Focusing methods for drifting relativistic electron beams for radiography at Atomic Weapons Establishment

Summary form only given. Minimization of the radiographic spot size is a continuing long-range goal for development of pulsed-power X-ray radiography sources. In these devices, a high current (10´s of kA´s), high voltage (multi-MV) electron beam is created in a vacuum diode. X-ray radiation is produced when the electron beam impinges on a high-Z target. The target may be located directly on the anode, or the anode may be a thin metallic foil through which the beam passes; in the latter case, the beam enters into a transport/focus region, is focused, and then impinges on a target assembly. It is the purpose of this paper to investigate several transport/focus methods and compare them with experimental data obtained with a paraxial diode on the E-Minor Blumlein machine at AWE. Transport/focus methods assessed here include: (1) ballistic focusing in a gas cell (with essentially complete charge and current neutralization), (2) /spl lambda//4 focusing in a gas cell with only partial current neutralization, (3) cone focusing in a gas-filled tapered metallic cone, (4) pressure-gradient focusing in a gas-filled drift tube with an axial pressure gradient, (5) foil focusing, in which one or more foils in vacuum are used to short out the beam´s radial electric field, and (6) a tapered z-discharge, in which a preformed current-carrying axial discharge produces a focusing effect. Each method is analyzed and assessed for radiographic spot size optimization.