Focusing methods for drifting relativistic electron beams for radiography at AWE

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. Analysis of experimental data from spot-size reduction and electron-beam propagation experiments on E-minor is presented. Analytical results, and computational results using the IPROP computer code, are presented for various experimental configurations including ballistic focusing, {lambda}/4 focusing, and cone focusing. In addition, possible methods for improving the spot size are proposed, in relation to all six transport/focus methods discussed above.