Longitudinal Ion Acceleration From High-Intensity Laser Interactions With Underdense Plasma

Longitudinal ion acceleration from high-intensity (I~10
²⁰
Wcm
^-2
) laser interactions with helium gas jet targets (n
_e
ap0.04 n
_c
) has been observed. The ion beam has a maximum energy for He
²⁺
of (40
_-8

⁺³
) MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10deg. Two-dimensional particle-in-cell simulations have been used to investigate the acceleration mechanism. The time-varying magnetic field associated with the fast electron current provides a contribution to the accelerating electric field as well as a collimating field for the ions. A strong correlation between the plasma density and the ion acceleration was found. A short plasma scale length at the vacuum interface was observed to be beneficial for the maximum ion energies, but the collimation appears to be improved with longer scale lengths due to enhanced magnetic fields in the ramp acceleration region.