Description
Understanding how cosmic rays gain energy and scatter in astrophysical environments remains one of the central open questions in plasma astrophysics. We report on an experiment at the GSI Helmholtz Centre for Heavy Ion Research, in which a monoenergetic 450 MeV chromium ion beam was propagated through a magnetized plasma formed by the collision of two counter-propagating, laser-ablated flows. The plasma, diagnosed using interferometry and ion deflectometry, showed an interaction region largely free from fluid-scale turbulence, consistent with magnetohydrodynamic simulations performed with FLASH [Moczulski et al., Phys. Plasmas 31, 122105 (2024)]. Despite this, time-of-flight diagnostics revealed both velocity-space diffusion and bulk acceleration of the ion beam.
The absence of strong, fluid-scale turbulence rules out Fermi acceleration as the cause of the observed scattering and energization. An analytical treatment, constrained by experimental observations, suggests wave–particle scattering by short-wavelength electrostatic kinetic turbulence as an alternative explanation. A forthcoming beamtime campaign at GSI will directly probe these electrostatic fluctuations using collective optical Thomson scattering, providing a quantitative test of the proposed mechanism. These experiments establish a new laboratory platform for studying cosmic-ray-like transport and pre-acceleration in magnetized plasmas. See [arXiv:2509.07880] for initial results.