Results 1 - 10 of 104
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[en] This work deals with atomic diffusion in solids and the investigation thereof by scattering methods, specifically using X-ray photon correlation spectroscopy (XPCS). Contrary to conventional methods, such as investigating diffusion via the analysis of the depth-dependent concentration of radioactive isotopes diffused into the sample during annealing, XPCS is able to detect the single atomic jump from lattice site to lattice site. Starting with a review of the concepts used for describing diffusion on a lattice it is demonstrated how to interpret the results of an XPCS experiment by the pair correlation function. To this end also the temporal evolution of the pair correlation function under the influence of short-range order is derived. Different aspects of atomic diffusion are simulated and discussed for selected exemplary systems. Results from experiments on these systems are presented and promising directions of future research are proposed. As the technique of X-ray photon correlation spectroscopy with atomic resolution was developed and for the first time successfully performed in the frame of this work, a substantial point is dedicated to practical considerations and calculations concerning the experiment, such as the optimal experimental set-up, the evaluation of the data, and the expected statistical significance of the results. (author)
[de]Diese Arbeit beschaeftigt sich mit der atomaren Diffusion in Festkoerpern und ihrer Untersuchung mittels Streumethoden, im konkreten Fall der Roentgenphotonenkorrelationsspektroskopie (XPCS). Anders als konventionelle Methoden wie die Untersuchung der Diffusion mittels tiefensensitiver Konzentrationsanalyse radioaktiver Isotope nach Auslagerung der Probe kann XPCS den einzelnen Sprung des Atoms von Gitterplatz zu Gitterplatz detektieren. Ausgehend von einem ueberblick ueber die Begriffe zur Beschreibung von Diffusion auf einem Gitter wird dargelegt, wie die Ergebnisse eines XPCS-Experiments mit Hilfe der Paarkorrelation interpretiert werden koennen. Dazu wird auch die zeitliche Entwicklung der Paarkorrelation unter dem Einfluss von Nahordnung abgeleitet. Anhand ausgewaehlter Systeme werden verschiedene Aspekte der atomaren Diffusion simuliert und diskutiert. Die Resultate der Experimente an diesen Systemen werden praesentiert und lohnende zukuenftige Richtungen vorgeschlagen. Da im Rahmen dieser Arbeit die Technik der Roentgenphotonenkorrelationsspektroskopie mit atomarer Aufloesung entwickelt und erstmals durchgefuehrt wurde, ist ein wesentlicher Punkt auch praktischen ueberlegungen und Rechnungen gewidmet, wie dem optimalen Einstelen der experimentellen Konfiguration, der Auswertung der Daten und der erwarteten statistischen Signifikanz des Ergebnisses. (author)
[en] The Linac Coherent Light Source at the SLAC National Accelerator Laboratory (operated by Stanford University for the US Department of Energy) is the world's first hard X-ray Free Electron Laser machine. It uses high energy electrons delivered by a linac to create ultrafast and brilliant X-ray pulses that can be used as a 'high-speed' camera to obtain images of atoms and molecules. LCLS is a pioneer machine and, as such, its design has encountered unprecedented challenges, the solutions to which will benefit future facilities of its kind across the globe. This article describes the radiation protection aspects of LCLS electron beamlines. Special emphasis is put on the successful commissioning of the LCLS electron line, where, for all examined loss sources, the measured prompt and residual dose rates are in agreement with or below the values predicted through detailed Monte Carlo simulations, used earlier to design the shielding
[en] Facility for Advanced Accelerator Experimental Tests (FACET) in SLAC will be used to study plasma wakefield acceleration. FLUKA Monte Carlo code was used to design a maze wall to separate FACET project and LCLS project to allow persons working in FACET side during LCLS operation. Also FLUKA Monte Carlo code was used to design the shielding for FACET dump to get optimum design for shielding both prompt and residual doses, as well as reducing environmental impact. FACET will be an experimental facility that provides short, intense pulses of electrons and positrons to excite plasma wakefields and study a variety of critical issues associated with plasma wakefield acceleration (1). This paper describes the FACET beam parameters, the lay-out and its radiological issues.
[en] The Heavy Ion Fusion Science Virtual National Laboratory (HIFS-VNL) is currently finalizing the design of NDCX-II, the second phase of the Neutralized Drift Compression Experiment, which will use an ion beam to explore Warm Dense Matter (WDM) and Inertial Fusion Energy (IFE) target hydrodynamics. The ion induction accelerator will include induction cells and Blumleins from the decommissioned Advanced Test Accelerator (ATA) at Lawrence Livermore National Laboratory (LLNL). A test stand has been built at Lawrence Berkeley National Laboratory (LBNL) to test refurbished ATA induction cells and pulsed power hardware for voltage holding and ability to produce various compression and acceleration waveforms. The performance requirements, design modifications, and test results will be presented.
