Compression of plasma in laser-driven implosions has been characterized as "ablative" or "exploding pusher" according to the relative importance of surface ablation pressure and bulk pressure due to preheating by electrons. To date, experimental study of ablative implosions has been limited, 1 partly because their high-density low-temperature plasma yields little diagnostic information in the form of x-ray or fusion-product emission compared with exploding-pusher implosions. 2 The development of x-ray probing techniques 3 has alleviated this problem and permitted study of the implosion dynamics of spherical-shell targets under ablative acceleration with use of streak time-resolved x-ray shadowgraphy. 4 Elementary mechanics show that for negligible mass loss, a spherical shell with ratio of radius to wall thickness (aspect ratio) r/Ar and density p can be accelerated to a velocity v of the order (p a r/&r) l/2 by a surface ablation pressure P a . Since the stagnation pressure P f of the implosion is of the order pv 2 it follows that P f^Pa r/&r. High aspect ratio therefore leads to high stagna-reduce direct fast-electron preheat of the fuel for laserfusion targets. See for example, K 0 Lee, Do W.
Plasma densities and temperatures are deduced from soft X-ray emission spectra of neon-filled glass microballoons imploded by a 0.2 terawatt laser. Spatial resolution allows separate characterisation of the ablation plasma, the imploded glass and the compressed gas. Comparison with computer simulations of the implosion shows sensitivity to models for energy transport by hot electrons.
Studies of ion flows from laser irradiated plane aluminium targets show that the ion velocity V scales with irradiance level I and optical focal spot diameter Phi , as V varies as I0.2 Phi 0.3. Edge effects due to lateral heat conduction cannot be ignored for focal spot sizes Phi
A laser-based method of preparing a plasma with only a single atomic-state population significantly out of equilibrium is described. This technique allows determination of collision rates for first excited states largely free from the problems of multiple-level excitation and cascade inherent in most fluorescent scattering techniques in plasmas, or from the absolute intensity measurements in the VUV inherent in emission spectroscopic techniques. The results obtained for the n=2 state of hydrogen, however, pose severe problems in interpretation in terms of presently accepted electron collision rates.
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