Steve R. Cordova

Overview of Self-Magnetically Pinched-Diode Investigations on RITS-6

The electron-beam-driven self-magnetically pinched diode is a candidate for future flash X-ray radiographic sources. As presently fielded on Sandia Laboratories six-cavity Radiographic Integrated Test Stand (RITS-6), the diode is capable of producing sub 3-mm radiation spot sizes and greater than 350 rads of hard X-rays at 1 m. The diode operates between 6 and 7 MV with a slowly decreasing impedance that falls from approximately 65 to 40 Ω during the main pulse. Sensitivity in diode operation is affected by the interaction of evolving plasmas from the cathode and anode, which seem to limit stable diode operation to a narrow parameter regime. To better quantify the diode physics, high-resolution time-resolved diagnostics have been utilized which include plasma spectroscopy, fast-gated imaging, X-ray p-i-n diodes, X-ray spot size, and diode and accelerator current measurements. Data from these diagnostics are also used to benchmark particle-in-cell simulations. An overview of results from experiments and simulations is presented.

Linear Transformer Driver (LTD) research for radiographic applications

URSA Minor is a 21-cavity Linear Transformer Driver (LTD) that has been built to evaluate LTD technology as a driver for flash radiography diode research. The LTD has been assembled and tested with initial testing being conducted at ±75 kV charge. In this configuration the generator has produced 1.7 MV on a 30-ohm MITL when driving an electron beam diode. Diagnostics during initial testing include cavity voltage, MITL currents, x-ray dose using TLDs, and x-ray dose rate using PIN diodes.

Testing of a 1-MV linear transformer driver (LTD) for radiographic applications

A seven cavity LTD accelerator for radiography applications has been tested with a large area electron beam diode. Individual cavities have been tested with a resistive load. Results of these experiments have been compared to circuit simulations. The circuit model has been improved to provide good agreement with single cavity resistive load tests and seven cavity tests with an electron beam load. A simple modification was made to a single LTD cavity to reduce unwanted voltage oscillations.

Circuit Simulations of a 1 MV LTD for Radiography

A 1 MV linear transformer driver (LTD), capable of driving a radiographic diode load, has been built and tested. A circuit model of this accelerator has been developed using the BERTHA circuit simulation code. Simulations are compared to data from power-flow experiments utilizing a large area electron-beam diode load. Results show that the simulation model performs well in modeling the baseline operation of the accelerator. In addition, the circuit model has been used to predict several possible fault modes. Simulations of switch prefires, main capacitor failure, vacuum insulator flashover, and core saturation have been used to estimate the probability of inducing further failures and the impact on the load voltage and current.

Retrapping studies on RITS

Sandia National Laboratories (SNL) is developing intense sources for flash x-ray radiography. In collaboration with Mission Research Corporation, the Atomics Weapons Establishment, and Titan Pulsed Sciences Division, SNL has studied power flow into various electron beam diodes on the radiographic integrated test stand (RITS). Historically, high-impedance accelerators have driven radiographic loads with little excess power flow. However, a number of facilities now under design use lower impedance inductive voltage adder technology to drive these diodes, requiring control of the vacuum electron sheath flow. The diode configuration in this study uses a non-emitting field shaper or knob to redirect this excess electron flow away from the diode region. Experiments at SNL have demonstrated retrapping of sheath current along a magnetically insulated transmission line into a blade load. The goals of the experiments presented here were to assess power flow issues and to help benchmark the LSP particle-in-cell code used to design the experiment. Comparisons between LSP simulations and experimental data are presented.

Demonstration of the self-magnetic-pinch diode as an X-ray source for flash core-punch radiography.

Minimization of the radiographic spot size and maximization of the radiation dose is a continuing long-range goal for development of electron beam driven X-ray radiography sources. In collaboration with members of the Atomic Weapons Establishment(AWE), Aldermaston UK, the Advanced Radiographic Technologies Dept. 1645 is conducting research on the development of X-ray sources for flash core-punch radiography. The Hydrodynamics Dept. at AWE has defined a near term radiographic source requirement for scaled core-punch experiments to be 250 rads{at}m with a 2.75 mm source spot-size. As part of this collaborative effort, Dept. 1645 is investigating the potential of the Self-Magnetic-Pinched (SMP) diode as a source for core-punch radiography. Recent experiments conducted on the RITS-6 accelerator [1,2] demonstrated the potential of the SMP diode by meeting and exceeding the near term radiographic requirements established by AWE. During the demonstration experiments, RITS-6 was configured with a low-impedance (40 {Omega}) Magnetically Insulated Transmission Line (MITL), which provided a 75-ns, 180-kA, 7.5-MeV forward going electrical pulse to the diode. The use of a low-impedance MITL enabled greater power coupling to the SMP diode and thus allowed for increased radiation output. In addition to reconfiguring the driver (accelerator), geometric changes to the diode were also performed which allowed for an increase in dose production without sacrificing the time integrated spot characteristics. The combination of changes to both the pulsed power driver and the diode significantly increased the source x-ray intensity.

