A direct measurement of a relativistic pair-plasma beam instability at The HiRadMat Facility (CERN)*

Relativistic electron-positron plasmas are believed to occur in extreme astrophysical environments such as the jets from black-holes and neutron-star magnetospheres. Their behaviour is quite different from typical electron-ion plasmas due to matter-antimatter symmetry and they are believed to be fundamental in determining the dynamics of such astrophysical systems.

In experiments performed in 2023, we demonstrated the first experimental realisation of relativistic electron-positron pair plasmas [1], driven by the 440 GeV/c proton beam at CERN’s HiRadMat Facility [2]. The pair-plasma beam was directed through an inductive plasma discharge to explore kinetic instabilities mediating its interaction with an ambient plasma.

In our previous work, limited diagnostic sensitivity meant we were unable to directly measure signatures of kinetic instabilities. This presentation includes new results from a follow-up experiment in which we fielded a refined in-crystal Faraday rotation instrument. This enabled the direct diagnosis of magnetic field amplification by the filamentation instability. To our knowledge this is the first direct measurement of a pair beam instability in the laboratory.

[1] Arrowsmith, et al. Laboratory realization of relativistic pair-plasma beams. Nat Commun 15, 5029 (2024).

[2] I. Efthymiopoulos et al. (2011) “HiRadMat: A new Irradiation Facility for Material Testing at CERN” Proc. 2nd Int. Particle Accelerator Conf. (IPAC’11).