Observation of spectral modulations in laser ion acceleration from underdense plasmas

Ions have been accelerated radially by irradiating a supersonic helium gas jet ($400\mu m$ diameter) with ultrashort, relativistic laser pulses ($I\lambda^{2}>10^{18}\mathrm{Wcm^{-2}\mu m^{2}}$). Accelerated ions have been measured over a wide range of densities ($0.01 n_{c} < n_{e} < 0.1 n_{c}$). Strong modulations in the ion spectra have been observed, depending on the interaction conditions. Whereas Monte-Carlo particle tracking indicates recombination of ions accelerated by Coulomb explosion, due to charge exchange with neutral gas, detailed 2D1/2 PIC simulations suggest the trapping and acceleration of ions from plasma edges by radially propagating waves. These results evidence new insights on the laser gas interaction in this density range.