Laser wakefield: Experimental study of nonlinear radial electron oscillations

The plasma electron density oscillation produced in the wake of a narrow (beam waist≪plasma wavelength) ultrashort laser pulse is measured by frequency-domain interferometry with a temporal resolution much better than the electron plasma period, and a spatial resolution across the laser focal spot. The absolute density perturbation is observed to be maximum when the pulse duration equals half the plasma 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 radial oscillations [J. M. Dawson, Phys. Rev. 113, 383 (1959)]. The damping of the oscillations is observed. It is very rapid (a few periods) when the oscillation is nonlinear. Comparison with the code WAKE [P. Mora and T. M. Antonsen, Jr., Phys. Rev. E 53, R2068 (1996)] indicates that the gas ionization creates a steep radial density gradient near the edge of the focus and that the electrons oscillating near this density gradient are responsible for the damping.