H/sup +/ emission at an electron-photon conversion target

Summary form only given. A high-intensity (3 kA) relativistic (6 MeV) electron beam is focused (during 65 ns) on a tantalum target and a large fraction of the incident energy is converted into photons via the bremsstrahlung process. The X-ray emission, directed inside a cone centered along the electron beam axis, is used for radiographic purposes. The energy deposited by the beam focused on a 2 mm diameter spot size, causes a local target vaporisation and production of ions. A beam charge neutralisation then occurs, cancelling the average radial electrical field. Therefore, the beam pinches due to the remaining azimuthal magnetic field. The ions are subsequently accelerated downstream in the strong axial electric field induced by the electron beam. Thus, the displacement of the waist from the target along with the ions leads to an increase of the spot size diameter /spl phi/, at the target and would result in a degradation of the sharpness of the radiographic image. The ions involved are H/sup +/ light ions, issued from hydrogen atoms contaminating the target because they are much more efficient to defocus the beam on this time scale, in reason of their greater velocity, as opposed to Ta/sup +/ heavy ions constituting the target material. The H/sup +/ ions where experimentally identified on the PIVAIR accelerator prototype, with a time-of flight diagnostic.