The shock pressures generated in the interaction of lasers and light ion beams with thin foils are calculated using a two-dimensional, axisymmetric hydrodynamic code. For lasers, finite spot sizes produce a significant decrease in the pressure generated, compared to previous 1-D estimates. The effect is closely related to the coupling of the critical density with the ablation surface and is less important at shorter wavelengths, the appropriate parameter being Iλ2. For 0·5 TW red light and a 100 μm radius spot pressures are limited to 30 Mb. Perturbations across the beam profile are discussed and it is demonstrated that beam uniformity of 30% (25% for green light) is required to achieve velocity modulation of less than 2%, such as would be necessary for an equation of state measurement.
For the larger targets used with light ion beams, two-dimensional effects prove to be less of a problem, and for high-Z materials the initial 1-D estimates of peak pressure are adequate. In order to obtain the shock planarity requisite for equation of state measurements in a high-Z target an intensity uniformity of ±15% would be required. However, use of a composite target enables this to be relaxed to ±25% whilst obtaining slightly higher pressures.