New Study Finds Potential for Spin-Polarised Beams in Plasma Acceleration

Spin-polarised particle beams have long been used in particle and nuclear physics as a means of testing the Standard Model and studying hadronic resonances. Traditionally, these beams were produced using radio-frequency-based accelerators. However, new research shows that laser-plasma interactions and beam-driven plasma acceleration could offer a more efficient alternative, capable of generating high-energy particle beams over shorter distances.

A decade ago, a group of researchers from Forschungszentrum Jülich and Heinrich-Heine University Düsseldorf in Germany proposed a concept for producing highly polarised electron, proton, or ion beams through plasma acceleration. Their method involved aligning the spins of the particles before plasma formation, a seemingly straightforward idea that presented numerous technical challenges. Maintaining and utilizing spin alignment in a plasma environment, known for its high temperatures, required careful consideration.

In 2020, a theoretical study examined the depolarisation times and scalability of polarised particle acceleration in strong plasma fields. Numerous numerical simulations indicated that polarised beams from plasma acceleration could be within reach, with hadron beams offering the simplest implementation due to their smaller magnetic moments. Polarised nuclei were also found to be easier to obtain than polarised electrons.

To provide evidence for the persistence of nuclear polarisation after plasma acceleration, the Jülich-Düsseldorf group conducted an experiment at the PHELIX petawatt laser at GSI Darmstadt. They utilized a 50% polarised 3He gas-jet target, which was irradiated by 2.2 ps laser pulses with approximately 50 J of energy. Two polarimeters, optimized for short ion bunches from plasma acceleration, were used to measure the polarisation of the accelerated 3He ions. When the nuclear spins in the target gas were aligned perpendicular to the flight direction of the helium ions, an angular asymmetry of the scattered particles in the polarimeters confirmed transversal polarisation. No such asymmetries were observed for the unpolarised gas.

The researchers intend to repeat the experiments at PHELIX using higher gas polarisation and a shorter gas-jet target. This adjustment would result in 3He ions dominantly emitted in the direction of the laser beam, at significantly higher energies between 10-15 MeV. The team has even proposed a scheme based on shock acceleration, which could potentially produce polarised 3He beams with energies exceeding 100 MeV using laser intensities greater than 10 PW. They are also developing a polarised hydrogen-chloride gas target for the laser- or beam-driven acceleration of polarised proton and electron beams.