New results mark progress towards polarized ion beams in laser-induced acceleration

The field of laser-induced relativistic plasmas and, in particular, laser-driven particle acceleration, has undergone impressive progress in recent years. Despite many advances in understanding fundamental physical phenomena, one unexplored issue is how the particle spins are influenced by the huge magnetic fields inherently present in the plasmas.

Laser-induced generation of polarized-ion beams would without doubt be important in research at particle accelerators. In this context, ³He²⁺ ions have been discussed widely. They can serve as a substitute for polarized neutron beams, because in a ³He nucleus the two protons have opposite spin directions, so the spin of the nucleus is carried by the neutron. However, such beams are currently not available owing to a lack of corresponding ion sources. A promising approach for a laser-based ion source would be to use pre-polarized ³He gas as the target material. Polarization conservation of ³He ions in plasmas is also crucial for the feasibility of proposals aiming at an increase in efficiency of fusion reactors by using polarized fuel, because this efficiency depends strongly on the cross-section of the fusion reactions.

A group from Forschungszentrum Jülich (FZJ) and Heinrich-Heine University Düsseldorf has developed a method to measure the degree of polarization for laser-accelerated proton and ion beams. In a first experiment at the Arcturus Laser facility, protons of a few million electron volts – generated most easily by using thin foil targets – were used to measure the differential cross-section d²σ/dϑdφ of the Si(p, p´)Si reaction in a secondary scattering target. The result for the dependence on scattering angle is in excellent agreement with existing data, demonstrating the feasibility of a classical accelerator measurement with a laser-driven particle source.

The azimuthal-angle (φ) dependence of the scattering distributions allowed the degree of polarization of the laser-accelerated protons to be determined for the first time. As expected from computer simulations for the given target configuration, the data are consistent with an unpolarized beam. This “negative” result indicates that the particle spins are not affected by the strong magnetic fields and field gradients in the plasma. This is promising for future measurements using pre-polarized targets, which are underway at Arcturus.

The polarization measurements are also an important step towards JuSPARC, the Jülich Short Pulse Particle and Radiation Centre at FZJ. This proposed laser facility will provide not only polarized beams but also intense X-ray and thermal neutron pulses to users from different fields of fundamental and applied research.