Strong self-focusing of a 7.2 MeV electron beam striking an aluminized mylar target

A substantial reduction in the spot size of a 7.2 MeV, 3 kA electron beam illuminating a thin aluminized mylar target has been observed during a 60 ns pulse. We attribute this to the emission of positive ions from the target, under the influence of the high energy flux of the beam, which perforates the target, These ions 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. After the pinch, the beam expands and forms filaments at the end of the pulse. By varying the initial beam transverse size, we have studied the influence of the beam density on the effect. A simulation of the effect with a particle-in-cell code provides a qualitative description and suggests that the ions responsible for the effect are Al⁺ and O⁺