Daniel S. Nelson

Characterization of the rod-pinch diode x-ray source on Cygnus

The rod-pinch diode[1] is a self-magnetically insulated electron beam diode that is capable of producing a very bright source of hard x-rays. As fielded on the Cygnus accelerator[2], the diode operates at an impedance of 50 Ohms and produces short pulse ( ∼50 ns) bremsstrahlung radiation with a 2 MeV photon endpoint energy and dose of 4 rad measured at one meter, with an x-ray spot size ∼ 1mm. The source can be used to image through ∼ 40 g/cm2 of material with spatial resolution of order 300 µm. Recently, a series of experiments on Cygnus have been conducted to better characterize the diodes operation and x-ray output. In particular, the x-ray spectral content, source spot-size, and shot-to-shot reproducibility have been diagnosed. The intent of these experiments is to enable improvements that may extend the diodes radiographic utility. An array of diagnostics have been utilized which include, end-on and side view x-ray pin hole imaging, time resolved and time integrated spot size measurements, step wedges, x-ray p-i-n diodes, and diode/MITL current measurements. High fidelity, PIC/Monte-Carlo simulations have also been conducted to help analyze the data. An overview of these experiments, simulations, and the conclusions from analysis is presented.

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.

Cygnus PFL Switch Jitter

The Cygnus Dual Beam Radiographic Facility consists of two identical radiographic sources: Cygnus 1 and Cygnus 2. Each source has the following X-ray output: 1-mm diameter spot size, 4 rads at 1 m, 50-ns full-width-half-maximum. The diode pulse has the following electrical specifications: 2.25 MV, 60 kA, 60 ns. This Radiographic Facility is located in an underground tunnel test area at the Nevada Test Site (NTS). The sources were developed to produce high-resolution images on subcritical tests performed at NTS. Subcritical tests are single-shot, high-value events. For this application, it is desirable to maintain a high level of reproducibility in source output. The major components of the Cygnus machines are Marx generator, water-filled pulse forming line (PFL), water-filled coaxial transmission line, three-cell inductive voltage adder, and rod-pinch diode. A primary source of fluctuation in Cygnus shot-to-shot performance may be jitter in breakdown of the main PFL switch, which is a “self-break” switch. The PFL switch breakdown time determines the peak PFL charging voltage, which ultimately affects the source X-ray spectrum and dose. Therefore, PFL switch jitter may contribute to shot-to-shot variation in these parameters, which are crucial to radiographic quality. In this paper we will present PFL switch jitter analysis for both Cygnus machines and present the correlation with dose. For this analysis, the PFL switch on each machine was maintained at a single gap setting, which has been used for the majority of shots at NTS. In addition the PFL switch performance for one larger switch gap setting will be examined.

High performance vacuum system for a Radiographic diode

The Cygnus Dual Beam Radiographic Facility consists of two identical radiographic sources each with a dose rating of ~4-rad at 1 m, and a ~1-mm diameter spot size. The development of the rod pinch diode (RPD) was responsible for the ability to meet these criteria. The RPD in each machine uses a 0.75-mm tungsten diameter tapered anode rod extended through a 9-mm diameter cathode aperture. When properly configured, the electron beam born off the aperture edge can self-insulate and pinch onto the tip of the rod creating an intense, small x-ray source. These sources are utilized in concert with an imaging system on complex experiments, which are single-shot, high-value events. In such an application there is an emphasis on machine performance (reliability and reproducibility). Vacuum quality is a significant determinator for source performance. In this paper we discuss the Cygnus Dual Beam Radiographic Facility, the associated vacuum systems, vacuum monitor calibration and a multi-purpose vacuum chamber. Vacuum system performance, using various enhancements, will be reported. Using the vacuum system enhancements, as well as other machine tweaks, recent source performance has improved. This new level of performance will be presented via dosimetry and electrical measurements.

Lithium Fluoride TLD dose quality

The use of Lithium Fluoride (LiF) Thermoluminescent Dosimeters (TLD) have been in use at a radiographic facility for over three years. The facility consists of two radiographic sources each with a dose rating of ~4-rad at 1 m. The calibration and fielding of LiF TLDs will be examined for accuracy and long term standard deviation of these measurements. LiF TLDs will be evaluated in single point measurements and multi-point arrays. Improved multi-point arrays will be compared to previous array data. The LiF TLDs will also be compared to Pin Diodes for routine measurements and evaluation of shielding around the sources.

