Long plasma channels formed by axicon-focused filaments
Résumé
We show that axicon-focusing of ultrashort laser pulses facilitates the generation of filaments with specific spatial profiles in the form of nonlinear Bessel beams. For conditions ensuring that the energy arriving from each Bessel ring is in quasi equilibrium with that absorbed by multiphoton processes, these Bessel filaments generate meter long plasma channels whereas the same pulses focused by a standard lenses generate plasma channels which do not exceed a few centimeters long. Measurements with different lasers show that the length and the homogeneity of the plasma channels are enhanced by the use of large beams and sharp-tip (ideal) axicons whereas blunt-tip axicons induce a lens effect leading to oscillations of the plasma density along the propagation axis.