G. Cooper

Intense electron beam sources for flash radiography

High intensity pulsed electron beams are used to create bremsstrahlung x-ray sources for flash radiographic interrogation of dynamic experiments. Typical industrial sources operate below 200 GW/cm2 intensities, while experimental requirements can demand above 50 TW/cm2. Recent developments in pulsed power-driven high intensity electron beam systems have significantly increased these operating regimes, demonstrating 20 TW/cm2, and computations predict successful extrapolation to higher intensities. Detailed studies of electron beam configurations, both theoretical and experimental, and the prognosis for each to increase to the required levels is discussed.

Recent immersed Bz x-ray diode experiments

The immersed Bz diode is being fielded on one of the AWE Superswarf machines which provides a 55ns, 5.5MV, 35kA electron beam. The external magnetic field, up to 25Tesla, is produced by a solenoid which is driven by a 624juF, 22kV capacitor bank. The magnetic field constrains the electron beam to a small diameter at the target which results in a small x-ray source size.

Development of the Self Magnetic Pinch Diode as a High Brightness Radiographic Source

Summary form only given. The self magnetic pinch (SMP) diode has been developed from a low voltage (<2 MV) to a high voltage (7-8 MV) radiographic source as part of a program to build a hydrodynamics research facility (Hydrus) at AWE Aldermaston. Development of the initial AWE diode design has used the facilities and assistance of a number of UK and US laboratories and companies to carry out both experimental and theoretical investigations into the operation of the diode. Experimental campaigns have been carried out to both investigate the physics operation and to demonstrate performance on the RITS-3 and RITS-6 drivers at SNL, the Mercury and Gamble II drivers at NRL and the Mogul D and EROS drivers at AWE. Modelling of the diode has been carried out mainly using the particle in cell (PiC) code LsP. It has been demonstrated that the diode has achieved the required radiographic performance for Hydros and it will be used as the initial operational diode when the facility is commissioned. Research to develop a long term diode with improved radiographic performance is still continuing.

Recent paraxial diode experiments on RITS-6

Summary form only given. Development of intense electron beam-driven diodes for flash X-ray radiography is being carried out at 7.5-12 MV on the RITS-6 accelerator at Sandia National Laboratories. One of several diodes under investigation is the paraxial diode, which employs a gas-filled transport cell to focus an electron beam onto a high-atomic-number target to generate X-rays. Three key objectives are to produce a small spot size, <5 mm, high forward-directed dose, >600 rads at 1 m, and efficiently couple this relatively high-impedance diode to the lower-impedance RITS-6 accelerator. Particle-in-cell (PIC) simulations have shown that the primary limitation in spot size is due to the finite decay of the plasma return current which causes the beam focal location to sweep axially during the timescale of the pulse, hence leading to an increasing spot size. Time-resolved measurements of the spot size which convey this trend are reported. Interpretation of these results was aided by PIC simulations in which additional physics was included. Other outstanding issues for the paraxial diode on RITS-6 are also presented which include electron powerflow coupling to the diode, cavity modes which may affect focusing, and gas-breakdown models. Measurements of dose, dose rate, time-integrated (and time-resolved) spot size, and current are reported for experimental results at 7.5 and 10.5 MV.

Initial findings for paraxial diode shielding studies on RITS-3

Summary form only given. The paraxial diode has been identified as one of the potential candidates capable of reaching the short-term radiographic source requirement for the proposed Hydrodynamic Research Facility at AWE. The off axis dose produced in the AWE paraxial diode is partly attenuated by a local Tungsten shielding cone. Shielding requirements to minimise the off axis dose reaching detectors has been calculated for the paraxial diode at 14 MeV, which suggest a minimum areal mass of 350g/cm/sup 2/ (17 cm of Tungsten) is required. This thickness is required at all angles from the X-ray source outside of the main collimated beam. Clearly, it would be beneficial to place as much of the shielding as possible around the X-ray source. The simplest way to achieve this is to increase the angle of the shielding cone (45/spl deg/ for Mogul E), however, this is likely to alter the electric field distribution in the A-K region and possibly the electron beam trajectories at the anode foil plane. This paper describes the work carried out on RITS-3 to determine if increasing the diode shielding cone angle affects the performance of the diode.