Stochastic Acceleration of Heavy Ions in a Magnetized and Turbulent Plasma

The exact mechanism that enables the acceleration of highly energetic charged particles in the Universe, cosmic rays (CR), remains controversial. Although many processes may result in CR acceleration, turbulence is generally accepted to be essential to energizing the ions and electrons in the interstellar medium. Indeed, the original mechanism of CR acceleration proposed by Fermi theorised that energetic charged particles gain energy in random scattering events with magnetized clouds. Given that the standard origin mechanism for these B fields is the turbulent dynamo mechanism, clearly a key process governing CR acceleration is related to how charged particles interact with stochastic B fields embedded in turbulent plasma. We have performed an experiment at GSI to investigate the interaction of fast heavy ions and turbulent magnetized plasma. Two opposing plastic targets, with textured surfaces, were laser-driven such that ablated plasma collided in the central region, creating a turbulent, magnetized plasma. As this occurs, collimated pulses of ions from UNILAC traverse the central region and the change in their energy profile is extracted from their time-of-flight (ToF). Our ToF data shows that the mean energy of ion pulses crossing the turbulent magnetized plasma increases. Our experimental results are supported by 3D magneto-hydrodynamics simulations.