A. D. White

The advanced helical generator

A high explosive pulsed power generator called the advanced helical generator (AHG) has been designed, built, and successfully tested. The AHG incorporates design principles of voltage and current management to obtain a high current and energy gain. Its design was facilitated by the use of modern modeling tools as well as high precision manufacture. The result was a first-shot success. The AHG delivered 16 MA of current and 11 MJ of energy to a quasistatic 80 nH inductive load. A current gain of 160 times was obtained with a peak exponential rise time of 20 micros. We will describe in detail the design and testing of the AHG.

Advances in optical fiber-based Faraday Rotation Diagnostics

In the past two years, we have used optical fiber-based Faraday Rotation Diagnostics (FRDs) to measure pulsed currents on several dozen capacitively driven and explosively driven pulsed power experiments. We have made simplifications to the necessary hardware for quadrature-encoded polarization analysis, including development of an all-fiber analysis scheme. We have developed a numerical model that is useful for predicting and quantifying deviations from the ideal diagnostic response. We have developed a method of analyzing quadrature-encoded FRD data that is simple to perform and offers numerous advantages over several existing methods. When comparison has been possible, we have seen good agreement with our FRDs and other current sensors.

Explosive flux compression generators at LLNL

At Lawrence Livermore National Laboratory we have developed a coupled helical-coaxial FCG device called the Full Function Test (FFT). This device was used to deliver 98 MA of current and 66 MJ of energy to an inductive load. The successful testing of the FFT represented the culmination of an effort to establish a high-energy pulsed power program that would greatly exceed the performance of capacitor bank facilities. Using the modeling, design, and experimental capabilities developed for the FFT, we have developed a new generator, the Mini-G. Based upon a half-scaling of the FFT device, the Mini-G is a coupled helical-coaxial FCG capable of delivering up to 60 MA of current and 8 MJ of energy. We will describe the design of this generator which involved the use of simulation codes as well as innovative pulsed power techniques to obtain a compact, optimized device.

Note: The full function test explosive generator

We have conducted three tests of a new pulsed power device called the full function test. These tests represented the culmination of an effort to establish a high energy pulsed power capability based on high explosive pulsed power (HEPP) technology. This involved an extensive computational modeling, engineering, fabrication, and fielding effort. The experiments were highly successful and a new U.S. record for magnetic energy was obtained.

Faraday rotation data analysis with least-squares elliptical fitting

A method of analyzing Faraday rotation data from pulsed magnetic field measurements is described. The method uses direct least-squares elliptical fitting to measured data. The least-squares fit conic parameters are used to rotate, translate, and rescale the measured data. Interpretation of the transformed data provides improved accuracy and time-resolution characteristics compared with many existing methods of analyzing Faraday rotation data. The method is especially useful when linear birefringence is present at the input or output of the sensing medium, or when the relative angle of the polarizers used in analysis is not aligned with precision; under these circumstances the method is shown to return the analytically correct input signal. The method may be pertinent to other applications where analysis of Lissajous figures is required, such as the velocity interferometer system for any reflector (VISAR) diagnostics. The entire algorithm is fully automated and requires no user interaction. An example of algorithm execution is shown, using data from a fiber-based Faraday rotation sensor on a capacitive discharge experiment.

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 LLNLs 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.