Load line evaluation of a 1-MV linear transformer driver (LTD)

A seven cavity LTD system has been assembled and tested in a voltage adder configuration capable of producing approximately 1-MV into a 7-Omega, critically damped load. Individual cavities have been tested with a resistive load. The seven cavity adder has been tested with a large area electron beam diode. The output pulse when tested into a resistive load is that of an RLC circuit. When tested with a dynamic load impedance, the output voltages of the cavities have an added oscillation. The oscillation affects the output pulse shape but is not harmful to the cavity components.

Cygnus Performance in Subcritical Experiments

The Cygnus Dual Beam Radiographic Facility consists of two identical radiographic sources with the following specifications: 4-rad dose at 1 m, 1-mm spot size, 50-ns pulse length, 2.25-MeV endpoint energy. The facility is located in an underground tunnel complex at the Nevada Test Site. Here SubCritical Experiments (SCEs) are performed to study the dynamic properties of plutonium [1], [2]. The Cygnus sources were developed as a primary diagnostic for these tests. Since SCEs are single-shot, high-value events - reliability and reproducibility are key issues. Enhanced reliability involves minimization of failure modes through design, inspection, and testing. Many unique hardware and operational features were incorporated into Cygnus to insure reliability. Enhanced reproducibility involves normalization of shot-to-shot output also through design, inspection, and testing. The first SCE to utilize Cygnus, Armando, was executed on May 25, 2004. A year later, April - May 2005, calibrations using a plutonium step wedge were performed. The results from this series were used for more precise interpretation of the Armando data. In the period February - May 2007 Cygnus was fielded on Thermos, which is a series of small-sample plutonium shots using a one-dimensional geometry. Pulsed power research generally dictates frequent change in hardware configuration. Conversely, SCE applications have typically required constant machine settings. Therefore, while operating during the past four years we have accumulated a large database for evaluation of machine performance under highly consistent operating conditions. Through analysis of this database Cygnus reliability and reproducibility on Armando, Step Wedge, and Thermos is presented.

RITS-3 self-break water switch studies

The radiographic integrated test stand (RITS-3) is a 4-MV, 160-kA, 70-ns inductive voltage adder accelerator at Sandia National Laboratories designed to develop critical understanding of flash radiographic drivers and x-ray sources. It utilizes three pulse forming lines (PFLs) to drive three inductive voltage adder cavities. Each PFL is switched by a fast-pulse-charged, self-breaking, annular water switch. Good synchronization and low loss in the switches is essential to efficient operation of the accelerator. Data from RITS-3 is analyzed with respect to jitter and losses, and compared to computer electric field codes for accuracy. In an attempt to achieve better synchronization, each of the three switch gap sizes was adjusted and electrode geometries were changed to improve performance.

Recent progress in the development of immersed diodes

Sandia National Laboratories is investigating and developing high-dose, high-brightness flash radiographic sources. One of the diodes we are developing is the immersed or B/sub z/ diode. These diodes employ large-bore, high-field solenoid magnets to help produce an intense electron beam from a needle-like cathode immersed in the strong B/sub z/ field of the magnet. Both simulation and experiments suggest that the capability of these diodes to achieve high dose from a small spot has been limited by deleterious effects associated with nonprotonic anode plasmas evolving from the electron target or high-atomic-number (high-Z) converter. We are in the process of developing and testing the next generation of immersed diodes. The goals of these diodes are to improve our understanding of the evolution of the anode plasmas and avoid or delay the onset of nonideal diode behavior by controlling the amount of nonprotonic species accelerated off the anode. These diodes employ improved plasma diagnostics for studying the evolution of the anode plasmas and novel cryogenic concepts for forming solid hydrogenous anodes. Recent progress in the development, testing and understanding of immersed diodes at Sandia is described.