Data on the emission of energetic ions produced in laser–matter interactions have been analyzed for a wide variety of laser wavelengths, energies, and pulse lengths. Strong correlation has been found between the bulk energy per AMU for fast ions measured by charge cups and the x-ray-determined hot electron temperature. Five theoretical models have been used to explain this correlation. The models include (1) a steady-state spherically symmetric fluid model with classical electron heat conduction, (2) a steady-state spherically symmetric fluid model with flux limited electron heat conduction, (3) a simple analytic model of an isothermal rarefaction followed by a free expansion, (4) the lasnex hydrodynamics code [Comments Plasma Phys. Controlled Fusion 2, 85 (1975)], calculations employing a spherical expansion and simple initial conditions, and (5) the lasnex code with its full array of absorption, transport, and emission physics. The results obtained with these models are in good agreement with the experiments and indicate that the detailed shape of the correlation curve between mean fast ion energy and hot electron temperature is due to target surface impurities at the higher temperatures (higher laser intensities) and to the expansion of bulk target material at the lower temperatures (lower laser intensities).

Experimental and theoretical results on the properties of CO2 laser-induced carbon and polyethylene (CH2) plasmas at laser intensities of ∼1015 W/cm2 are presented. The Thomson parabola technique is used to measure the ion velocity distribution in the underdense expanding plasma which is collisionless and isothermal. A model which treats the problem of the collisionless expansion of an isothermal electrostatic multi-cold-ion quasineutral plasma will be used to interpret the experimental results. Experiments and theory show that the effect of hydrogen in the CH2 target induces a cutoff in the carbon ion velocity distribution. Theory suggests that acceleration field attenuation effects modify the behavior of these plasmas. Data suggest that the space–time evolution of the ion velocity distribution is completed before an ionization-recombination equilibrium is reached. Experimental results from the underdense region are used to estimate plasma parameters near the critical surface, which show that the presence of hydrogen in the target apparently greatly reduces the thermal temperature near the critical surface, probably due to enhanced lateral energy transport.

Fast ions produced by laser irradiance of the front side of a wire target also appeared on the back side. The speed of the energy transport around the wire was measured at approximately 2*108 cm s-1 and its temperature gradient was approximately 5*105 eV cm-1. The energy was transported a lateral distance of 1500 mu m away from the laser focal spot (100 mu m diam.) when protons were one of the plasma constituents. However, for a pure carbon target, the energy spread was not observed down to a distance of 300 mu m.

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The effects of target surface impurities on the laser-produced plasma were studied by means of Thomson ion spectrograms and time-of-flight measurements of the plasma ions. Hydrocarbons were believed to be the most likely surface impurity. Consequently, polyethylene was used to characterise an impure target. For comparison, pure targets (5 to 20 ppm impurities) of C, Ti, and Ta and a standard quality Ti target were used. The fastest Cz+ ions from the plasma expansion using a polyethylene target had the same peak velocity. When the hydrogen impurity was removed from a carbon target, the fastest CZ+ ions had an energy proportional to their charge in agreement with the isothermal expansion model. Also, the presence of hydrogen affected the lateral transport of the plasma.