COMPUTATIONAL STUDY OF LASER IMPRINT MITIGATION IN FOAM-BUFFERED INERTIAL CONFINEMENT FUSION-TARGETS

Recent experiments have shown that low density foam layers can signifi cantly mitigate the perturbing effects of beam nonuniformities affecti ng the acceleration of thin shells. This problem is studied parametric ally with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Com ments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targ ets are employed, consisting typically of 250 Angstrom of gold, and 50 mu m of 50 mg/cm(3) C10H8O4 foam attached to a 10 mu m foil. In simul ation these were characteristically exposed to 1.2 ns, flat-topped gre en light pulses at 1.4 x 10(14) W/cm(2) intensity, bearing 30 mu m lat eral perturbations of up to 60% variation in intensity. Without the bu ffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown t o derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding 5 x 10(13 ) W/cm(2) are needed for assured stability beyond 2 ns; (3) longer foa ms at lower densities are needed for effective mitigation with shorter wavelength light; (4) the gold layer hastens conversion of the struct ured foam to a uniform plasma. (C) 1998 American Institute of Physics.