Multifoil scattering and transport of an intense relativistic electron beam after foil scattering and ionized gas focusing

Intense relativistic electron beams such as those produced by the DARHT accelerators, consist of large total currents of 1.6-2 kA at modest energies in the range of 16-20 MeV. Beam envelope codes are generally used to predict the evolution of the first moment of the radial distribution (i.e. beam 2-RMS radius) assuming a constant emittance. Upon passing through a thin metal foil, such as may be used in a vacuum window, the beam experiences multiple Coulomb scattering and foil focusing effects which modify downstream transport. We measure the 4-RMS emittance of the beam with and without a material scattering foil using the solenoid sweep method with a downstream magnet. A 50 um Ti foil and 25.4 um Al foil are used to increase the emittance from about 0.09 cm-rad to as much as 2.9 cm-rad after 1.5 m of transport. We also make measurements with aluminum scattering foils 1.5 m upstream and 3 cm upstream of the imaging foil to examine the beam dynamics after scattering. Simulations at elevated pressure show that the beam is already neutralized by the proximity to the foils and so plasma transport does not cause significant pinching (~5%). We find envelope codes are able to describe the 2-RMS radius of the beam after foil interaction taking into account appropriate weights on the scattering angle after 1.5 m of transport.