Evan Rose

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, < 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.

KrF amplifier design issues and application to inertial confinement fusion system design

Los Alamos National Laboratory has assembled an array of experimental and theoretical tools to optimize amplifier design for future single-pulse KrF lasers. The next opportunity to exercise these tools is with the design of the second-generation NIKE system under construction at the Naval Research Laboratory with the collaboration of Los Alamos National Laboratory. Major issues include laser physics (energy extraction in large modules with amplified spontaneous emission) and diode performance and efficiency. Low cost is increasingly important for larger future KrF single-pulse systems (low cost and high efficiency is important for larger repetitively pulsed applications such as electric power production). In this article, we present our approach to amplifier scaling and discuss the more important design considerations for large single-pulse KrF amplifiers. We point out where improvements in the fundamental database for KrF amplifiers could lead to increased confidence in performance predictions for large amplifiers and address the currently unresolved issues of anomalous absorption near line center and the possibility of diode instabilities for lowimpedance designs. Los Alamos has applied these amplifier design tools to the conceptual design of a 100-kJ Laser Target Test Facility and a 3-MJ Laboratory Microfusion Facility.

A new method for imaging nuclear threats using cosmic ray muons

Muon tomography is a technique that uses cosmic ray muons to generate three dimensional images of volumes using information contained in the Coulomb scattering of the muons. Advantages of this technique are the ability of cosmic rays to penetrate significant overburden and the absence of any additional dose delivered to subjects under study above the natural cosmic ray flux. Disadvantages include the relatively long exposure times and poor position resolution and complex algorithms needed for reconstruction. Here we demonstrate a new method for obtaining improved position resolution and statistical precision for objects with spherical symmetry.

Foil support structuree for large electron guns

Abstract : This paper describes a novel support structure for a vacuum diode used to pump a gaseous laser with an electron beam. Conventional support structures are designed to hold a foil flat and rigid. This new structure takes advantage of the significantly greater strength of metals in pure tension, utilizing curved shapes for both foil and support structure. The shape of the foil is comparable to the skin of a balloon, and the shape of the support structure is comparable to the cables of a suspension bridge. This design allows a significant reduction in foil thickness and support structure mass, resulting in a lower electron-beam loss between diode and laser gas. In addition, the foil is pre-formed in the support structure at pressures higher than operating pressure. Therefore, the foil is operated far from the yield point. Increased reliability is anticipated.

Testing Metglas for use in DARHT accelerator cells

The Dual Axis Radiographic Hydrotest Facility [DARHT] at Los Alamos will use two induction linacs to produce high-energy electron beams. The electron beams will be used to generate X-rays from bremsstrahlung targets. The X-rays will be used to produce radiographs. The first accelerator is operational now, producing a 60-nanosecond electron beam. The second accelerator is under construction. It will produce a 2-microsecond electron beam. The 78 induction cells of the second axis accelerator require a total Metglas capacity of approximately 40 volt seconds of flux. Four Metglas cores are used in each of the 5-foot diameter accelerator cells. Each Metglas core weighs approximately 3000 pounds. This paper presents the measurement techniques and results of the Metglas tests. Routine automated analysis and archival of the pulse data provided hysteresis curves, energy loss curves and total flux swing in the operating regime. Results of the tests were used to help the manufacturer improve quality control and increase the average flux swing of the cores. Results of the tests were used to match Metglas cores and to assemble accelerator cells with equal volt-second ratings.

Qualification of diode foil materials for excimer lasers

The Aurora facility at Los Alamos National Laboratory uses KrF excimer lasers to produce 248 nm light for inertial confinement fusion applications. Diodes in each amplifier produce relativistic electron beams to pump a Kr-F-Ar gas mixture. A foil is necessary to separate the vacuum diode from the laser gas. High tensile strength, high electron transmission, low ultraviolet reflectivity, and chemical compatibility with fluorine have been identified as requisite foil properties. Several different materials were acquired and tested for use as diode foils. Transmission and fluorine compatibility tests were performed using the Electron Gun Test Facility (EGTF) at Los Alamos. Off-line tests of tensile strength and reflectivity were performed. Titanium foil, which is commonly used as a diode foil, was found to generate solid and gaseous fluoride compounds, some of which are highly reactive in contact with water vapor. 6 refs., 6 figs., 1 tab.

