Eugene C. Ormond

Performance of the Cygnus X-ray source

Summary form only given. Cygnus is a radiographic X-ray source developed for support of the Sub-Critical Experiments Program at the Nevada Test Site. Major requirements for this application are: a dramatically reduced spot size as compared to both Government Laboratory and existing commercial alternatives, layout flexibility, and reliability. Cygnus incorporates proven pulsed power technology (Marx Generator, Pulse Forming Line, Water Transmission Line, and Inductive Voltage Adder sub-components) to drive a high voltage vacuum diode. In the case of Cygnus, a relatively new approach (the rod pinch diode) is employed to achieve a small source diameter. Design specifications are: 2.25 MeV peak energy, < 1 min source diameter, and 5-10 rads dose at 1 meter. The pulsed power and system architecture design plan has been previously presented (Weidenheimer et al., 2001). The first set of Cygnus shots are now underway and are geared to verification of electrical parameters and, therefore, use a large area diode configuration offering increased shot rate as compared to that of the rod pinch diode. Later tests incorporate the rod pinch diode and will concentrate on X-ray production with time resolved measurements of X-ray dose and spot size. In this work we present results of initial operation in terms of electrical and radiation parameters. In addition, the issues associated with static and time resolved radiographs may be included. Performance of the pulse power system is being evaluated by comparison of measured to design output parameters. This is accomplished by comparison of multiple voltage and current measurements throughout the system with various circuit model codes such as MicroCAP and T-Line.

Cygnus Dual Beam Radiography Source

The subcritical experiment (SCE) program was initiated after the 1992 moratorium on underground nuclear testing in support of stockpile stewardship. The dynamic material properties of plutonium are a major topic of exploration for the SCE program. In order to provide for a multilayered containment of plutonium, the SCEs are executed in the Ula underground tunnel complex at the Nevada Test Site (NTS). Cygnus is a new radiographic X-ray source developed for diagnostic support of the SCE Program at NTS. Typically, SCEs have been limited to surface diagnostics. Cygnus radiography was developed to complement the existing surface diagnostics, provide a more extensive spatial view (albeit temporally limited), and provide internal (penetrating) measurements. The Stallion series of SCEs consists of the following four shots listed in chronological order: Vito, Rocco, Mario, and Armando. Armando was the initial experiment for Cygnus radiography. The Rocco, Mario, and Armando tests use identical physics packages, permitting the correlation of Armando radiographic results with surface results from all three shots. The main X-ray source requirements for an SCE involve spot size, intensity, penetration, and duration. To this end Cygnus was designed to satisfy the following specifications: ~1 mm source diameter, 4 Rads dose at a distance of 1 meter, ~2.25 MeV endpoint energy, and < 100 ns pulse length. Two Cygnus sources (Cygnus 1, Cygnus 2) were fielded at NTS providing two views separated in space by 60deg and in time by 2 mus. Cygnus performance as a dual beam radiography source at NTS is highlighted in this paper.

Performance of the Cygnus x-ray source

Cygnus is a radiographic x-ray source developed for support of the Sub-Critical Experiments Program at the Nevada Test Site. Major requirements for this application are: a dramatically reduced spot size as compared to both Government Laboratory and existing commercial alternatives, layout flexibility, and reliability. Cygnus incorporates proven pulsed power technology (Marx Generator, Pulse Forming Line, Water Transmission Line, and Inductive Voltage Adder sub-components) to drive a high voltage vacuum diode. In the case of Cygnus, a relatively new approach (the rod pinch diode [1]) is employed to achieve a small source diameter. Design specifications are: 2.25 MeV endpoint energy, &#60; 1 mm source diameter, and >3 rads dose at 1 meter. The pulsed power and system architecture design plan has been previously presented [2]. The first set of Cygnus shots were geared to verification of electrical parameters and, therefore, used a large area diode configuration offering increased shot rate as compared to that of the rod pinch diode. In this paper we present results of initial rod pinch operation in terms of electrical and radiation parameters.

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.