B. M. Huhman

High-Power Self-Pinch Diode Experiments for Radiographic Applications

We report here on self-magnetic-pinch diode experiments at voltages from 3.5 to 6 MV. In addition to electrical diagnostics, the diode is characterized as a radiation source by dose and spot-size measurement. As the operating voltage increases, we find that a given diode geometry tends to produce a smaller spot but suffers from the reduced impedance lifetime. Optimization involves increasing the cathode diameter and diode gap as the voltage increases. We find a good quantitative agreement with the Monte Carlo code integrated tiger series over the entire data set, assuming an effective electron incidence angle of 20deg. Over this range, we observe favorable dose and spot scaling of optimized diode performance with voltage. Our best results are roughly 200-rad at 1 m with an ~2-mm-diameter spot. These were obtained at diode parameters of roughly 6 MV, 150 kA, and 30-ns radiation full-width at half-maximum.

NRL Materials Testing Facility

The Naval Research Laboratory performs basic research on high power railgun electric launchers. The program uses a 1.5-MJ, 2.5 km/s launch velocity railgun located in NRLs Materials Testing Facility. The railgun consists of an 11-MJ capacitive energy store configured as 22, 0.5-MJ modules. Each bank module has an independently triggered thyristor switch, series inductor, and crowbar diode and is joined to the railgun breech with coaxial cables. Individual bank timing and charge levels can be set to produce up to 1.5 MA peak current and 4-5 ms long current pulses. The 6-m long railgun used a nominally 5 cm bore diameter with steel or copper rails and epoxy laminate insulators. The muzzle contains a Tungsten-Copper arc horn to minimize damage from residual drive current upon launch. Aluminum armatures with acrylic bore riders are used for the launch package. Launch data is recorded digitally and analyzed using in-house computer codes. The system design and operation will be discussed.

EM Gun Bore Life Experiments at Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5-MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical finite element analysis models of railgun operation. Results of several experiments on rail wear will be discussed.

EM gun bore life experiments at the Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5 MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical Finite Element Analysis (FEA) models of railgun operation. Results of several experiments on rail wear will be discussed.

Design of a battery intermediate storage system for rep-rated pulsed power loads

The U.S. Naval Research Laboratory (NRL) is developing a battery-powered, rep-rate charger for a 60-kJ capacitor bank. The capacitor will be charged with a bank of LiFePO4 batteries in conjunction with a DC-DC converter. During discharge, the batteries will generate heat from the internal resistance. If the heat is not addressed, damage to the cell may occur, leading to degraded cell lifetime and the potential for cell venting. NRL has developed an integrated cooling system for high power batteries that can limit the residual heat in the battery cells. Results from experiments will be presented, both at the cell and the module level.

Investigations Into the Design of a Compact Battery-Powered Rep-Rate Capacitor Charger

The U.S. Naval Research Laboratory is developing a battery-powered rep-rate charger for a 60-kJ capacitor bank. The goal is to charge a 4800-μF capacitor to 5 kV in for a 50-shot burst at 10 shots/min. A bank of LiFePO4 batteries is used with a full H-bridge converter, a transformer, and a rectifier to transform the 600 V battery voltage to 5-kV secondary voltage. There are two major aspects to the charger design: battery energy store and the dc-dc converter. A stable battery pack requires active cooling, management, and feedback for proper and safe operation. A design study has been performed to identify an appropriate dc-dc converter that can minimize the weight and volume of the system while maintaining high electrical efficiency. An LLC is being evaluated, as it has the possibility of eliminating a discrete inductor using the magnetizing inductance of the transformer in the resonant circuit. Work has also been performed in the design and evaluation of a battery pack, specifically on characterizing the operational parameters of the pack as applied to the desired load. This paper will present simulation data and results from the experiments.

Design and implementation of an advanced X-ray trigger generator for EML test facilities

The Materials Testing Facility (MTF) at the U.S. Naval Research Laboratory (NRL) routinely captures orthogonal x-ray images of launch packages (LP) as they travel through the flight tube after a launch. Located about a meter from the target plates, the x-ray images provide a useful diagnostic for determining the performance of the materials used during the test. The images are captured on digital image plates, a reusable technology that uses a laser to scan the photo-stimulated phosphor particles, thus rendering a digital image that can be processed by a computer software package.

Effect of resistance modification on EML capacitor bank performance

The U.S. Navy is considering the development of an electromagnetic launcher (EML) for surface-fire support and other missions [1]. The Naval Research Laboratory has initiated a program to develop and test materials to achieve these fire rates and lifetimes [2]. The U.S. Naval Research Laboratory has assembled a facility to develop and test materials for the study of barrel lifetime in electromagnetic launchers (EML) for surface-fire support and other missions [3].

Design of a Computer-Based Control System using Labview for the Nemesys Electromagnetic Launcher Facility

The U.S. Naval Research Laboratory has assembled a facility to develop and test materials for the study of barrel lifetime in electromagnetic launchers (EML) for surface-fire support and other missions1. The pulsed power system utilizes 12 500-kJ modules that can be individually triggered to shape the output current pulse2. Each bank module consists of four 130 kJ/can 11-kV capacitors from General Atomics Electronics Systems. The switching thyristors and crowbar diodes are from ABB. A series inductor of approximately 80 μH is used to limit the peak current to 100 kA, isolate modules from each other, and ensure the current is delivered to the test system. LabVIEW from National Instruments (NI) was selected as the control software for the EML system. All facility operations are handled through LabVIEW and controlled by a single operator. The software controls the safety systems; programs and monitors the three CCS High Voltage Power Supplies from General Atomics Electronic Systems; and triggers the capacitor banks. Projectile position status inside the barrel is also monitored in 25-ns steps using the PXI-7811R FPGA module. An overview of the EML facility with respect to control issues is presented. In addition to the software code, circuit diagrams of conditioning hardware will also be discussed. Results from test shots will be shown and discussed.

High-power self-pinch diode experiments for radiographic applications

Summary form only given. The self-magnetic-pinch [SMP] diode is a high impedance (~40-Ohm), low R/D (typically 4 mm/8 mm) pinched-beam diode that shows promise for high-power X-radiography. The scaling of this diode to higher voltage (and thus power) is critical to the development of next generation radiographic sources. We report here on SMP experiments on the NRL Mercury generator at voltages from 3.5-6 MV and impedances from 35-50 Ohms. Measurements include diode electrical behavior, time-integrated and time-resolved X-ray dose, and time integrated radiographic spot size. We have studied the effects of several variations in electrode geometry, surface coating, gap; and the level of machine prepulse. Extensive modeling using the Sandia ITS codes is used to help interpret the X-ray dose and spot measurements. As the operating voltage increases, we find that a given diode tends to produce a smaller spot but also suffer reduced impedance lifetime, and optimization involves increasing the cathode diameter and diode gap as the voltage increases. We find good quantitative agreement with ITS predictions over the entire data set, assuming an electron incidence angle of 20 degrees. This gives a dose rate that scales (over the range examined) as IV2.2. Over this range, we observe favorable scaling of optimized diode performance with voltage, with good dose scaling and a slight spot size decrease with voltage. Our best results comprise roughly 200 rads at 1 meter with a ~2 mm diameter spot