The potential of the paraxial diode as a focussing element for X-radiography

Summary form only given. The paraxial diode is one of several candidate diodes being investigated to meet the X-radiography requirements of the HRF at AWE. The short term requirement is to produce a 5 mm X-ray source with a dose of 600 Rads at 1 metre. To help meet this requirement the paraxial diode is currently being studied in detail with the aid of the pic code Lsp. The paraxial diode is a gas filled focussing cell which transports and focusses a relativistic electron beam to a high atomic target producing bremsstrahlung radiation. The diode typically generates currents of the order of 30 kA at voltages up to 10 MeV. The diode acts as a 1/4 betatron focussing element with a focal length F proportional to the square root of the beam energy and inversely proportional to the square root of the net current in the gas cell F /spl prop/ (/spl gamma//I/sub net/)/sup 1/2/. The time integrated radiation spot is determined both by finite beam emittance and by focal sweeping due to variations in net current. The Lsp code was used to calculate the electron beam time integrated spot, dose, mean angle, focal plane and emittance. Lsp calculations for the spot, dose and focal plane compare well with experimental data obtained from a variety of diodes fielded at the Atomic Weapons Establishment and from diodes fielded on the RITS3 accelerator at Sandia National Laboratories. Suggestions to improve the X-ray intensity are presented.