The Multi-mEgavolt Radiographic Linear INductive voltage adder (MERLIN) is a pulsed power machine developed future AWE hydrotrials. A Marx generator charges ten Pulse-Forming Lines (PFLs), which subsequently discharge into induction cells. The induction cell voltages add along the stalk of the Inductive Voltage Adder (IVA). The output pulse of ≈ -7 MV, ≈ 230 kA, ≈ 60 ns drives a Self-Magnetic Pinch (SMP) diode.

The MERLIN accelerator being commissioned at AWE in a new Technology Development Centre will provide one of the flash radiographic sources at a joint UK/French facility for hydrodynamic testing in support of the two nations' nuclear deterrents. The ten module Induction Voltage Adder (IVA) has been designed to provide a 60 ns long TerraWatt pulse to drive a Self Magnetic Pinch (SMP) electron beam diode at 7.5 MV. The design work for MERLIN was carried out by L3 Pulse Sciences in San Leandro, California and builds on previous IVA experience in the USA. Prototyping of sub-systems was also carried out by L3 to confirm that the performance and reliability requirements for the overall accelerator can be met. However, it is only now that all the components of the accelerator have been brought together and its overall function can be characterised and compared with predictions. Commissioning of the accelerator has involved setting to work the ancillary systems which provide and control oil, deionised water, sulphur hexafluoride gas, vacuum, control and instrumentation, diagnostics and data acquisition. With these operating satisfactorily testing of the pulsed power systems was able to commence. Commissioning of the pulsed power systems started with a run up of the Marx generator into a resistive load to its operating voltage of 2.5 MV, including characterisation of the trigger systems and the diverter switches. These are intended to short the Marx output after it reaches peak voltage, or if a prefire occurs, in order to reduce the risk of electrical breakdowns. The waveforms produced during factory tests in the US were successfully reproduced and the jitter of the trigger systems shown to meet specification. This allowed the commissioning programme to proceed to the active commissioning phase where an X-ray output is generated. Active commissioning is enabled by the Marx generator being connected via an oil insulated transfer line to the Pulse Forming Lines (PFLs). Each module of MERLIN comprises an induction cell driven by one of these PFLs. The upstream section of each PFL receives its 2.5 MV charge from the Marx generator on a microsecond timescale before its pulse forming action is initiated by a laser triggered gas switch. The laser triggering should provide nanosecond order synchronisation, and hence excellent pulse reproducibility, when the pulses are combined in the adder. The 60ns duration 1.1 MV outputs of the PFLs are fed to their corresponding induction cells which act to perform voltage addition along a 28 metre long 80 Ohm MITL. This delivers an 11 MV forward going wave to the e-beam diode. The pulsed radiographic source driven by MERLIN will be a SMP diode developed in an AWE led research programme in collaboration with US National Laboratories. This diode operates at approximately 40 Ohms with the result that retrapping of the MITL sheath current occurs transforming the 11 MV forward wave down to ~ 7.5 MV while increasing the load current to ~ 200 kA. The PFL's configuration tailors the output pulse to compensate for the SMP diode's intra-pulse impedance droop and hence generate a relatively constant voltage during the X-ray flash. Since the SMP diode is a single shot device (due to the energy density incident at the anode/X-ray converter) a Large Area Diode (LAD) of similar impedance is utilised to allow repeated testing of the pulsed power systems. By mid-2017 the testing of the Pulsed Power systems and MITL with the LAD is due to have been completed and the optimisation of the SMP diode should be in progress.

Summary form only given. Direct irradiation of materials by electron beams (e-beams) has been used to study material response. The desire to utilize high-power (~ TW) generators to achieve higher specific energy deposition over larger areas has led to several approaches. One approach utilizes a monolithic e-beam diode with an external magnetic field (B field). The external B field allows the diode to operate in the bipolar, space-charge-limited regime without the current being magnetically limited to a lower value. The field also is used to guide the e-beam through the gas-filled region between the vacuum diode and the object to be irradiated. An alternate approach, discussed in this presentation, utilizes multiple diodes electrically in parallel, with each diode running below the critical current to obtain a high current. The e-beams are then scattered in foils and combined in the gas-transport region to achieve the desired irradiation uniformity and area. We report on experiments that have been performed on the Gamble II generator at NRL (~ 1 MV, ~ 800 kA, ~ 60 ns) designed to study this second approach. Diagnostics include diode voltage and current, a net-current monitor, interferometry, spectroscopy, an axial array of Ta-strip x-ray witness plates, and a segmented calorimeter. Experiments have been performed with a single ring diode and two nested ring diodes. Where possible, the measurements are compared with results from ITS and from the recently-developed ABC model4 for the interaction of the e-beam with the gas. Results show that the beam is very nearly charge and current neutralized as it propagates ~ 20 cm in 1-Torrr N2 gas. Beam scattering from both the anode and a Ti scattering foil in the gas results in a relatively uniform radial beam profile.

