I. Crotch

High-Power Self-Pinch Diode Experiments for Radiographic Applications

We report here on self-magnetic-pinch diode experiments at voltages from 3.5 to 6 MV. In addition to electrical diagnostics, the diode is characterized as a radiation source by dose and spot-size measurement. As the operating voltage increases, we find that a given diode geometry tends to produce a smaller spot but suffers from the reduced impedance lifetime. Optimization involves increasing the cathode diameter and diode gap as the voltage increases. We find a good quantitative agreement with the Monte Carlo code integrated tiger series over the entire data set, assuming an effective electron incidence angle of 20deg. Over this range, we observe favorable dose and spot scaling of optimized diode performance with voltage. Our best results are roughly 200-rad at 1 m with an ~2-mm-diameter spot. These were obtained at diode parameters of roughly 6 MV, 150 kA, and 30-ns radiation full-width at half-maximum.

Self magnetic pinch diode experiments at AWE

For many years AWE has used self magnetic (SM) pinch diodes on its lower voltage flash X-ray machines (MEVEX @ 0.8 MV and Mini-B @ 2 MV). With the recent emphasis on reduction of the X-ray spot size, one part of the diode research project has been to field SM pinch diodes at higher voltages. A series of experiments has been conducted on the Mogul D and Eros pulsed power drivers to in an attempt to meet the HRF source term requirements. The charging voltage on Mogul D was increased to its operational maximum and 62R with a 2.1 mm spot (AWE definition) was produced at 4.2 MV. Further shots on the EROS driver failed to match the Mogul D results because of the 110 kV prepulse on EROS. MCNP has been used to calculate dose scaling as a function of charge and voltage for the SM pinch diode. This has shown that long term HRF dose goals are achievable with this diode.

Characterization of a self-magnetic-pinched diode

Self-magnetic-pinched diode behavior at 1.5-2 MV was diagnosed using a variety of electrical, radiation, and optical diagnostics. Results are compared with predictions of the LSP particle-in-cell code, and shown to be in good agreement. A practical diagnostic of electron incidence angles is demonstrated. A quadrature interferometer is shown to be capable of measuring the time-dependent position of the effective electrode-plasma boundaries. Both one-dimensional (1-D) and two-dimensional (2-D) interferometry show the importance of anode plasma expansion in such diodes with high anode-power concentration. Not only does the anode plasma contribute significantly to gap closure, but there is evidence that anode plasma expansion results in a distortion of the effective anode shape, which can significantly affect the diode performance.

Status of the AWE Hydrus IVA fabrication

The ten-module Hydrus Induction Voltage Adder, designed by L3 Communications - Pulse Sciences Division for AWE, builds on previous IVA experience in the US. Each of the ten modules comprises a 1.4 MV induction cell driven by a laser triggered gas switched Pulse Forming Line (PFL) in order to provide nanosecond order synchronisation, and hence excellent pulse reproducibility. The PFLs are charged by a single Marx through an oil-insulated transmission line. The outputs of the cells are added along a 22 metre long 80 ohm MITL to deliver an 11 MV forward going wave to the e-beam diode. The accelerator will be used for flash radiography by AWE utilising a Self Magnetic Pinch diode as the radiographic source. This diode operates at approximately 40 Ohms with the result that retrapping of the MITL sheath current occurs, reducing the diode voltage to ∼ 7.5 MV, but increasing the load current to 200 kA. The detailed PFL design was previously prototyped and has been chosen to tailor the output pulse to compensate for the SMP diodes intra pulse impedance reduction and hence generate a relatively constant voltage during generation of the X-ray flash. The components of the Hydrus IVA are approaching completion at L3 Pulse Sciences at San Leandro, CA. All parts of the IVA are being procured by L-3 PS and delivered to San Leandro for subassembly. The major IVA subassemblies being fabricated comprise the Marx, oil line, PFL, cell, and stalk. Ancillary systems being fabricated comprise the control software, vacuum, water processing, oil processing, magnetic core reset, gas processing, data acquisition, and power supply. Subassemblies and subsystems are subject to a variety of QA tests which include high voltage testing of the Marx and its trigger, and a first-article PFL driving both a dummy load and a first-article cell. The status of the in-progress fabrication and QA testing for each of the major subsystems is described in this paper. The complete system will not be assembled and tested in the US. All components of the IVA are to be delivered as subassemblies to AWE in the UK in mid 2012 for assembly and commissioning.

2-D simulations of the self magnetic pinch diode

For many years AWE has used self magnetic (SM) pinch diodes on its lower voltage flash x-ray machines (MEVEX @0.8 MV and Mini-B @ 2 MV). The SM pinch diode has proved to be a reliable source for providing small diameter radiographic spot sizes (2.0 - 2.5 mm). With the recent emphasis on reduction of the x-ray spot size at higher voltages, one part of the diode research project has been to field SM pinch diodes at higher voltages. An electromagnetic PIC code, LSP, has been used to carry out 2-D simulations of the diode to support this project. Results of these code simulations will be presented. The simulations show good agreement with measured experimental machine voltage and current records. Good correlation is also achieved with old experimental data on diode performance. The simulations suggest further improvements in spot size reduction could be achieved with changes in the diode geometry.

