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Parylene and tungsten-glass disks have been irradiated with a 1.06-/um laser at fluxes of 10^^-10^^ W/cm^ Analysis of the experimental results indicates that the laser-plasma interaction was characterized by critical surface steepening, ripplii^ and/or filamentation, polarization-dependent scattering, and efficient generation of superthermal electrons, These effects were significantly stronger in the tungsten-glass experiments.Extensive e2q)erimental studies^ of the interaction of laser radiation with low Z (^3. 5) targets, covering a wide range of intensities (/^ ^ 10^^ W/ cm^) and wavelengths (^ lOo 6 jum), indicate that the interaction process is generally strongly influenced by collective plasma processes. This Letter presents the results of 1. 06-ium laser-irradiation experiments^ with the Cyclops laser^ in which Z was varied significantly by using disk targets of Parylene (CgHg) and W glass (0. 75 W2O, 0. 25 PgOg). Detailed measurements of the scattered laser light and x-ray and charged-particle emission characteristics show strong evidence that the interaction process was dominated by critical surface steepening, rippling and/or filamentation, polarization-dependent scattering, and efficient generation of superthermal electrons. Furthermore, the strongest noncollisional behavior occurred in the high-Z W-glass irradiations. Analysis suggests that the observed behavior of the two target types can be unified by the parameter 77 = Fo/^r? ^^^ ratio of the electron oscillatory and thermal velocities.The planar (650--950 \im diamx? \im thick) disks were located ^ 10^ /im inside the focal waist in the near field of an//2. 5 aspheric lens. The flux level (10^^-10^^ W/cm^) was varied by changing the laser pulse length (150-400 ps) and energy (5-75 J). Target damage due to amplified spontaneous emission and prepulses was found to be unimportant. An extensive array of diagnostics^'^ observed laser and plasma behavior during each experiment The time-integrated laser energy distribution in the target plane was measured during each experiment with an equivalent-lens-multiple-image camera system., The focused laser beam, 250-300 /im FWHM (full width at half-maximum) in diameter, exhibited a ring structure, rising (50-100)% above the central minimum. Small-scale fluctuations, 30-40 iixa in diameter and (30-50)% in relative 7^, were superimposed on these gross features.Figvire 1 shows the measured absorption T]^ VS Ii, Both target types absorbed comparable fractions. For the ~ 200-ps experiments, ri^ for W glass (Parylene) varied from ~ 38^o (r 30%) at 5 X10^^ W/cm^ to -30% {-25%) at 5 x lO^^* W/cm^ The discrepancy between the photodiode and box calorimeter measurements should be taken as indicative of the accuracy of the energy balance 70 60 50 40 c 30h 20 lOh-

In order to obtain the pusher attenuation length (ρΔR) of inertial confinement fusion targets by neutron activation, it is necessary to know what fraction of the target debris is counted. One method of measuring this quantity experimentally is by means of a radioactive tracer. We demonstrate the usefulness of this technique by experiments utilizing a 24Na tracer.

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