Laser-intensity scaling experiments in long-scalelength, laser-produced plasmas

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Abstract
An experimental technique is demonstrated that allows variation of the average laser intensity by more than two orders of magnitude while producing much smaller changes in the other parameters that determine the laser–plasma interactions. By irradiating exploding‐foil targets with 0.35 μm laser light in flat‐topped pulses of variable duration, the high‐power Nova laser [Rev. Sci. Instrum. 5 7, 2101 (1986)] produced plasmas with electron temperatures of order 1 keV and with scale lengths of the (radial and axial) electron‐density gradients of order 1000 laser wavelengths. By using a constant target thickness and systematically decreasing the pulse length and spot size as laser intensity increased, the changes in the temperature and in the scale lengths were minimized. The time‐resolved spectrum of the Raman‐scattered light was used to measure both the electron temperature and the maximum density of the expanding plasmas. In this paper, these measurements are compared to both 1‐D models and 2‐D simulations. The expected trends of slower burnthrough and lower temperature with lower laser intensity were observed. However, the inferred temperatures are lower and the burnthrough of the target (at high laser intensities) is slower than the modeling predicts. Possible sources of these discrepancies are discussed.