A novel inductive probe, termed MIDOT, was developed for monitoring high-current flat transmission lines. While being inexpensive the probe does not require calibration, is resistant to both shock waves and temperature variations, and it is easy to manufacture and mount. It generates strong output signals that are relatively easy to interpret and has a detection region limited to a pre-defined part of the transmission line. The theoretical background related to the MIDOT probes, together with their practical implementation in both preliminary experimentation and high-current tests, is also presented in the paper. The novel probe can be used to benchmark existing 2D numerical codes used in calculating the current distribution inside the conductors of a transmission line but can also easily detect an early movement of a transmission line component. The probe can also find other applications, such as locating the position of a pulsed current flowing through a thin wire.

AWE is replacing its current Mevex X-ray machine (~800kV, 35kA, 60ns pulse) to improve reliability and maintainability. A design using a Linear Transformer Driver (LTD) insulated with dry air at atmospheric pressure has been developed by ITHPP. A testbed using 3 cavities, made from 2 bricks of two GA35460M 8nF, 100kV capacitors, a multigap multichannel switch and a magnetic core, has been developed and extensively tested [1]. This led to the development of the full generator that uses 17 cavities in series to produce an 800kV output voltage on a vacuum insulated line and a 23-25Ω diode. The ~4.3m long inner stalk is cantilevered by the back of the pulser and is positioned by a 3 point adjustable support. This setting is completed by 2 internal tensioning cables to compensate for flexing and provide fine adjustment of the diode end. The assembly of the generator has been done on the 1 st half of 2015. The machine can be operated with negative and positive polarity outputs to satisfy operational requirements given by AWE. The tests results of this generator in both polarities are presented in terms of output performances. The reproducibility and reliability of the full system is also analyzed.

This paper describes a novel MOSFET gate driver circuit design for pulsed power application. It is shown that MOSFET switching speed can be enhanced by using an energised inductor as a high current source in series with the gate terminal of a power MOSFET. This topology demonstrates switching speeds of less than 10ns for MOSFETs with high input gate capacitance. It is also shown that by increasing the current in the gate drive and with an optimised layout, a single gate driver can be used to drive multiple parallel connected MOSFETs synchronously without compromising their performance. The circuit design is analysed and experimental results presented when operating at an instantaneous power of 25 kW.

Summary form only given. A prototype two-stage Linear Transformer Driver (LTD) system based on power MOSFETs has been designed and tested for single shot and multi-pulse burst mode operation. Each adder stage consists of two custom designed MOSFET driver boards with four parallel connected MOSFETs and two high voltage MKP capacitors per board. The two boards are connected on either side of the toroid core. The custom current source gate driver was designed to drive four parallel connected MOSFETs synchronously at switching speeds of less than 10ns. The induction core is made of Nanoperm and is 60mm wide and 20mm high. It is fully enclosed in an aluminium cavity. The overall width of the system is 200 mm. A compact LTD system is established with a high power density and ultra-fast switching. Experimental results demonstrate pulse widths of 100ns at a peak voltage of 1.5 kV and a peak current of 300 A with switching times of less than 12 ns.

MOSFETs and IGBTs currently have limited use in high current pulsed power applications due to their lower power density and the complexity involved in developing an array of these devices. This paper proposes a compact 600V, 1kA module using power MOSFETs with specifically designed MOSFET driver boards and 8 parallel MOSFETs. The proposed set up offers a higher power density using minimal components along with ultrafast repetitive switching. Experimental results demonstrate pulse widths of 100ns and switching times <; 10ns under single shot and burst mode operation.

A common initiative for the research into the physics and technology of a flyer plate electromagnetic accelerator has been started at AWE, Aldermaston and Loughborough University. Two electromagnetic accelerators have been constructed and tested: AMPERE, a 120 kJ / 40 kV capacitor bank system at AWE and QUATTRO a 100 kJ/30 kV capacitor bank system at Loughborough Pulsed Power Laboratory. Two numerical models for simulating the foil-flyer accelerator have been developed: a 0-D anda 2-D. The 0-D model is providing a useful tool for designing and for parametric studies and the 2-D model for accurately calculating all the details of the accelerator such as: the velocity, acceleration and temperature of the flyer the current distribution and the overall distribution of the magnetic and electric fields generated during a shot. The paper will present the most relevant experimental data obtained during the first phase of the joint programme and compare it with theoretical predictions.

