Optimization of plasma uniformity in laser-irradiated underdense targets M. S. Tillack, K. L. Sequoia, B. O’Shay University of California, San Diego 9500 Gilman Drive La Jolla, CA USA H. A. Scott Lawrence Livermore National Laboratory P.O. Box 808 Livermore, CA USA C. A. Back General Atomics P.O. Box San Diego, CA USA Objectives: Studies of atomic processes in laser plasma require uniform conditions: a) Predict the degree of uniformity in n e and T e 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 If t W/cm 2, the targets expand too quickly Key Physics Issue: Choice of Opacity and Ionization Models Inverse bremsstrahlung, non-LTE Introduction Inverse bremsstrahlung in Hyades Comparison with experiment Hyades (Cascade Applied Sciences) 1D rad-hydro Gray (Sesame) or multi-group diffusion Saha or average atom ionization model Helios (Prism Computational Sciences) 1D rad hydro 5000-group computed opacities Numerical Simulation Experimental Geometry (NIKE) 1.6 kJ, 248 nm 4 ns 12˚ cone angle 5x10 12 –5x10 14 W/cm 2 McWhirter condition The density is uniform when Z eff is near a maximum and hydro expansion is small (I 1 mm) Density uniformity (n e >1.4x10 14 T e 1/2 (  E) 3 cm –3 ) It’s difficult to achieve optically thin plasma with 2 mg/cc (5x10 20 ) SiO 2 targets 1 mm T e <300 eV (note: n cr =16x10 21 /cm 3 ) Pillbox Target Most of the plasma is non-LTE Cases analyzed 0– 6 at% Ti dopant 2–8 mg/cc 1–2.2 mm thickness 5x10 12 – 4x10 14 W/cm 2 Experimental Parameters: High Fluence: 2.2 mm 3% Ti dopant 2.7 mg/cc 5.7x10 13 W/cm 2 (248 nm) Low Fluence: 1 mm 6% Ti dopant 2.5 mg/cc 4.6x10 12 W/cm 2 (248 nm) LTE non-LTE 2.5 ns Non-LTE ionization balance of Ti in 2 mg/cc SiO 2 (Cretin) 2 ns data courtesy of Prism Comp. Sci. (2.5 at%) The radiation mean free path at 150 eV is several mm Hyades 35-group, non-LTE avg. atom Helios predicts much higher temperatures Double-Sided Illumination 1 mm thick 2.6 mg/cc SiO 2 Same total laser input (2 x 2.5e 12 or 2 x 3e 13 ) 2-sided illumination provides a more uniform temperature profile at lower intensity I=6x10 13 W/cm 2 T e, eV Time, ns I=5x10 12 W/cm 2 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 01 mm2 Time, ns R, cm Ne, cm -3 Time, ns Te, eV 2 ns 35 photon energy groups Doping affects rad-hydro Opacity and Ionization Options in Hyades (pure SiO 2 ) High Fluence Modeling Results High IntensityBase Case Results I=6x10 13 W/cm 2 Z eff Time, ns Higher laser intensity gives higher, slightly flatter temperature and faster, stronger ionization 2.5 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 SiO 2 aerogel with Ti dopant } Energy Balance 2.5 ns Z eff 5x10 12 W/cm 2 Time, ns Zone Coordinate, cm 6x10 13 W/cm 2 Time, ns Zone Coordinate, cm