X-ray pinhole camera measurements

The development of the rod pinch diode has lead to high resolution radiography used on contained explosive experiments. The rod pinch diodes use a small diameter anode rod, which extends through a cathode aperture. Electrons borne off the aperture edge can self-insulate and pinch onto the tip of the rod, creating an intense, small x-ray source. This source is utilized as the primary diagnostic on numerous experiments that include high-value, single-shot events. In such applications there is an emphasis on machine reliability, x-ray reproducibility, and x-ray quality. We have observed that an additional pinch occurs at the interface near the anode rod and the rod holder. This suggests that there are stray electrons emitted from the surfaces of the surrounding area. In this paper we present results of x-ray measurements using a pinhole camera. The camera geometry used is an upstream view 30° with respect to the diode centerline. This diagnostic will be employed to: (1) diagnose x-ray reproducibility and quality, and (2) investigate the effect of different diode configurations.

Cygnus source emission

The Cygnus Dual Beam Radiographic Facility consists of two identical radiographic sources each with a dose rating of 4 Rads at 1 m, and a 1 mm diameter spot size. The development of the rod pinch diode was responsible for the ability to meet these criteria. The rod pinch diode in a Cygnus machine uses a small diameter anode rod, which extends through a cathode aperture. The electron beam born off the aperture edge can self-insulate and pinch onto the tip of the rod, creating an intense, small x-ray source. The Cygnus sources are utilized as the primary diagnostic on numerous experiments which include high-value, single-shot events. In such applications there is an emphasis on reliability and reproducibility. A shot-to-shot evaluation of the machine performance will be conducted and evaluated using two x-ray pinhole cameras.

Cygnus dose symmetry

The Cygnus Dual Beam Radiographic Facility consists of two identical radiographic sources each with a dose rating of 4 Rad at 1 m, and a 1 mm diameter spot size. The development of the rod pinch diode was responsible for the ability to meet these criteria. The rod pinch diode in the Cygnus machines use a 0.75 mm diameter tungsten tapered anode rod extended through a 9 mm diameter cathode aperture. When properly configured, the electron beam born off the aperture edge can self-insulate and pinch onto the tip of the rod creating an intense, small x-ray source. The Cygnus sources are utilized as the primary diagnostic on experiments which are single-shot, high value events. In such an application there is an emphasis on reliability and reproducibility. The azimuthal quality and reproducibility of the Cygnus sources will be analyzed using lithium fluoride thermoluminescent dosimeters. Photogrammetry will be used to quantify the precision of the rod pinch diode build (i.e. the centeredness of the anode rod). One goal of these tests will be investigation of the relationship of dose symmetry to anode rod position (i.e. the correlation of off-center dose to an off-center anode rod).

Cygnus Trigger System

The Cygnus Dual Beam Radiographic Facility consists of two radiographic sources (Cygnus 1, Cygnus 2) each with a dose rating of 4 rads at 1 m, and a 1-mm diameter spot size. The electrical specifications are: 2.25 MV, 60 kA, 60 ns. This facility is located in an underground environment at the Nevada Test Site (NTS). These sources were developed as a primary diagnostic for subcritical tests, which are single-shot, high-value events. In such an application there is an emphasis on reliability and reproducibility. A robust, low-jitter trigger system is a key element for meeting these goals. The trigger system was developed with both commercial and project-specific equipment. In addition to the traditional functions of a trigger system there are novel features added to protect the investment of a high-value shot. Details of the trigger system, including elements designed specifically for a subcritical test application, will be presented. The individual electronic components have their nominal throughput, and when assembled have a system throughput with a measured range of jitter. The shot-to-shot jitter will be assessed both individually and in combination. Trigger reliability and reproducibility results will be presented for a substantial number of shots executed at the NTS.

Cygnus Diverter Switch Analysis

Summary form only given. The Cygnus Dual Beam Radiographic Facility consists of two 2.25-MV, 60-kA, 50-ns X-ray sources fielded in an underground laboratory at the Nevada Test Site. The tests performed in this laboratory invoke study of the dynamic properties of plutonium and are called subcritical experiments, from end-to-end. the Cygnus machines utilize the following components: Marx generator, water-filled pulse-forming line (PFL), water-filled coaxial transmission line, 3-cell inductive voltage adder (IVA), and rod-pinch diode. The upstream coaxial transmission line interface to the PFL is via a radial insulator with coaxial geometry. The downstream coaxial transmission line terminates in a manifold where the center conductor splits into three lines which individually connect to each of the IVA cell inputs. There is an impedance mismatch at this juncture. It is a concern that a reflected pulse due to anomalous behavior in the IVA or diode might initiate breakdown upon arrival at the upstream PFL/coax insulator. Therefore near the beginning of the coaxial transmission line a radial diverter switch is installed to protect the insulator from over voltage and breakdown. The diverter has adjustable gap spacing, and an in-line aqueous-solution (sodium thiosulfate) resistor array for energy dissipation. There are capacitive voltage probes at both ends of the coaxial transmission line and on the diverter switch. These voltage signals will be analyzed to determine diverter performance. Using this analysis the usefulness of the diverter switch is evaluated.