Density Profile Steepening by the Ponderomotive Force of an Intense Picosecond Laser,

We report theoretical and experimental results indicating that the ponderomotive force of a high-intensity laser significantly modifies the thermal expansion of a laser-plasma in the regime where the density scale length is much less than the laser wavelength. Our theoretical calculations predict that the effect should become important when the average electron quiver energy mv sub os sq.2 becomes comparable to the electron thermal energy kTe. Doppler measurements of 0.053 microns light reflected from a solid target of aluminum agree with this prediction. After at ime equal to a laser pulsewidth 1 ps, at an incident laser intensity of 2 x 10 to the 15th Wsq. cm., and a peak electron temperature of 200 eV, the critical surface expands at only approximately 30 of the velocity it would if it were driven by free expansion due to thermal pressure alone. Agreement is found with a hydrodynamic numerical code using similar input parameters.