Experimental Observation of Anomalous Stopping of Mega-ampere Electron Current in Porous Materials

Energy loss of fast electrons in materials is a fundamental process in many fields spanning fusion research, high-energy-density physics, astrophysics, and material science. Contrary to conventional collisional theory that predicts greater energy loss in denser materials, we experimentally observe an anomalous phenomenon: high-current-density electron beams undergo significantly stronger deflection and stopping in low-average-density porous foams than in their denser counterparts. Pore-resolved simulations and theoretical analysis demonstrate that return currents along the foam's skeleton generate multi-kilo-Tesla magnetic fields in the vacuum pores. These intense fields strongly scatter beam electrons, enhancing stopping by orders of magnitude beyond collisional predictions. The derived conditions for this anomalous stopping agree with experiments and simulations, establishing a new microstructure-mediated regime of beam-matter interaction with potential applications in charged-beam control for fusion and laboratory astrophysics.