Radiographic performance of Cygnus 1 and the Febetron 705

Spot sizes are measured for medium energy X-ray generators. The Cygnus X-ray source [J. R. Smith et al., 2002] is developed for support of the Sub-Critical Experiments Program at the Nevada Test Site. Cygnus uses proven pulsed power technology to drive a rod pinch diode at 2.25 megavolts. Four rads at one meter is achieved in a 50-ns FWHM pulse and the radiographic spot size is as small as 1 mm-diameter. Radiographic spot size is measured employing roll bars and resolution targets. Images are recorded on film and storage phosphor, digitized, and analyzed. The Febetron 705 X-ray source operates at 2.3 megavolts and is rated at 500 millirads at one meter. A demountable diode tube provides the ability to replace the standard diode with custom hardware. Spot sizes for standard hardware are measured. New diode designs are investigated in an effort to generate smaller radiographic spot sizes to produce improved radiographic images.

Testing pulse forming networks with DARHT accelerator cells

The Dual Axis Radiographic Hydrotest Facility [DARHT] at Los Alamos will use two induction linacs to produce high-energy electron beams. The electron beams will be used to generate X-rays from bremsstrahlung targets. The X-rays will be used to produce radiographs. The first accelerator is operational now, generating a 60-nanosecond electron beam. The second accelerator is under construction. It will generate a 2-microsecond electron beam. The 78 induction cells of the second axis accelerator will be driven by an equal number of pulse forming networks. Each pulse forming network [PFN] generates a nominal 200-kV, 2-microsecond pulse to drive an accelerator cell. Each pulse forming network consists of a set of four equal-capacitance sub-PFNs, stacked in a Marx configuration. The PFN Test Stand was configured to test newly constructed accelerator cells under conditions of full voltage and pulse duration. The PFN Test Stand also explored jitter, prefire and reliability issues for a pulse forming network operated into a purely resistive load. The PFN Test Stand provided experience operating a simple subsystem of the DARHT accelerator. This subsystem involved controls, diagnostics, data acquisition and archival, power supplies, trigger systems, core reset and a gas flow system for the spark gaps. Issues for the DARHT accelerator were investigated in this small-scale facility.

Beam emittance diagnostic for the DARHT second axis injector

Low beam emittance is key to achieving the required spot size at the output focus of the DARHT Second Axis. The nominal electron beam parameters at the output of the injector are 2 kA, 4.6 MeV, 2-microsecond pulse width and an rms radius less than 1 cm. Emittance is measured by bringing the beam to a focus in which the emittance is a dominant influence in determining the spot size. The spot size is measured from Cerenkov or optical transition radiation (OTR) generated from a target intercepted by the beam. The current density in the focused DARHT beam would melt this target in less than 1/2 microsec. To prevent this we have designed a DC magnetic transport system that defocuses the beam on the emittance target to prevent overheating, and uses a 125-ns half period pulsed solenoid to selectively focus the beam for short times during the beam pulse. During the development of the fast-focusing portion of this diagnostic it has been determined that the focusing pulse must rapidly sweep through the focus at the target to an over-focused condition to avoid target damage due to overheating. The fast focus produces /spl sim/1 kilogauss field over an effective length of /spl sim/50 cm to bring the beam to a focus on the target. The fast focus field is generated with a 12-turn coil located inside the beam-transport vacuum chamber with the entire fast coil structure within the bore of a D.C. magnet. The pulsed coil diameter of /spl sim/15 cm is dictated by the return current path at the nominal vacuum wall. Since the drive system is to use 40 kV to 50 kV technology and much of the inductance is in the drive and feed circuit, the coil design has three 120 degree segments. The coil, driver and feed system design, as well as beam envelope calculations and target heating calculations are presented below. Operation of the OTR imaging system will be discussed in separate publication.

Improved KrF laser design for the Laboratory Microfusion Facility

A conceptual design of the KrF laser-driven Laboratory Microfusion Facility (LMF) has been completed. LASNEX calculations predict an indirect-drive target yield of 400 MJ from the 3-MJ, 480-beam driver system. Nine final amplifiers with individual output energy of 412 kJ are used. The 480 beams are transmitted through helium to reduce losses and are delivered to target through a series of buildings designed for radiation safety. The total cost of the KrF laser-driven LMF is estimated by an independent cost assessment to be