Optimization of plasma uniformity in laser-irradiated underdense targets

1. SiO2 aerogel with Ti dopant Zone Coordinate, cm Zone Coordinate, cm • Cases analyzed • 0– 6 at% Ti dopant • 2–8 mg/cc • 1–2.2 mm thickness • 5x1012– 4x1014 W/cm2 5x1012 W/cm2 6x1013 W/cm2 Time, ns Time, ns Indirect radiation heating from end zones also can produce uniform temperature and density 3 mg/cc SiO2 2.6 mg/cc SiO2 3% Ti 3 mg/cc SiO2 Laser Laser 0 1 mm 2 Te, eV Time, ns Ne, 1020 cm-3 Time, ns R, cm Time, ns Optimization of plasma uniformity in laser-irradiated underdense targets M. S. Tillack, K. L. Sequoia, B. O’Shay H. A. Scott C. A. Back University of California, San Diego9500 Gilman Drive La Jolla, CA 92093-0438 USA Lawrence Livermore National Laboratory P.O. Box 808 Livermore, CA 94561 USA General Atomics P.O. Box 85608 San Diego, CA 92186-5608 USA Inverse bremsstrahlung, non-LTE Density uniformity Introduction Objectives: Studies of atomic processes in laser plasma require uniform conditions: a) Predict the degree of uniformity in ne and Te for directly-heated underdense (non-LTE) targets b) Explore the impact of physics models on the results c) Propose design solutions to improve the uniformity Under conditions of direct heating, the value of absorption coefficient is critical The density is uniform when Zeff is near a maximum and hydro expansion is small (I<1014, t>1 mm) Experimental Geometry (NIKE) (note: ncr=16x1021/cm3) 1.6 kJ, 248 nm 4 ns 12˚ cone angle 5x1012–5x1014 W/cm2 Inverse bremsstrahlung in Hyades 2.5 ns Zeff Numerical Simulation I=5x1012 W/cm2 It’s difficult to achieve optically thin plasma with 2 mg/cc (5x1020) SiO2 targets 1 mm thick @ Te<300 eV Time, ns McWhirter condition Hyades (Cascade Applied Sciences) 1D rad-hydro Gray (Sesame) or multi-group diffusion Saha or average atom ionization model (ne>1.4x1014 Te1/2(DE)3 cm–3) LTE non-LTE Helios (Prism Computational Sciences) 1D rad hydro 5000-group computed opacities Most of the plasma is non-LTE If t<1 mm or I>1014 W/cm2, the targets expand too quickly Key Physics Issue: Choice of Opacity and Ionization Models Comparison with experiment • Experimental Parameters: • High Fluence: • 2.2 mm • 3% Ti dopant • 2.7 mg/cc • 5.7x1013 W/cm2 (248 nm) • Low Fluence: • 1 mm • 6% Ti dopant • 2.5 mg/cc • 4.6x1012 W/cm2 (248 nm) Opacity and Ionization Options in Hyades (pure SiO2) The radiation mean free path at 150 eV is several mm Doping affects rad-hydro 2 ns 2.5 ns 2 ns } 35 photon energy groups High Fluence Modeling Results Non-LTE ionization balance of Ti in 2 mg/cc SiO2 (Cretin) Energy Balance (2.5 at%) data courtesy of Prism Comp. Sci. Helios predicts much higher temperatures Hyades 35-group, non-LTE avg. atom Double-Sided Illumination Pillbox Target High IntensityBase Case Results Higher laser intensity gives higher, slightly flatter temperature and faster, stronger ionization 2-sided illumination provides a more uniform temperature profile at lower intensity 1 mm thick 2.6 mg/cc SiO2 Same total laser input (2 x 2.5e12 or 2 x 3e13) 2.5 ns 2.5 ns Zeff Te, eV I=6x1013 W/cm2 Time, ns Time, ns Conclusions: • In this regime, results are sensitive to models used • LTE and non-LTE results are quite different • Doping has a significant effect on the radiation hydrodynamics • Double-sided and indirect illumination both show promise • More data are needed to help understand the underlying physics I=6x1013 W/cm2