Observation of strong self-focusing of an intense relativistic electron beam impinging on a SiO2 target

A substantial reduction in the spot size of an electron beam striking a SiO₂ target has been observed during a 60 ns pulse. We attribute this to the emission of positive ions, most probably protons, from the target. These are then accelerated upstream and focused along the axis by the electron beam, where they partially neutralize the space-charge of the beam, and produce a sharp pinch. A simulation of the effect with a particle-in-cell code provides an adequate quantitative description, and a simple model, based on space-charge-limited emission of protons by the target confirms qualitatively the results of the simulation. The presence of an aluminum foil on the upstream side of the target is shown to suppress the effect, probably by stopping the protons