Collisionless shocks are common phenomena in space physics, astrophysics and inertial confinement fusion, and have long been an important fundamental research topic in the field of plasma. Limited by space observation methods and numerical simulation technologies, there is still a lack of systematic research on the pre-acceleration process of particles in magnetized shocks. Fermi acceleration is believed to be the primary mechanism for producing high - energy charged particles in the Universe. In this process, charged particles gain energy successively through multiple reflections in the collisionless shock. Here, we present direct laboratory experimental evidence of ion energization in a supercritical collisionless shock. A quasi - monoenergetic ion beam with a velocity two to four times that of the shock was observed, which is consistent with the fast ion component observed in the Earth's bow shock. Our simulations reproduced the energy gain and showed that ions were accelerated mainly by the motional electric field during reflection. The results identify shock drift acceleration as the dominant ion acceleration mechanism, which is consistent with satellite observations in the Earth's bow shock. Our observations pave the way for laboratory investigations of cosmic accelerators and are also beneficial to laser fusion and laser - driven ion accelerators.
 

collisionless shocks,ion acceleration,laser plasma