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.