J. Threadgold

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.

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.

Pulsed power active interrogation of shielded fissionable material

We report on a collaborative test campaign conducted at the Naval Research Laboratorys Mercury pulsed power facility in December of 2012. The experiment sought to use Mercury in the Intense Pulsed Active Detection (IPAD) [1] mode to interrogate a fissionable material target (depleted uranium, DU) and benchmark the effects of shielding the target with either a low-Z (2% borated high-density polyethylene, BPE) or high-Z (steel) material. A large suite of instrumentation, including 3He, BF3, NaI(Tl), and liquid scintillation detectors were used to measure the delayed γ and neutron signatures from the DU. The test campaign consisted of a series of single IPAD pulses, i.e., “shots,” employing incremental shielding configurations of BPE (up to 50 g/cm2) and steel (up to 150 g/cm2) encapsulating the DU target. We show the results from each detector array, for varying amounts of shielding, in terms of the signal-to-noise vs. time.

Critical Current Operation of the Optimized Self-Magnetic-Pinch Radiographic Diode

The self-magnetic-pinch (SMP) diode is being developed as an intense X-ray source for pulsed power driven radiographic systems. This diode is a critical current limited device described using equations derived from analytic approximations to large aspect ratio (cathode diameter to anode-cathode gap) variant diodes. The radiographic and large-aspect ratio variant of the SMP diode, differ in aspect ratio by around an order of magnitude. Derivation of the critical current limit for the large aspect ratio variant SMP diode requires a number of assumptions; not all of which are valid for the radiographic SMP diode. The applicability of existing critical current approximations to the radiographic SMP diode is discussed. A new critical current description for an optimized radiographic SMP diode configuration is proposed. This new critical current approximation is shown to account well for the radiographic performance (impedance and spot size) of optimized radiographic SMP diodes without the need for an experimentally derived empirical scaling factor.

Active Interrogation of Depleted Uranium Using a Single Pulse, High-Intensity Photon and Mixed Photon-Neutron Source

Active interrogation is a method used to enhance the likelihood of detection of shielded special nuclear material (SNM); an external source of radiation is used to interrogate a target and to stimulate fission within any SNM present. Radiation produced by the fission process can be detected and used to infer the presence of the SNM. The Atomic Weapons Establishment (AWE) and the Naval Research Laboratory (NRL) have carried out a joint experimental study into the use of single pulse, high-intensity sources of bremsstrahlung x-rays and D(\gammab, n)H photoneutrons in an active interrogation system. The source was operated in both x-ray-only and mixed x-ray/photoneutron modes, and was used to irradiate a depleted uranium (DU) target which was enclosed by up to 150 g·cm - 2 of steel shielding. Resulting radiation signatures were measured by a suite of over 80 detectors and the data used to characterise detectable fission signatures as a function of the areal mass of the shielding. This paper describes the work carried out and discusses data collected with 3He proportional counters, NaI(Tl) scintillators and Eljen EJ-309 liquid scintillators. Results with the x-ray-only source demonstrate detection ( > 3\sigmab) of the DU target through a minimum of 113 g·cm - 2 of steel, dropping to 85 g·cm- 2 when using a mixed x-ray/photoneutron source. The 3He proportional counters demonstrate detection ( > 3\sigmab) of the DU target through the maximum 149. 7 g·cm - 2 steel shielding deployed for both photon and mixed x-ray/photoneutron sources.

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.

Prompt sub-MeV neutron production from the 7Li(p,n)7Be reaction on mercury

Summary form only given. Proton-beam-generation experiments have been conducted on the NRL Mercury pulsed-power generator operating in positive polarity with a lithium metal target embedded in the cathode. The accelerating voltage was limited to below 2.7 MV in order to limit the energy of neutrons produced in the 7Li(p,n)7Be reaction (Q = 1.88 MeV) to below 1 MeV. Analyses based on published results1 and calculations presented here are used to predict the angular distribution of neutron yield and spectrum for each shot. Predicted neutron yields are compared to Rh-counter and Al-activation measurements. The results of these comparisons are quite encouraging, showing better than factor-of-2 agreement between the two sets of measurements and the analysis over the voltage range of the shot series. In order to achieve this level of agreement, a series of MCNPX computations has been carried out to determine the spectral contribution of neutrons reflected from the Mercury test-cell environment2 and the associated changes in detector calibrations. The agreement between measurements and modeling provides a check on the voltage calculated using a positive-polarity ion-diode model. For operation at 2.5–2.6 MV, on-axis neutron yields from the p-Li reaction are in the 1011 neutrons/steradian range.

Particle in cell modelling of the quantitative evolution of a pinched electron beam across the a-k gap of a self magnetic pinch diode

Summary form only given. Particle in Cell (PiC) simulations using the Large Scale Plasma (LSP) code of a Self Magnetic Pinch (SMP) diode have been carried out to investigate the electron beam conditions present in the SMP anode-cathode (a-k) gap. The probability distributions of radial charge, macro particle energy and macro particle velocity are described as a function of emission region and position across the a-k gap. The conclusions of these investigations are that the top surface of cathode contributes around 50% of the electron flow while the lower surface accounts for only around 5% with the remaining flow coming from the tapered cathode tip. The energy of the electron beam starts as a low energy peaked distribution at the start of the a-k gap and goes through a broad distribution to one with a high energy bias, retaining a low energy tail largely independent of emission site. The angles which the electrons have across the a-k gap follow distributions which show structure for the emission sites on the top half of the cathode with the quantitative change in angle now being obtainable as a function of position. For emission sites on the lower half of the cathode, and as a position across the a-k gap, the angle distribution data has a broad shape biased to low angle with no clearly observable peaks.

Self magnetic pinch diode experiments at 4-6 MV

Summary form only given. Self Magnetic Pinch diode experiments carried out at voltages of 4-6 MV using a variety of different pulse power drivers. Experiments were carried out using the single pulse forming line Asterix driver at CEG Gramat in France and both the high and low impedance versions of the Inductive Voltage Adder (IVA) RITS3 driver at Sandia National Labs in the USA. The experiments showed successful operation of the diode on all the drivers with doses in excess of 150 Rads @ 1 m and spot sizes of less than 3 mm (AWE definition). Record current densities on the target for a radiographic diode were produced at approximately 350 kA/cm. Results of the experiments are presented and compared to code predictions. The Self Magnetic Pinch diode has been used at lower voltages (<2 MV) as a radiographic source for hydrodynamic experiments.