R.L. Carlson

Design of a driver for the Cygnus X-ray source

Cygnus is the prototype of a radiographic x-ray source leveraging existing hardware and designs to drive a rod-pinch diode at 2.25 MV. This high-resolution x-ray source is being developed to support the Sub-Critical Experiments Program (SCE) at the Nevada Test Site (NTS), and as such employs a modular technology that is scaleable to higher voltages and can be readily deployed underground. The diode is driven by three Induction Voltage Adder (IVA) cells from the Sandia SABRE [1] accelerator, threaded by a positive polarity vacuum coax that extends 2 meters to the diode and is designed to operate below electron emission on the anodized outer electrode. The /spl sim/40 ohm diode impedance requires a 40/3/sup 2/ or /spl sim/4.5 ohm source to drive the three IVA cavities in parallel; a convenient impedance for a single water coax. The water coax is designed to function as a two-step impedance transformer as well as a long, passive water cable, accommodating several bends along its length. The latter feature allows independent positioning of the pulsed power driver, IVA and diode x-ray source. The long water coax is driven by a PFL originally developed for Sandias Radiographic Integrated Test Stand (RITS) and a low-inductance commercial Marx charges the single PFL. The accelerator design is a result of a cooperative effort by Titan-PSI and Maxwell (now collectively Titan-PSD) SNLA, LANL, NRL, and Bechtel-Nevada.

Guiding of an electron beam from a rf accelerator by a laser-ionized channel

The first guiding of multiple pulses from a rf accelerator by an ultraviolet‐laser‐ionized channel in low‐pressure benzene has been demonstrated. In particular, two 5‐ns, 750‐A, 30‐MeV pulses separated by 20 ns were transported a maximum distance of 13.5 m. Detailed measurements of the temporal behavior of the beam as it propagated versus distance, laser‐channel parameters, and accelerator conditions were made.

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.

Induction voltage adder architectures and electrical characteristics

In this paper we use three different designs for induction voltage adders to be used in radiographic X-ray sources to illustrate the differences between possible IVA architectures, and between their electrical characteristics as these affect the radiographic diodes that the IVAs drive. The architectures are chosen considering the spaces in which the FVAs must fit, and the electrical characteristics are determined by the approaches chosen for the IVA electrical subsystems. We describe the status of the various IVA technologies. The IVAs, (RITS, the AWE HRF and URSA) are large systems with similar output parameters in the 12-16 MV range.

High‐brightness electron‐beam generation and transport

Experimental results for a high‐brightness electron‐beam source are compared to results of numerical and analytic calculations. The 4‐MV, 5‐kA beam is generated from a velvet cathode and focused by a solenoidal magnet. Data for the diode impedance and the beam focusing length and spot size agree well with numerical simulations. The minimum spot size is consistent with a normalized Lapostolle emittance of 0.12 cm rad, yielding a brightness of 3.5×108 A/m2  rad2 . Transverse beam oscillations are observed in streak photographs and are thought to be caused by electromagnetic dipole modes in the diode cavity. The oscillation amplitude is significantly reduced by making the current rise more slowly.

Intense electron beam sources for flash radiography

High intensity pulsed electron beams are used to create bremsstrahlung x-ray sources for flash radiographic interrogation of dynamic experiments. Typical industrial sources operate below 200 GW/cm2 intensities, while experimental requirements can demand above 50 TW/cm2. Recent developments in pulsed power-driven high intensity electron beam systems have significantly increased these operating regimes, demonstrating 20 TW/cm2, and computations predict successful extrapolation to higher intensities. Detailed studies of electron beam configurations, both theoretical and experimental, and the prognosis for each to increase to the required levels is discussed.

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.

Rex, A 5-MW Pulsed-power Source For Driving High-brightness Electron Berm Diodes

The Relativistic Electron-beam Experiment, or REX accelerator, is a pulsed-power source capable of driving a 100-ohm load at 5 MV, 50 kA, 45 ns (FWHM) with less than a 10-ns rise and 15-ns fall time. This paper describes the pulsed-power modifications, modeling, and extensive measurements on REX to allow it to drive high impedance (100s of ohms) diode loads with a shaped voltage pulse. A major component of REX is the 1.83-m diam x 25.4-cm-thick Lucite insulator with embedded grading rings that separates the output oil transmission line from the vacuum vessel that containing the re-entrant anode and cathode assemblies. A radially tailored, liquid-based resistor provides a stiff voltage source that is insensitive to small variations of the diode current and, in addition, optimizes the electric field stress across the vacuum side of the insulator. The high-current operation of REX employs both multichannel peaking and point-plane diverter switches. This mode reduces the prepulse to less than 2 kV and the postpulse to less than 5% of the energy delivered to the load. Pulse shaping for the present diode load is done through two L-C transmission line filters and a tapered, glycol- based line adjacent to the water PFL and output switch. This has allowed REX to drive a diode producing a 4-MV, 4.5-kA, 55-ns flat-top electron beam with a normalized Lapostolle emittance of 0.96 mm-rad corresponding to a beam brightness in excess of 4.4x10/sup 8/ A/m/sup 2/-rad/sup 2/[1,2].

Pulse power performance of the Cygnus 1 and 2 radiographic sources

Cygnus is a two-axis radiographic X-ray facility designed to drive rod-pinch diode loads at 2.25 MV with a spot size of about 1 mm producing 4 Rads at 1 meter. This x-ray source was developed to support the Sub-Critical Experiments Program (SCE) at the Nevada Test Site (NTS) and is distinguished from other commercially available sources by a dramatically reduced spot size for high resolution radiography, higher reliability, and compact size and modularity for greater layout flexibility to fit within the size constraints of its ultimate under-ground site location [D. Weidenheimer et al., June 17-22, 2001]. The facility is composed of two virtually identical machines referred to as Cygnus 1 and Cygnus 2 that incorporate proven pulsed power technology. Each machine employs a Marx generator, pulse forming line (pfl), water coax transmission line, and inductive voltage adder (IVA) that drive a high vacuum rod-pinch diode. The pfl design was originally developed for the radiographic integrated test stand (RITS) [I. D. Smith et al., October 2002] and the IVA cells are from the Sandia SABRE [J. P. Corley et al., 1991] accelerator. The Cygnus 1 machine was constructed and fielded at the Los Alamos National Laboratory to undergo pulsed power component and reliability testing and for use to develop and optimize the rod-pinch diode load [J. R. Smith et al., June 23-28, 2002]. Later, Cygnus 2 was constructed and fielded at Titan-PSD for testing employing the changes and modifications that resulted from of the Cygnus 1 tests. At the time of this writing, Cygnus 2 has undergone testing of the pulsed power components up through the output of the water line where a dummy load was placed. A pulse has not yet been propagated through the water coax to the diode. This paper describes and compares the pulsed power performance of both Cygnus machines up to the output of the water line. The Cygnus testing program is a result of the cooperative effort of Titan PSD, Sandia National Laboratory, Los Alamos National Laboratory, and Bechtel Nevada.