Explosive pulsed power experimental capability at LLNL

LLNL has developed a family of advanced magnetic flux compression generators (FCGs) used to perform high energy density physics experiments and material science studies. In recent years we have performed these experiments at explosive test sites in New Mexico and Nevada. In 2011, we re-established an explosive pulsed power test facility closer to Livermore. LLNLs Site 300 is a U.S. DOE-NNSA experimental test site situated on 7000 acres in rural foothills approximately 15 miles southeast of Livermore. It was established in 1955 as a non-nuclear explosives test facility to support LLNLs national security mission. On this site there are numerous facilities for fabricating, storing, assembling, and testing explosive devices. Site 300 is also home to some of DOEs premier facilities for hydrodynamic testing, with sophisticated diagnostics such as high-speed imaging, flash X-ray radiography, and other advanced diagnostics for performing unique experiments such as shock physics experiments, which examine how materials behave under high pressure and temperature. We have converted and upgraded one particular firing bunker at Site 300 (known as Bunker 851) to provide the necessary infrastructure to support high explosive pulsed power (HEPP) experiments. In doing so, we were able to incorporate our established practices for handling grounding, shielding, and isolation of auxiliary systems and diagnostics, in order to effectively manage the large voltages produced by FCGs, and minimize unwanted coupling to diagnostic data. This paper will discuss some of the key attributes of the Bunker 851 facility, including the specialized firesets and isolated initiation systems for multistage explosive systems, a detonator-switched seed bank that operates while isolated from earth and building ground, a fiber-optic based timing, triggering and control system, an EMI Faraday cage that completely encloses diagnostic sensors, cabling and high-resolution digitizers, optical fiber-based velocimetry and current sensor systems, and a flash X-ray radiography system. The photos and experimental results from recent FCG experiments will also be shown and discussed.

Measuring helical FCG voltage with an electric field antenna

A method of measuring the voltage produced by a helical explosive flux compression generator using a remote electric field antenna is described in detail. The diagnostic has been successfully implemented on several experiments. Measured data from the diagnostic compare favorably with voltages predicted using the code CAGEN [1], validating our predictive modeling tools. The measured data is important to understanding generator performance, and is measured with a low-risk, minimally intrusive approach.

The application of Kiuttu's formulation to study coaxial Flux Compression Generators

A class of flux compression generators (FCGs) is based on the compression of the cross-sectional area of a coaxial geometry where the current flows along the outer conductor and returns through the inner conductor. This compression causes an increase in current since magnetic flux must be conserved. Kiuttus inductive electric-field formulation is a powerful tool for the conceptual design of coaxial FCGs. The usefulness of this formulation is demonstrated in this paper for a simplified geometry using a finite-element partial differential equation solver (FlexPDE) for calculation of the inductive electric field. A time-varying applied current or a moving surface creates the nonconservative electric field. Losses due to diffusion of magnetic flux into conducting surfaces can also be accounted for and modeled in this setting. This analytical-computational approach serves as an important step in validating the magnetohydrodynamic (MHD) portion of the complex multiphysics parallel Lawrence Livermore code, Arbitrary Lagrangian-Eulerian (ALE3D). The nonintuitive boundary conditions involved in solving the otherwise straightforward partial differential equations are described in detail and illustrated in a simple model. The physical parameters used in the simulations are not based on a specific design.

Bonded penetration analysis for a severe lightning strike to a facility

Lightning strikes pose a serious threat to facilities and their subsystems. If a facility takes a direct strike, large amounts of pulsed electromagnetic (EM) energy can radiate into the interior of the facility. This energy can couple into electronic systems causing failures. Often, proper shielding of the facility can reduce the radiated energy by an order of magnitude. In an attempt to reduce pulsed EM energy, facilities are built to resemble a Faraday cage. However, most facilities have several imperfections which limit the effectiveness of their shielding capabilities. Penetrations into the facility are a type of imperfection that allows EM fields to be produced in the interior of the facility. Therefore, penetrations must be connected to the Faraday cage through bond wires to maintain the shields integrity and protect sensitive components. Finite element computer simulations have been performed to determine the effects of bonded penetrations, using 6 AWG bond wires. In an attempt to offer guidelines, which optimize the facilitys shielding effectiveness; several bond wire configurations have been investigated. Bond wire lengths, bond wire orientation, single and multiple bond wire configurations and varying the angle between bond wires have been investigated. Simulation results have shown that multiple bond wires result in greater than 40dB reduction of pulsed EM fields in the interior of the facility and a spacing of greater than 45° is optimum for bond wire spacing, for the simulated facility. In addition, the penetration current diverted by the bond wire was monitored. For severe direct lightning strikes, i.e. Ipeak=200 kA and dI/dt=400 kA/μs, the simulation suggest greater than 90% of the lightning current is diverted through the bond wire into the Faraday cage for the configurations examined. The high current nature of the severe lightning pulse produces large Lorentz forces on the bond wire. Laboratory experiments are being developed at the LLNL pulsed power lab to ensure that bond wires maintain proper connection when exposed to high currents, ensuring desired shielding throughout a direct strike.