Studies into the Time Resolved Source Diameter of a Self Magnetic Pinch Radiographic Diode

The self magnetic pinch diode is being considered as a possible source for future high voltage (10MV) flash radiographic systems being developed. Previous studies at both the Atomic Weapons Establishment (AWE) Aldermaston in the UK and Sandia National Labs (SNL) in the US have investigated the time integrated dose and source diameter produced by the diode. Presented here are results from investigations into the dynamic time history of the source diameter. Experimental investigations have been carried out at both AWE and SNL with the image of the source projected onto fast decay scintilators viewed by nanosecond gating and streak cameras. Experimental measurements are compared with results from 2D and 3D electromagnetic particle in cell codes.

High-power self-pinch diode experiments for radiographic applications

Summary form only given. The self-magnetic-pinch [SMP] diode is a high impedance (~40-Ohm), low R/D (typically 4 mm/8 mm) pinched-beam diode that shows promise for high-power X-radiography. The scaling of this diode to higher voltage (and thus power) is critical to the development of next generation radiographic sources. We report here on SMP experiments on the NRL Mercury generator at voltages from 3.5-6 MV and impedances from 35-50 Ohms. Measurements include diode electrical behavior, time-integrated and time-resolved X-ray dose, and time integrated radiographic spot size. We have studied the effects of several variations in electrode geometry, surface coating, gap; and the level of machine prepulse. Extensive modeling using the Sandia ITS codes is used to help interpret the X-ray dose and spot measurements. As the operating voltage increases, we find that a given diode tends to produce a smaller spot but also suffer reduced impedance lifetime, and optimization involves increasing the cathode diameter and diode gap as the voltage increases. We find good quantitative agreement with ITS predictions over the entire data set, assuming an electron incidence angle of 20 degrees. This gives a dose rate that scales (over the range examined) as IV2.2. Over this range, we observe favorable scaling of optimized diode performance with voltage, with good dose scaling and a slight spot size decrease with voltage. Our best results comprise roughly 200 rads at 1 meter with a ~2 mm diameter spot

Further work on the implementation of rod pinch diodes on negative polarity pulse power drivers

Recent work by the AWE Pulsed Power Group has been to implement a rod pinch diode on some of the UK lower voltage (800kV to 4MV) negative polarity pulse power drivers. The rod pinch diode is normally fielded on positive polarity drivers which eliminates stray emission from the cathode emitter to the surrounding support structure. All the UK radiographic machines operate in negative polarity and cannot readily be switched to positive. This has consequences for electron trajectories, potential loss current paths and x-ray photon attenuation in the anode support structure. Although successful in its implementation, the diode performance is partly limited by the x-ray photon attenuation in the anode support structure. This poster reports on the further work undertaken in implementing the existing diode design on higher voltage drivers as well as experiments with an alternative diode designed to overcome attenuation from the anode structure.

Overview of self-magnetic pinch diode investigations on RITS-6

The self-magnetic pinch diode is currently fielded on the RITS-6 accelerator at Sandia National Laboratories operating between 7-12 MV and is the leading candidate for future radiographic source development at the Atomic Weapons Establishment. The diode is capable of producing sub 3-mm radiation spot sizes and greater than 350 Rads measured at 1m. Complex physical processes affect the diode operation which in turn may affect its radiographic potential. High-resolution, time-resolved diagnostics have been utilized to help quantify the diode physics which include plasma spectroscopy, gated imaging, X-ray p-i-n diodes, spot size, and diode current measurements. The data from these diagnostics are also used to benchmark particle-in-cell simulations in order to better understand the underlying physics of operation. An overview of these experiments and simulations including future plans is presented.

Status of self-magnetic pinch diode investigations on RITS-6

The electron beam-driven self-magnetic pinch diode is presently fielded on the RITS-6 accelerator at Sandia National Laboratories and is a leading candidate for future flash x-ray radiographic sources. The diode is capable of producing sub 3-mm radiation spot sizes and greater than 350 rads measured at 1 m from the x-ray source. While RITS-6 is capable of delivering up to 11.5 MV using a magnetically insulated transmission line (MITL), the diode typically operates between 6 – 7 MV. Because the radiation dose has a power-law dependence on diode voltage, this limits the dose production on RITS. Coupling this low-impedance (∼ 40–60 ohms) diode to a MITL with similar or higher impedance affects its radiographic potential. The sensitivity in diode operation is compounded by the interaction of evolving plasmas from the cathode and anode, which seem to limit stable diode operation to a narrow regime. To better quantify the diode physics, high-resolution, time-resolved diagnostics have been utilized which include plasma spectroscopy, fast-gated imaging, x-ray p-i-n diodes, x-ray spot size, and diode and accelerator current measurements. Data from these diagnostics are also used to benchmark particle-in-cell simulations in order to better understand the underlying diode physics. An overview of these experiments and simulations is presented.