Compressed-shell integrity measurements in spherical implosion experiments

The shell integrity near peak compression of spherical implosions using the 60-beam, 30-kJ UV OMEGA laser system [Opt. Commun. 133, 495 (1997)] has be en measured. Hot core emission backlights a shell with a thin titanium-dope d layer that is imaged at x-ray photon energies above and below the titaniu m K edge. The x-ray intensity ratio between the two images is related to pe rturbations in the cold, or absorbing, part of the shell. The measured cold -shell areal-density modulations, integrated over the time of peak compress ion, are of the order of 25% to 50% with nonuniformity spectra peaked at sp atial wavelengths of 30 to 50 mum and with the smallest detectable nonunifo rmity features extending down to spatial wavelengths of 12 to 15 mum. Hot-s hell areal-density modulations of the emitting part of the shell (inner edg e) are of the order of 13% to 20%. The measured shell modulations are in ag reement with the results of two-dimensional simulations that include initia l shell perturbations, imprinted shell modulations due to nonuniformities i n a single laser beam, and a beam-to-beam energy imbalance in the laser dri ve. (C) 2001 American Institute of Physics.