Laboratory evidence for proton energization by collisionless shock\n surfing

Abstract

Charged particles can be accelerated to high energies by collisionless shock\nwaves in astrophysical environments, such as supernova remnants. By interacting\nwith the magnetized ambient medium, these shocks can transfer energy to\nparticles. Despite increasing efforts in the characterization of these shocks\nfrom satellite measurements at the Earth's bow shock and powerful numerical\nsimulations, the underlying acceleration mechanism or a combination thereof is\nstill widely debated. Here, we show that astrophysically relevant\nsuper-critical quasi-perpendicular magnetized collisionless shocks can be\nproduced and characterized in the laboratory. We observe characteristics of\nsuper-criticality in the shock profile as well as the energization of protons\npicked up from the ambient gas to hundreds of keV. Kinetic simulations\nmodelling the laboratory experiment identified shock surfing as the proton\nacceleration mechanism. Our observations not only provide the direct evidence\nof early stage ion energization by collisionless shocks, but they also\nhighlight the role this particular mechanism plays in energizing ambient ions\nto feed further stages of acceleration. Furthermore, our results open the door\nto future laboratory experiments investigating the possible transition to other\nmechanisms, when increasing the magnetic field strength, or the effect induced\nshock front ripples could have on acceleration processes.\n