LASER WAKEFIELD - EXPERIMENTAL-STUDY OF NONLINEAR RADIAL ELECTRON OSCILLATIONS

The plasma electron density oscillation produced in the wake of a narr ow (beam waist much less than plasma wavelength) ultrashort laser puls e is measured by frequency-domain interferometry with a temporal resol ution much better than the electron plasma period, and a spatial resol ution across the laser focal spot. The absolute density perturbation i s observed to be maximum when the pulse duration equals half the plasm a period. The relative density perturbation varies from a few percent at high density to 100% at low density. For nonlinear oscillations we measure the increase of the electron plasma frequency predicted for ra dial oscillations [J. M. Dawson, Phys. Rev. 113, 383 (1959)]. The damp ing of the oscillations is observed. It is very rapid (a few periods) when the oscillation is nonlinear. Comparison with the code WAKE [P. M ora and T. M. Antonsen, Jr., Phys. Rev. E 53, R2068 (1996)] indicates that the gas ionization creates a steep radial density gradient near t he edge of the focus and that the electrons oscillating near this dens ity gradient are responsible for the damping. (C) 1998 American Instit ute of Physics.