Experimental studies of direct-drive, low-intensity, low-adiabat spherical implosions on OMEGA

Low-adiabat (high-compressibility) implosions were performed with ∼860 μm diam, 24- and 35-μm-thick shells filled with 15 atm of D2 gas driven with ∼12.5 kJ, 3-ns-square pulse shapes, and a laser intensity of ∼2x1014 W/cm2. Initial target modulations, imprinted on the target surface by laser nonuniformities, were varied by using drive with and without smoothing by spectral dispersion. The performance of the spherical implosions (with a gas fuel convergence of ∼16) was correlated with measured target modulations at the end of the acceleration phase performed in a planar target with similar conditions using through-foil radiography. The neutron yield decreases sharply when target modulations become comparable with the target thickness at the end of the acceleration phase. The measured target areal densities are less sensitive to target modulations. Implosions with 35-μm-thick shells produced neutron-averaged areal densities that are among the highest on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]