Abstract

Ablative acceleration of thin foils heated by laser irradiation is of great interest in connection with laser fusion as well as with generation of strong shocks. Taking into consideration these applications of the accelerated foil, the most important parameters are final velocity, mass, and density, the last being decreased as the foil thermally expands. Optical diagnostic methods are convenient to use and provide good spatial and temporal resolution; however, in their application to dense plasmas from accelerated foils an obstacle arises in that light does not penetrate into plasma with a density ⪞10²² cm^-3. Nevertheless, combining several optical methods and diagnosing different foil regions at the front and rear sides of the foil makes this problem soluble. Actually, three methods have been used:(1) the space-time diagram of the critical density surface movement at the front (irradiated) side of the foil was traced by integrating the velocity time dependence obtained from the time-resolved second harmonic spectrum; (2) the movement of the plasma density surface corresponding to an optical shadow (~10²⁰ cm^-3) at the rear side of the foil was monitored by optical probing using multiframe shadowgraphy and interferometry; (3) the foil’s center of mass velocity was measured with a hollow ballistic pendulum allowing considerable improvement in accuracy. Experiments were performed with two channels of the UMI-35 laser facility with an intensity of 5 × 10^,2W/cm² and a pulse duration of 6-8ns. Thin Mylar foils with thicknesses ranging from 6 to 25μm have been accelerated to velocities of the order of 100 km/s. The dependence of accelerated foil velocity and thermal expansion on foil thickness is discussed.

© 1986 Optical Society of America