High-resolution spectrometry of charged particles from inertial-confinement-fusion ͑ICF͒ experiments has become an important method of studying plasma conditions in laser-compressed capsules. In experiments at the 60-beam OMEGA laser facility ͓T. R. Boehly et al., Opt. Commun. 133, 495 ͑1997͔͒, utilizing capsules with D 2 , D 3 He, DT, or DTH fuel in a shell of plastic, glass, or D 2 ice, we now routinely make spectral measurements of primary fusion products ͑p, D, T, 3 He, ␣͒, secondary fusion products ͑p͒, ''knock-on'' particles ͑p, D, T͒ elastically scattered by primary neutrons, and ions from the shell. Use is made of several types of spectrometers that rely on detection and identification of particles with CR-39 nuclear track detectors in conjunction with magnets and/or special ranging filters. CR-39 is especially useful because of its insensitivity to electromagnetic noise and its ability to distinguish the types and energies of individual particles, as illustrated here by detailed calibrations of its response to 0.1-13.8 MeV protons from a Van de Graaff accelerator and to p, D, T, and ␣ from ICF experiments at OMEGA. A description of the spectrometers is accompanied by illustrations of their operating principles using data from OMEGA. Sample results and discussions illustrate the relationship of secondary-proton and knock-on spectra to capsule fuel and shell areal densities and radial compression ratios; the relationship of different primary fusion products to each other and to ion temperatures; the relationship of deviations from spherical symmetry in particle yields and energies to capsule structure; the acceleration of fusion products and the spectra of ions from the shell due to external fields; and other important physical characteristics of the laser-compressed capsules.

Spherical shock-ignition experiments on OMEGA used a novel beam configuration that separates low-intensity compression beams and high-intensity spike beams. Significant improvements in the performance of plastic-shell, D 2 implosions were observed with repointed beams. The analysis of the coupling of the high-intensity spike beam energy into the imploding capsule indicates that absorbed hot-electron energy contributes to the coupling. The backscattering of laser energy was measured to reach up to 36% at single-beam intensities of $8 Â 10 15 W/cm 2. Hard x-ray measurements revealed a relatively low hot-electron temperature of $30 keV independent of intensity and timing. At the highest intensity, stimulated Brillouin scattering occurs near and above the quarter-critical density and the two-plasmon-decay instability is suppressed. V

Direct drive implosions of targets filled with different mixtures of D 2 and 3 He gas on the OMEGA laser system [T.R. Boehly et al., Opt. Commun. 133, 495 (1997)] have shown an unexpected scaling of experimental nuclear yields. At temperatures above a few eV, D 2 and 3 He gasses are fully ionized; and hydrodynamically-equivalent fuels with different ratios of D 2 and 3 He can be chosen to have the same mass density, total particle density and equation of state. Implosions with a 50-50 mixture of D: 3 He by atom consistently result in measured nuclear yields half of that anticipated by scaling from measured yields of implosions with pure D 2 and nearly pure 3 He. This observation is seen over a wide range of experimental configurations, including targets with a variety of shell thicknesses and fill pressures, simultaneously for two different nuclear yields (DD and D 3 He), as well as for shock and compression yields. A number of possible mechanisms to cause the scaling are considered, but no dominant mechanism has been identified. 1 also Visiting Senior Scientist at LLE.