Self-focused transport of a high ν/γ electron beam for materials surface modification

Experimental and computational studies have been performed to examine the dynamics of self-focused transport of a 0.5 MeV, 18 kA cylindrical electron beam. Beam dynamics are complicated by the fact that the beam voltage and current required for the application means the Alfven limit is approached (ν/γ ∼ 0.7). The goal of the study is to passively transport and sufficiently focus the beam to induce changes in the surface layer (50 – 500 µm) of various metals. The electron beam is produced in a vacuum diode at a beam radius where the resultant current density (∼120 A/cm 2 ) is compatible with anode foil lifetime requirements. Beam transport and focusing is done in a low-pressure, neutral gas-filled, conducting tube in the ion-focused regime 1 . Here, beam space charge forces are neutralized as a function of time to allow progressive focusing of the beam by its self-magnetic field. The electron beam evolves to a smaller radius, higher current density distribution capable of bringing the surface of the metals to near melting temperatures. This interaction must take place far enough downstream of the anode foil to allow front-surface temperature diagnostics and to maintain anode foil integrity for many shots. Beam transport efficiency and envelope evolution are studied as a function of input emittance, neutral gas pressure, and transport distance. The beam current is measured with Rogowski coils, the beam current density with a vacuum Faraday cup, and the beam spatial profile with a gated, intensified camera. Computational studies are performed using LSP 2 to analyze the experiments. Measurements of the evolution of the beam radial profile agree well with the numerical predictions. Results of the experiments show that the half-current radius of the input electron beam can be reduced by a factor of about 2.84. This results in a current density of about 1 kA/cm 2 which is sufficient to produce desired surface effects in various metals.