Laser-plasma interaction experiments have been performed with both 1.05-and 0.53-jLim-wavelength light incident on spherical glass targets. Comparisons of hard x-rayspectra and fast-ion energy imply a substantial reduction of hot-electron levels at the shorter wavelength. Increased absorbed energy fractions at the shorter wavelength are in agreement with the expected scaling of inverse bremsstrahlung absorption.

Numerical hydrodynamic simulations which include absorption and transport modifications due to ion-acoustic turbulence have been performed. These simulations can reproduce the important features of the fast-ion velocity distributions as measured with biased charge collectors and a magnetic spectrograph in spherically illuminated microballoon experiments. Experiments with hemispherical targets show that a substantial amount of fast-ion energy is directed inward and may contribute to driving the implosion.Measurements of ion expansion energy from laser-heated plasmas show an anomalous fast-ion component. 1 Charge collectors biased to record ion current as a function of time show that in a typical experiment approximately half of the absorbed energy resides in only a few percent of the target mass. 2 This result implies that the absorbed energy is being coupled into a very thin layer at the target surface. This mode of coupling leads to poor momentum transfer between the expanding plasma and the compressed region of the pellet and could place a fundamental limitation on the efficiency of laser-driven ablative compression. Figure 1 presents data from two instruments used to obtain the ion velocity distribution. The target for this laser shot was a 54-jtim-diam spherical glass shell with 0.8-jum wall thickness filled with 10 atm of DT gas mixture. The laser energy on the target was 21 J at 1.06-/im wavelength in a 70-psec flat pulse. A lens-ellipsoidalmirror illumination system provided near-normal illumination of the pellet. 3 Figure 1(a) shows the voltage trace from a conventional charge collector. The initial sharp spike is produced by photoelectron emission from the collector surface due to the ultraviolet and x-ray flash when the laser pulse hits the target. The second peak results from the fast ions, and the broad feature represents the arrival of the bulk of the ionized target material. Using previously measured values for the mean ionic charge and secondary-emission coefficient, 4 this voltage trace can be converted into an ion-velocity spectrum. Because of uncertainty in the secondary-emission correction, the velocity spectrum above 2x 10 8 cm/sec is obtained with a small magnetic spectrograph which uses cellulose nitrate foils as detectors. 5 Figure 1(b) shows the ion-velocity spectrum. At high velocities, the spectrum is adequately 10 4 10

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A compact, flexible design for a parallel-fields ion analyzer is presented. Accurate ion velocity and charge state measurements can be obtained over a wide range without the need for calibration sources. Etchable cellulose-nitrate foil is used to record individual ion tracks.

Pinhole-camera images formed by fast ions emitted from spherical-glass-shell targets uniformly irradiated by intense 1.06-μm laser light have been recorded on cellulose-nitrate films. The spatial resolution of the images is consistent with the 5- and 9-μm pinhole size. The size of the image relative to the target size provides the first measurement of the angular distribution of fast-ion emission from spherical targets. The observed images indicate a forward-peaked distribution, but with very broad angular spread. Image sizes increase with laser intensity and pulse length.