NUMERICAL-SIMULATION OF LIMITING CURRENTS FOR TRANSPORT OF INTENSE RELATIVISTIC ELECTRON-BEAMS IN CONDUCTING WAWEGUIDES

The space-charge limiting current for an intense, magnetized, relativistic electron beam injected into a grounded metallic pipe is investigated with a 2(1/2)-dimensional particle-in-cell code. Comparisons between the simulation results, the well known Bogdankevich-Rukhadze limiting current, and more recent theoretical estimates of the limiting current are presented. Transmitted currents > 15% above those predicted by the Bogdankevich-Rukhadze and other limiting current estimates are observed. However, good agreement between the simulation results and the analytic estimates of Uhm [Phys. Fluids B 5, 1919 (1993)] and Fessenden [Lawrence Livermore Lab. Rep. No. UCID-16527 (1974)] is found. For an injected current above the limiting value, a virtual cathode is formed which alters the transmitted current density profile of the beam. A theoretical estimate of the magnitude of the transmitted current under this condition is compared with simulation results. The predicted transmitted current is found to be valid only for injected currents slightly above the limiting current. In addition, the transition between vacuum and ion-focused-regime transport, with and without an applied axial magnetic field is simulated. For ion-focus-regime densities (n(p) similar to n(b)), the effect of the virtual cathode in limiting the beam transmission is greatly diminished as expected. (C) 1995 American Institute of Physics.