Enhancement of target normal sheath acceleration in laser multi-channel target interaction

Target-normal sheath acceleration (TNSA) of ions by >100-fs relativistic laser pulses irradiating a multichannel target consisting of a row of parallel long wires and a plane back foil is studied. Two-dimensional particle-in-cell simulations show that the laser light pulls out from the wires a large number of dense hot attosecond electron bunches, which are synergetically accelerated forward by the relativistic ponderomotive force of the laser as well as the longitudinal electric field of a transverse magnetic mode that is excited in the vacuum channels between the wires. These electrons are characterized by a distinct two-temperature energy spectrum, with the temperature of the more energetic electrons close to twice the ponderomotive potential energy. After penetrating through the foil, they induce behind its rear surface a sheath electric field that is both stronger and frontally more extended than that without the channels. As a result, the TNSA ions have much higher maximum energy and the laser-to-ion energy conversion efficiency is also much higher. It is found that a laser of intensity 1.37 x 10(20) W/cm(2), duration 165 fs, and energy 25.6 J can produce 85 MeV protons and 31 MeV/u carbon ions, at 30% laser-to-ion energy conversion efficiency. The effects of the channel size and laser polarization on the TNSA ions are also investigated.