Observation of the transverse modulation effect of two-stream instability on low-energy proton beam in low-density plasma

In the field of inertial confinement fusion and ion beam-driven high-energy-density physics, the energy deposition along the beam trajectory is a critical physical parameter. Beyond particle collisions, the collective effects of plasma can significantly affect ion transport. The development of two-stream instability induced by beam-plasma interaction plays a crucial role, as it simultaneously affects the beam structure and its energy transfer. This paper presents an experimental investigation into the transverse modulation effects with a 100 keV proton beam passing through low-density hydrogen plasma. The results demonstrate that the observed beam defocusing phenomenon can be attributed to the excitation of two-stream instability: the diffusion of returning electrons leads to the formation of a radial electric field, which ultimately induces the transverse divergence of the proton beam. Both the numerical simulations based on analytical theoretical methods and particle-in-cell simulations are quantitatively validated, yielding results consistent with the experimental observations. Furthermore, the particle-in-cell simulation results indicate that wave-particle interactions can lead to additional energy loss, which is more significant than collision term. Experimental investigations of the two-stream instability are essential for revealing its underlying dynamics, validating theoretical descriptions, and improving the reliability of the ion beam transport modeling in plasmas.