Ion acceleration in underdense plasmas by ultra-short laser pulses

We report on the ion acceleration mechanisms that occur during the interaction of an intense and ultrashort laser pulse ( I lambda(2) > 10(18) W cm(-2) mu m(2)) with an underdense helium plasma produced from an ionized gas jet target. In this unexplored regime, where the laser pulse duration is comparable to the inverse of the electron plasma frequency omega(pe), reproducible non-thermal ion bunches have been measured in the radial direction. The two He ion charge states present energy distributions with cutoff energies between 150 and 200 keV, and a striking energy gap around 50 keV appearing consistently for all the shots in a given density range. Fully electromagnetic particle-in-cell simulations explain the experimental behaviors. The acceleration results from a combination of target normal sheath acceleration and Coulomb explosion of a filament formed around the laser pulse propagation axis.