The Z machine is a 36-module, multi-megavolt, low impedance magnetic pressure driver for high-energy-density physics experiments. In 2007, a major re-build doubled the stored energy and increased the peak current capability of Z. The upgraded system routinely drives 27 MA through low inductance dynamic loads with 110 nanosecond time to peak current. The Z pulsed power system is expected to be prepared for a full-energy experiment every day, with a small (<2%) chance of pulsed power system failure, and ±2 ns timing precision. To maintain that schedule with 20 MJ stored, it becomes essential to minimize failures that can damage hardware. We will show the results of several improvements made to the system that reduce spurious breakdowns and improve precision. In most cases, controlling electric fields is key, both to reliable insulation and to precision switching. The upgraded Z pulsed power system was originally intended to operate with 5 MV peak voltage in the pulse-forming section. Recent operation has been above 6 MV. Critical items in the pulsed power system are the DC-charged Marx generators, oil-water barriers, laser-triggered gas switches, and the vacuum insulator. We will show major improvements to the laser-triggered gas switches, and the water-insulated pulse forming lines, as well as delivered current reproducibility results from user experiments on the machine.
The Z pulsed power driver at Sandia National Laboratories is used for a wide range of high energy density physics experiments in areas such as inertial confinement fusion, radiation effects, and dynamic material properties. Experimental demands are pushing for the highest energy attainable with more reliability and precision in timing and pulse compression. A previous version of the laser-triggered gas switch had been made reliable at voltages up to 5.7 MV, allowing 5 nanosecond load accuracy. The desire for higher energy and higher precision dictated a new laser-triggered switch design. In Z, 36 DC-charged Marx generators pulse-charge water-insulated capacitors in 1.5 microseconds. The laser-triggered gas switch commutes the energy stored in the water-insulated capacitor to subsequent pulse compression stages that utilize self-closing water switches. The laser-triggered switch is the last command triggered switch in the chain, and largely determines the temporal accuracy of the total load current. Both switches consist of a laser triggered section and a self-closing cascade section. The previous design required a trigger plate to provide mechanical support for the cascade section. With fixed laser energy, it was impossible to increase the triggered fraction of the switch. Because of the trigger support plate that affects the field distribution after triggering, establishing an operating pressure that provides a reliable balance between low pre-fire rate and low jitter becomes difficult, and more so at higher voltage. The new switch uses a cantilevered design that increases the electric stress in the self-closing section after triggering, even with a slightly-reduced triggered gap. It was required that the new design work within the same operating space and infrastructure as the previous. We will show details of the design and features necessary for reliable operation in the extreme electrical and mechanical environment presented by daily operation on Z.
Low carbon, high strength steel alloys such as Vascomax steels are used in a wide variety of extreme environments due to their high strength, high fracture toughness, and stability over a wide range of temperatures. In this study, Vascomax® C250 steel was dynamically characterized in compression using Kolsky compression bar techniques at two strain rates of 1000 and 3000 s-1. A pair of impedance-matched tungsten carbide platens were implemented to protect damage to the bar ends. The tungsten carbide platens were experimentally calibrated as system compliance which was then properly corrected for actual specimen strain measurements. In addition, elastic indentation of the high-strength compression sample into the platens was also evaluated and showed negligible effect on the specimen strain measurements. The Vascomax® C250 steel exhibited strain-rate effects on the compressive stress-strain curves. The dynamic yield strength was approximately 18% higher than quasi-static yield strength obtained from hardness tests. The dynamic true stress-strain curves of the Vascomax® C250 steel in compression were also computed and then compared with the previously obtained true tensile stress-strain curves at the same strain rates. The Vascomax® C250 steel exhibited a reasonable symmetry in dynamic compression and tensile stress-strain response. However, the fracture strains in dynamic compression were smaller than those in dynamic tension probably due to different fracture mechanisms in the different loading modes.
Vascomax® maraging C250 and C300 alloys were dynamically characterized in tension with Kolsky tension bar techniques. Compared with conventional Kolsky tension bar experiments, a pair of lock nuts was used to minimize the pseudo stress peak and a laser system was applied to directly measure the specimen displacement. Dynamic engineering stress–strain curves of the C250 and C300 alloys were obtained in tension at 1000 and 3000 s−1. The dynamic yield strengths for both alloys were similar, but significantly higher than those obtained from quasi-static indentation tests. Both alloys exhibited insignificant strain-rate effect on dynamic yield strength. The C300 alloy showed approximately 10 % higher in yield strength than the C250 alloy at the same strain rates. Necking was observed in both alloys right after yield. The Bridgman correction was applied to calculate the true stress and strain at failure for both alloys. The true failure stress showed a modest strain rate effect for both alloys but no significant difference between the two alloys at the same strain rate. The C250 alloy was more ductile than the C300 alloy under dynamic loading.
Electrical discharge in a fluid produces a transient pressure wave, which should be taken into account when designing structural components of pulsed power machines. By combining theoretical approaches for underwater explosive theory with a self-consistent approach for modeling water as a compressible non-flow material, finite element models can be used to investigate mechanical response to arc driven pressure waves. Water switch testing included impulse measurements of both multiple and single arc gaps to compare with theory. Pressure wave interactions seen in computer models were in good agreement with peak pressures measured from a three-electrode water switch. Pressure measurements from a single electrode water switch were in close approximation to predictions based on explosive theory. Structural damage tests were also conducted in which damage to machine parts were related to arc energies.