The 6th International Conference on Matter and Radiation at Extremes (ICMRE2023)

Laser-driven ion sources with unique features, such as small device size and high brightness, are useful in radiography, warm-dense-matter generation, fast ignition, etc. Target normal sheath acceleration (TNSA) mechanism has attracted wide research interest due to its robustness in terms of laser-target parameters. It suggests that the cutoff-energy of TNSA protons is proportional to \(\sqrt{n_eT_e}\), where \(n_e\) and \(T_e\) are the hot electron density and temperature, respectively. However, since hot electrons are usually generated from skin layers of the target’s front surface by direct laser action, both \(n_e\) and \(T_e\) are rather unsatisfactory. The energy of TNSA protons needs further improvement. Furthermore, the expansion of the sheath field due to divergent hot electrons can undesirably increase proton divergence.

To address the above problem, we propose a new scheme for the generation of high-energy collimated proton beams by irradiating micro-pillar targets with intense circularly-polarized lasers. The electrons at the outer surface of the target can be accelerated by the laser-excited surface plasma wave (SPW). During the acceleration, the angular momentum of the laser pulse is effectively transferred to the in-phase electrons, leading to the formation of dense electron vortex. As a result, gigagauss axial magnetic field is induced at the target rear. Such strong magnetic field can effectively confine subsequent hot electrons and significantly enhance the sheath field and associated proton quality. Three-dimensional particle-in-cell simulations show that for femtosecond lasers at ~\(10^{22}\) W/cm\(^2\), the sheath electron is about critical density with an effective temperature up to ~70 MeV. The cut-off energy of the TNSA proton is above 140 MeV with a divergence angle as low as 3°. Our work presents a new approach for producing high-quality proton beams that should be useful in many areas.