Flat plate FCG experimental system for material studies

Magnetic flux compression generators (FCGs) driven by high explosives can produce extremely high magnetic fields that are useful in accelerating metal liners and sample materials to high velocities to study their properties. For material studies requiring extremely high energy and applied pressures, explosive FCGs can far surpass the typical performance of capacitor based systems. Flat plate generators (FPGs) are useful in many flux compression applications. They are well suited for doing material studies in planar geometries, and they enable the use of certain diagnostic techniques, most notably flash X-ray radiography, which would be difficult if not impossible to utilize in coaxial geometries. Typical flat-plate generators have rather slow-rising output currents. This can cause loads to deform significantly before the highest rate of current gain from the generator can be reached. Shearer et al. at LLNL overcame this handicap by developing a version of FPG that used a flat plate armature and contoured stator. A rectangular block of high explosive (HE) is lit by a row of detonators placed across the width of the HE at a select location along the length of the generator. As the HE burns, the armature takes a characteristic shape determined by the line initiation location. At the appropriate time, the armature first contacts the stator near the input end, then continues to expand into a shape resembling the contoured stator. At late time, the armature contacts the stator at a shallow 1 to 2 degree phasing angle, which rapidly sweeps flux into the load, resulting in a fast current rise time. We have constructed a similar type generator for our present experimental work. It is capable of delivering 20 MA of current with a 2 to 4 μs exponential rise time into suitable loads. This paper describes the design of LLNL's flat-plate FCG, along with results of modeling and simulation performed for its development. Experiments have been carried out using the FPG with seed currents ranging from 0.75 to 1.6 MA using capacitor banks, and up to 2 MA using a helical FCG. Accurate measurements of input and output currents have been made and performance agrees remarkably well with MHD simulations. Challenges faced with calibrating diagnostics and fielding these types of experiments will also be discussed.