The modelling, design and optimisation of a hypervelocity pulsed-power projectile launcher for extreme state research

Thesis

Projectile launch techniques, such as gas guns and electromagnetic (EM) flyer plates, are capable of creating planar, high-pressure shocks across substantial material volumes, resulting in precise equation of state (EoS) measurements. However, achieving a quasi-equilibrated shock on impact requires the flyer to maintain constant velocity and near-constant density across its thickness, often limiting the maximum achievable velocity and pressure. The electric gun, a pulsed-power driven projectile launcher, was originally invented in the 1970s as a tool for making TPa EoS measurements in metals. However, the technique faces challenges in accelerating flyers thick enough to generate shock pulses with sufficient duration to make precise EoS measurements. Accelerating a thick flyer (> 0.5 mm) to hypervelocity can induce violent state change and disintegration. Despite its limitations, the electric gun’s mechanism offers unique advantages over the EM flyer plate, having higher efficiency and avoiding ohmic heating of the flyer. This thesis aims to exploit advances in magneto-hydrodynamic (MHD) modelling techniques and access to M3, a 2.5 MJ pulsed-power device, to understand the state change mechanisms occurring in thick flyers accelerated by the electric gun. This understanding is then used to guide the design of an electric gun load for M3 that mitigates these mechanisms. First, a 0D model of the electric gun was created to expediate the investigation of the effects of the current profile and load geometry on the pressure states in the flyer. This 0D model was then leveraged to inform an experimental study of the effect of the current rise-time on the acceleration of flyers up to 2 mm (twice as thick as had been achieved prior to this thesis) to hypervelocity, revealing two predominant state change mechanisms in the flyer; the build-up of thermal pressures resulting in violent spallation on launch and plasma breakthrough late in flight. Employing these experimental results as a benchmark, the modelling of the electric gun was explored in two-dimensions using the MHD code, B2. For completeness, a material strength algorithm was incorporated in B2, though its effect on the simulated electric gun load performance was found to be secondary to the power loss in the load. Finally, the tools and understanding developed in the preceding chapters were combined to assess the performance of an electric gun load for making EoS measurements on a fixed rise-time device, driving a 20 GPa shock in a PMMA target for 1000 ns over a 10×10 mm area. Overall, this thesis reveals the electric gun to be a complex but versatile projectile launcher, with many promising avenues for advancement remaining.

Authors: Fitzgerald, M

• DOI: 10.5287/ora-9eoogkv4b
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Eulerian hydrocode; shock impact; flyer acceleration; electrothermal; exploding foil; flyer launch; strength model; electric gun; electromagnetic; pulsed-power; dynamic behaviour of materials
• Subjects: Engineering; Physics