Stochastic Ion Acceleration in a Laser-driven Turbulent Plasma at the GSI Helmholtz Centre for Heavy Ion Research

We present results showing direct measurement of particle acceleration in an astrophysically relevant, laser-driven laboratory experiment. Ultra-relativistic particle acceleration is a longstanding astrophysical problem that remains a subject of intense study. A candidate explanation for the observed cosmic ray (CR) flux is 2^nd order Fermi acceleration, whereby charged particles gain energy through random scattering events with turbulent magnetized clouds in the interstellar medium. Although less efficient than its 1^st order counterpart, it is nonetheless expected to produce a significant modification to the CR spectrum due to the ubiquity of turbulence in our universe.

We performed an experiment to investigate the acceleration and diffusion of ions in a turbulent magnetized plasma, employing the unique capability of the GSI facility of delivering both a monoenergetic heavy ion beam and sufficiently high laser energies. Two opposing plastic foils with textured surfaces were laser-driven to produce a turbulent magnetized plasma in the colliding region. A pulsed beam of chromium ions from the UNILAC then traversed the interaction region before hitting a time-of-flight (ToF) diamond detector, which was used to extract the energy profile of each pulse. The ToF data shows evidence of an acceleration of the ion pulse coincident with the presence of plasma. To understand the underlying physics of the acceleration, various optical diagnostics were also fielded to characterize the turbulent plasma.