with size dispersion <3%. Hot electrons, high energy conversion efficiency Sphere Targets Pyramid and Wedge Targets Experimental Setup Studies were carried out using the THOR laser which delivers 35fs pulses at 800nm with energy 0.8J. To avoid generation of a plasma gradient at the target surface from prepulses, the 800nm laser light is frequency doubled. 12mJ of 400nm light in a 100fs pulse is focused by a f/2.8 parabolic mirror to a peak intensity of 2x1017 W/cm2 in a 6um, 1/e2 diameter spot. 600mJ of 800nm light in a 35fs pulse is focused by a f/3 parabolic mirror to a peak intensity of 2x1019 W/cm2 in a 10um, 1/e2 diameter spot.The preplasma scale length is found to be 2-3um, indicating that the wedge and pyramid target geometry remains intact until the arrival of the main pulse.Precise target alignment is achieved by means of a symmetric back reflection pattern of a collinear HeNe. To detect K-shell x-rays, a cylindrical PET crystal in von-Hamos geometry is employed. This detector is sensitive to the 1-100 keV electron population. To detect bremsstrahlung x-rays, NaI Scintillator Photomultiplier detectors with metal foil/slab filters are employed. This detector is sensitive to the >10-100keV hot electron population. Experimental Results: Sphere Targets Experimental Results: Pyramid and Wedge Targets The x-ray production strongly depends on the employed sphere size. The Ka yield peaks when 260nm spheres coat the target. The increase is 3 (10) compared to planar targets shot at 45 ° (0°) The hard x-ray yield above 22keV and 75keV peaks for 260nm spheres. Hard x-ray yield is increased 100 fold compared to planar targets. Ka yield from wedge targets shot in s-polarization is higher than for p-polarization. K-a yield from Pyramid targets is intermediate. All re-entrant targets produce less Ka radiation than flat targets. This applies both for 11 and 25um Titanium substrate. Hard x-ray yield from wedge targets shot in p-polarization is higher than for s-polarization, with pyramids producing intermediate yields. PIC simulations: Wedge Targets The shape of the Hea line differs for targets with and without spheres. For bare Silicon, the blue side of the Hea line is steep, indicating that the target expands during emission. The addition of a spheres layer acts as a tamper to the underlying substrate. The result is emission from denser regions of the plasma, flattening the blue wing of the Hea line. The measured line ratios of Li satellites are compared to non-LTE simulations from the code FLYCHK. The data is consistent with plasma densities of (1.9±0.3)x1022cm-3 and temperatures of (200 ±50)eV for untreated and (300 ±50)eV for treated surfaces. PIC simulations reveal higher laser energy absorption for p-wedges. Due to the large opening angle of the wedge, guiding of laser energy and electrons towards the tip is insignificant. Hot electrons (>100keV) are rather accelerated into the bulk silicon where they lead to the observed hard x-ray yield enhancement over s-wedges. For s-polarization, less energy is absorbed but the light pressure channels more electrons with 10-100KeV into the Ti substrate, leading the observed Ka yield enhancement over p-wedges. Simulations: Sphere Targets Mie Scattering [V]: Spheres alter the boundary condition of the laser field on the target, leading to local field enhancement (>10x) over the vacuum intensity. This leads to an increase in ponderomotive Energy and the observed enhancement of x-ray yield, temperature and electron Conclusions Targets coated with a monolayer of wavelength scale spheres show resonant enhancement of x-ray yield, temperature and electron temperature for 0.26um spheres. Local field enhancements and multipass vacuum heating are responsible [VI]. Depending on the laser polarization, wedge targets benefit hard x-ray or Ka emission. Re-entrant targets fail to enhance x-ray yield over planar targets. The opening angle of the pyramids is too large for cone guiding to become significant [VII]. temperature. Local field and electron energy enhancement are confirmed by PIC simulations. • G Mie, Ann. Phys, (Berlin) 25, 377 (1908) • B.N. Breizman, et. al., PoP 12, 056706 (2005) • T. Nakamura, et. al., PRL 59, 265002 (2004) • H.A. Sumeruk, S. Kneip, et. al., PRL 98, 045001 (2007) • H.A. Sumeruk, S. Kneip, et. al., PoP 14, 062704 (2007) • S. Kneip, B.I.Cho, et. al., HEDP 4, 41-48 (2008) • B.I.Cho, G.M. Dyer, S. Kneip, et. al., PoP 15, 052701 (2008) • present address: Department of Physics, Imperial College, London SW7 2AZ, United Kingdom • present address: STFC Rutherford Appleton Laboratory, Central Laser Facility, Chilton, Didcot, OX11 0QX, United Kingdom • present address: Physics Department, Harvey Mudd College, Claremont, California, USA • contact: stefan.kneip@imperial.ac.uk