J. McLean

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.

Design of a replacement Marx generator for the Mevex x-ray machines

A replacement Marx generator for one of AWEs flash radiographic machines is required to mitigate component obsolescence. The design uses twelve 250 nF capacitors to charge a 14 nF Blumlein to 700 kV. For ease of maintenance the generator is removable from the oil tank by use of a rail system. Electrostatic codes have been used to design a new earthing system with access to the hidden capacitor terminals. The paper describes the design and the results of the Marx construction and high voltage commissioning into a resistive load.

Towards Optimization of the Paraxial Diode for X-Ray Radiography

The paraxial diode is a gas-filled focusing cell which is routinely used to transport high energy density electron beams for use in pulsed-power driven flash X-ray radiography experiments. Minimization of the radiographic spot size and maximization of the radiation dose is a continuing long-range goal for development of this diode. Suggestions for optimizing the radiographic utility of the paraxial diode are presented within the constraints of the physics of beam focal sweep and emittance growth.

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.

Paraxial diodes on RITS-3

The paraxial diode is the workhorse X-ray source at AWE for flash radiography of hydrodynamic experiments. It uses a gas filled cone to focus a 30-40 kA, 5 to 10 MV electron beam onto a tantalum target to produce a source 5-7 mm across. Future plans for a new hydrodynamics facility at AWE call for this diode to be fielded at a higher voltage on an inductive voltage adder (IVA) machine. The current machines at AWE are based upon single pulse forming line (SPFL) technology. To gain some experience with IVA technology and paraxial diodes, a series of shots were fired on RITS-3 at Sandia National Labs. Over 30 shots were fired on RITS-3 to optimise and investigate aspects of paraxial diode operation. The main concern with using RITS-3 was the excess current (/spl sim/110 kA) that would be dumped into the vacuum envelope and whether it would effect diode performance. The results that were obtained showed that this was not the case. On RITS-3 a best performance of 57R@1 m from a 5.7 mm spot was recorded, very similar to performance seen on comparable X-ray machines at AWE.