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).

A time-of-flight ion collector was used to measure the energy of ions from the plasma produced by a high-power CO2 laser pulse focused on a flat polyethylene target. At laser irradiances exceeding 5×1012 W/cm2 at the target, high-energy ion current spikes appeared in addition to the ion current from an expanding thermalized plasma. The measured arrival time of these spikes was consistent with the assumption that ions of different charge and mass were accelerated by a high potential during the laser pulse which was produced by a steep pressure gradient at the plasma surface. The accelerating potential (equal to the measured energy of a singly charged ion) was proportional to the laser irradiance at the target to the 2/3 power. At an irradiance of 2×1014 W/cm2, the H+ ion energy was 140 keV. About 90% of the total ion energy appeared as high-energy ions.

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can make an order-of-magnitude estimate of r using a classical hard-sphere model for the collisions where the rotons are assumed to be stationary, and the He^ quasiparticles move with a mean speed {v) = {^kT/m^''Y''^, Thenwhere r'Ms the mean collision frequency, n is the number density of atoms, and m^'^-^2,Zm^ is the effective mass of the He^ quasiparticle. Using a value ofa = 1.6xl0"^^ cm^ measured by Herzlinger and King,^ this expression becomes r(°K) = 5.7X, in satisfactory agreement with our results.

Generation of vacuum ultraviolet radiation by plasma nonlinearities AIP Conf. Proc. 90, 224 (1982); 10.1063/1.33764Photoionization quantometer for absolute intensity measurements of vacuum ultraviolet and soft xray radiation from laser plasma Rev. Sci. Instrum. 52, 16 (1981);

Results are presented of an experimental study of the plasma produced by the interaction of the focused light from a Q-switched laser with a tungsten target in a vacuum. The plasma radius, temperature, and radiative properties were measured during the period of the laser pulse. In addition, the target mass loss and plasma velocity and impulse were observed. Based on these experimental results, a model was developed which provides an energy and power balance for the expanding, radiating plasma.

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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