A joint program involving the study and practical performance of a foil-flyer electromagnetic accelerator has recently been initiated by Atomic Weapons Establishment, Aldermaston, and Loughborough University. As an initial phase of the work, both 0-D and 2-D numerical models for the foil-flyer accelerator have been developed. The 0-D model, although very crude, is capable of providing an insight into the accelerator phenomena and is currently used for parametric design studies. The 2-D model is based on the well-proven Loughborough filamentary modeling technique and is capable of accurately calculating the 2-D distribution of current, velocity, acceleration, and temperature of the flyer, together with the complete distribution of the magnetic and electric fields generated during a shot. The paper presents the two models and compares typical theoretical predictions with the corresponding experimental results.

An experimental programme, leading to the development of a foil-flyer electromagnetic accelerator (FFEMA), is being conducted at AWE. A high current pulsed power generator driving a foil-flyer is used to provide tailored impulse profiles into targets for testing material properties at high strain rates. This work has required the design and construction of both an experimental platform to conduct the experiments, named AMPERE, as well as the development of a number of computer models to predict the electrical and mechanical performance of the foil-flyer and its interaction with a target.

Thehigh dielectricstrengthof sulphurhexafluoride (SF6) when comparedwith other gases, coupled with safety benefits such asnon-flammabilityand non-toxicity, has seen the widespread use of SF6 for the insulation of switching components.However, SF6 is now widely recognised as a highly damaginggreenhousegas, and investigationsof the switching propertiesof alternativegases to replace SF 6 within the bounds of existing system topologies are required. In the presentpaper, a comparativestudy has beencarriedout on atriggeredspark› gap of type presently deployed in industrial pulsed-power machines, todeterminethe suitability of nitrogen(N2) to replace SF6 as the switching medium,without compromising on functionality. Experimentswere performed with fast-rising trigger pulses to minimise the delay time to breakdownand jitter, and threedistinctoperationalregimes have been identified for both gases as the pressureinside the switch isincreased.The static breakdowncharacteristicsand upperpressureboundaries of operationhave beendeterminedfor both gases at arangeof de chargingvoltages. Measurementsof the time tobreakdown have shown jitters as low as 1.3 ns whenoperating in N2, highlighting the potentialof N2 to replaceSF6 without the need for re-design orreplacement of the presentlyused switch.

Summary form only given. The Hydrus project will provide an enhancement of the flash radiographic diagnostics available in the Hydrodynamics Department at AWE as part of new facilities for hydrodynamics research. The X-ray sources for the experiments carried out at AWE are provided by focused electron beam diodes driven by pulsed power accelerators. The most attenuating objects to be radiographed require an X-ray spectrum and dose that can only be provided by pulsed power machines operating at 7 to 14 MV. A design for a 14 MV IVA (induction voltage adder) has been completed by L3 Communications Pulse Sciences for AWE and three such machines are about to be manufactured to provide the main radiographic capability in the new facility. Research currently being conducted by AWE and international collaborators aims to also develop improved electron beam diodes to meet the requirements for higher radiographic resolution being demanded. Other experiments performed at AWE require a softer X-ray spectrum and hence a lower voltage pulsed power driver, two of which will be provided in a second area of the same facility. It has been determined that the optimum radiographic source will have a sub-100 ns pulse , a diameter of 1 mm or less, an X-ray dose of at least 15R at 1 metre and an X-ray spectrum end point energy of 1-1.5 MeV. An electron beam diode that has demonstrated performance o f this order is the plasma filled rod pinch driven by the Gamble II accelerator at NRL. After a concept study and discuss ions with possible vendors it has been decided procure two machines similar in performance to Gamble II. They will feature a Marx generator charging a single water pulse forming line which will in turn drive the diode via a 3 ohm water insulated output line.

Summary form only given. The plasma-filled rod-pinch diode (PFRP) is a new technique for making an intense, small, pulsed X-ray source for radiographic applications. The PFRP utilizes an injected plasma in a rod-pinch-diode configuration to concentrate a 1-2 MeV, 0.1-0.8 MA electron beam at the tip of a 1-mm diameter, tapered tungsten rod, producing an extremely intense X-ray pulse with parameters thought to be impossible with conventional vacuum-diode techniques. The small X-ray source diameter [0.4-mm full-width-at-half-maximum (FWHM) line-spread function] and high X-ray dose (24 R at 1 m) with 1-2 MeV electron energies result in a radiographic figure-of-merit (FOM = dose/FWHM 2 ) that exceeds by a factor-of-forty the FOM of conventional X-ray sources used for pulsed hydrodynamic radiography in this electron-energy range. The NRL Gamble II generator has recently been refitted with a 3-Omega output water line to test the PFRP under conditions similar to those required for AWE's Hydrodynamics Research Facility program. Results of this research effort will be reported.