[en] We report on the testing of optical diagnostics developed for warm-dense-matter (WDM) experiments on the Neutralized Drift Compression Experiments (NDCX-1) at Lawrence Berkeley National Laboratory (LBNL). The diagnostics consist of a fast optical pyrometer, a streak camera spectrometer, and a VISAR.While the NDCX is in the last stage of commissioning for the target experiments, the diagnostics were tested elsewhere in an experiment where an intense laser pulse was used to generate the WDM state in metallic and carbon samples
[en] The DIANA project (Dakota Ion Accelerators for Nuclear Astrophysics) is a collaboration between the University of Notre Dame, University of North Carolina, Western Michigan University, and Lawrence Berkeley National Laboratory to build a nuclear astrophysics accelerator facility 1.4 km below ground. DIANA is part of the US proposal DUSEL (Deep Underground Science and Engineering Laboratory) to establish a cross-disciplinary underground laboratory in the former gold mine of Homestake in South Dakota, USA. DIANA would consist of two high-current accelerators, a 30 to 400 kV variable, high-voltage platform, and a second, dynamitron accelerator with a voltage range of 350 kV to 3 MV. As a unique feature, both accelerators are planned to be equipped with either high-current microwave ion sources or multi-charged ECR ion sources producing ions from protons to oxygen. Electrostatic quadrupole transport elements will be incorporated in the dynamitron high voltage column. Compared to current astrophysics facilities, DIANA could increase the available beam densities on target by magnitudes: up to 100 mA on the low energy accelerator and several mA on the high energy accelerator. An integral part of the DIANA project is the development of a high-density super-sonic gas-jet target which can handle these anticipated beam powers. The paper will explain the main components of the DIANA accelerators and their beam transport lines and will discuss related technical challenges
[en] The magnet system of the VENUS ECR Ion Source at LBNL has two 1.5-watt cryocoolers suspended in the cryostat vacuum. Helium vapor from the liquid reservoir is admitted to a finned condenser bolted to the cryocooler 2nd stage and returns as liquid via gravity. Small-diameter flexible tubes allow the cryocoolers to be located remotely from the reservoir. With 3.1 watts load, the helium reservoir is maintained at 4.35 K, 0.05K above the cryocooler temperature. Design, analysis, and performance are presented
[en] At Lawrence Berkeley National Laboratory we are constructing an ECR ion source test facility for nuclear science experiments. For this purpose a single-stage 2.45 GHz electron cyclotron resonance ion source has been designed and fabricated. It features an axial magnetic field with a mirror ratio of up to 5.5 and a hexapole field produced by a novel Nd-Fe-B permanent magnet assembly. In order to enhance the ion confinement time the source plasma volume has been enlarged as much as possible while still maintaining a high mirror ratio. This paper describes the design of the source. Ion optics simulation of the extraction system currently under design will also be presented
[en] For over 20 years, Allison scanners have been used to measure emittances of low-energy ion beams. We show that scanning large trajectory angles produces ghost signals caused by the sampled beamlet impacting on an electric deflection plate. The ghost signal strength is proportional to the amount of beam entering the scanner. Depending on the ions, and their velocity, the ghost signals can have the opposite or the same polarity as the main beam signals. The ghost signals cause significant errors in the emittance estimates because they appear at large trajectory angles. These ghost signals often go undetected because they partly overlap with the real signals, are mostly below the 1% level, and often hide in the noise. A simple deflection plate modification is shown to reduce the ghost signal strength by over 99%
[en] During the last year, the VENUS ECR ion source was commissioned at 18 GHz and preparations for 28 GHz operation, which is set to begin early in 2004, are now underway. The goal of the VENUS ECR ion source project as the RIA R and D injector is the production of 240emA of U30+, a high current medium charge state beam. On the other hand, as an injector ion source for the 88-Inch Cyclotron the design objective is the production of 5emA of U48+, a low current, very high charge state beam. During the commissioning phase with 18 GHz, tests with various gases and recently metals have been performed with up to 2000 W RF power and the performance is very promising. For example, 1100 e mu A of O6+,180 e mu A of Ar12+, 150 emA of Xe20+ and 100 emA of Bi24+ were produced in the early commissioning phase, ranking VENUS among the currently highest performance 18 GHz ECR ion sources. The emittance of the beams produced at 18 GHz was measured with a two axis emittance scanner. In FY04 a 10 kW, 28 GHz gyrotron system will be added, which will enable VENUS to reach full performance. The performance of the VENUS ion source, low energy beam transport (LEBT) and its closed loop cryogenic system are described in the paper. Recently, a new high temperature axial oven has been installed in the source and the first results on metal beams such as bismuth are given. The design of the 28 GHz, 10 kW gyrotron system is also be described. During the last year, the VENUS ECR ion source was commissioned at 18 GHz and preparations for 28 GHz operation, which is set to begin early in 2004, are now underway. The goal of the VENUS ECR+, a high current medium charge state beam. On the other hand, as an injector ion source for the 88-Inch Cyclotron the design objective is the production of 5 emA of U48+, a low current, very high charge state beam. During the commissioning phase with 18 GHz, tests with various gases and recently metals have been performed with up to 2000 W RF power and the performance is very promising. For example, 1100 e mu A of O6+, 180 e muA of Ar12+, 150 emA of Xe20+ and 100 emA of Bi24+ were produced in the early commissioning phase, ranking VENUS among the currently highest performance 18 GHz ECR ion sources. The emittance of the beams produced at 18 GHz was measured with a two axis emittance scanner. In FY04 a 10kW, 28 GHz gyrotron system will be added, which will enable VENUS to reach full performance. The performance of the VENUS ion source, low energy beam transport (LEBT) and its closed loop cryogenic system are described in the paper. Recently, a new high temperature axial oven has been installed in the source and the first results on metal beams such as bismuth are given. The design of the 28 GHz, 10 kW gyrotron system is also be described