Search

Time‐resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at 1016 W/cm2, is used to study the basic processes which control the x‐ray pulse duration. Short x‐ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time‐dependent calculations of the atomic physics [Phys. Fluids B 4, 2007, 1992] coupled to a Fokker–Planck code [Phys. Rev. Lett. 53, 1461, 1984] indicate that it is essential to take into account the non‐Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.

Electrons of 2.3 MeV energy and light in the wavelength range from 2λ0 to 1.28λ0 scattered in the direction of incident laser have been observed from the plasmas of exploding thin foils created by a carbon dioxide laser. The correlation measurements of electrons and light, performed independently, pointed to a common physical origin for these near the quarter‐critical density region of the plasma where low phase velocity plasma waves are driven. This also indicated that low‐density forward Raman scattering with high phase velocity plasma waves may not be principally responsible for the energetic electrons. In order to interpret the experimental results a simple computer calculation has been performed obtaining the final energies of electrons trapped in freely propagating electron plasma waves with increasing phase velocity.

A 12 μm polyester foil is accelerated by a 0.26 μm wavelength laser and collides with a 15 μm thick molybdenum foil. The accelerating pressure is 45 Mbar (laser intensity≊3– 4×1014 W/cm2) and gives to the polyester foil a velocity of about 160 km/sec. The measurement of the shock pressure induced in the impacted foil is made with an improved step technique. When the initial spacing between the two foils is too large compared to the focal spot radius, i.e., larger than 20–30 μm, the different experimental results cannot be reproduced with one‐dimensional simulations; this is only possible by using a two‐dimensional Lagrangian code that has been developed and that takes into account the strong deformation of the accelerated foil. Finally, even with the low level of x‐ray heating due to the ablation plasma, multihundred megabar pressures can be obtained within a very short time.