Magnetic fields above the surface of a superconductor with internal magnetism

The author presents a method for calculating the magnetic fields near a planar surface of a superconductor with a given intrinsic magnetization in the London limit. He computes solutions for various magnetic domain boundary configurations and derives relations between the spectral densities of the magnetization and the resulting field in the vacuum half space, which are useful if the magnetization can be considered as a statistical quantity and its features are too small to be resolved individually. The results are useful for analyzing and designing magnetic scanning experiments. Application to existing data from such experiments on Sr$_2$RuO$_4$ show that a domain wall would have been detectable, but the magnetic field of randomly oriented small domains and small defects may have been smaller than the experimental noise level.Comment: 8 pages, 2 figures. Journal version. Added one figure, some discussion. A few typos correcte

Iron Dominated Electromagnets: Design, Fabrication, Assembly and Measurements

Medium energy electron synchrotrons used for the production of high energy photons from synchrotron radiation is an accelerator growth industry. Many of these accelerators have been built or are under construction to satisfy the needs of synchrotron light users throughout the world. Because of the long beam lifetimes required for these synchrotrons, these medium energy accelerators require the highest quality magnets of various types. Other accelerators, for instance low and medium energy boosters for high energy physics machines and electron/positron colliders, require the same types of magnets. Because of these needs, magnet design lectures, were organized and presented periodically at biennual classes organized under the auspices of the US Particle Accelerator School (USPAS). These classes were divided among areas of magnet design from fundamental theoretical considerations, the design approaches and algorithms for permanent magnet wigglers and undulators and the design and engineering of conventional accelerator magnets. The conventional magnet lectures were later expanded for the internal training of magnet designers at LLNL at the request of Lou Bertolini. Because of the broad nature of magnet design, Dr. S. Y. Lee, the former Director of the Particle Accelerator School, saw the need for a specialized course covering the various aspects of the design, engineering and fabrication of conventional magnets. This section of the class was isolated and augmented using the LLNL developed material resulting in the class on conventional magnet design. Conventional magnets are defined (for the purposes of this publication) as magnets whose field shape is dominated by the shape of the iron magnet yoke and are excited by coils, usually wound from solid or hollow water-cooled copper or aluminum conductors. This publication collects the lecture notes, written for the first course in the USPAS conventional magnet design course and evolved over subsequent presentations of this same course, and organizes the material roughly divided among two parts. One part is theoretical and computational and attempts to provide a foundation for later chapters which exploit the expressions and algorithms for the engineering and design calculations required to specify magnet conceptual designs. A chapter is devoted to the description and use of one of many magnet codes used to characterize the two dimensional field resulting from various magnet cross-sections. A chapter is included which exploits the two-dimensional theory and applies the mathematics to techniques and systems for magnet measurement. The second part of this publication ranges to practical issues associated with the fabrication of components, assembly, installation and alignment of magnets. This section also includes fabrication practices which respond to personnel and equipment protection needs. Required design calculations are supplemented by examples and problems. A CD is included with tools provided to simplify the computation of some of the more tedious relationships. This CD also includes useful photographs and pictures describing the high volume production of typical magnet types, which if included in the publication will add too many pages and increase the cost of publication. Styles among those facing similar problems will result in a wide variation of individual magnet designs. Designs and technologies will evolve and improve. This publication provides a snapshot of the present technology and presents as examples the magnet designs developed in response to the needs of several projects, the Advanced Light Source at LBNL, PEPII Low Energy Ring and SPEAR3 synchrotron light source at SLAC and the Australian Light Source, currently under construction in Melbourne. In each example, the reasons for fabrication design decisions are itemized and rationalized as much as is reasonable. The examples presented in this publication are provided as starting points which can be used as a design basis for magnets required for future projects. It is hoped that the listing of some design choices and the motivation for these choices will be useful. It is the intention of the author to publish a document collecting and archiving the tools and techniques learned from the past masters in the craft and to provide a useful reference for future magnet designers

Ultrafast X-ray Science at the Sub-Picosecond Pulse Source

The ultrafast, high brightness x-ray free electron laser (XFEL) sources of the future have the potential to revolutionize the study of time dependent phenomena in the natural sciences. These linear accelerator (linac) sources will generate femtosecond (fs) x-ray pulses with peak flux comparable to conventional lasers, and far exceeding all other x-ray sources. The Stanford Linear Accelerator Center (SLAC) has pioneered the development of linac science and technology for decades, and since 2000 SLAC and the Stanford Synchrotron Radiation Laboratory (SSRL) have focused on the development of linac based ultrafast electron and x-ray sources. This development effort has led to the creation of a new x-ray source, called the Sub-Picosecond Pulse Source (SPPS), which became operational in 2003 [1]. The SPPS represents the first step toward the world's first hard x-ray free electron laser (XFEL), the Linac Coherent Light Source (LCLS), due to begin operation at SLAC in 2009. The SPPS relies on the same linac-based acceleration and electron bunch compression schemes that will be used at the LCLS to generate ultrashort, ultrahigh peak brightness electron bunches [2]. This involves creating an energy chirp on the electron bunch during acceleration and subsequent compression of the bunch in a series of energy-dispersive magnetic chicanes to create 80 fs electron pulses. The SPPS has provided an excellent opportunity to demonstrate the viability of these electron bunch compression schemes and to pursue goals relevant to the utilization and validation of XFEL light sources

A Proposal for a New HOM Absorber in a Straight Section of the PEP-II Low Energy Ring

Attainment of high luminosity in storage ring colliders necessitates increasing stored currents and reducing bunch lengths. Consequently, intense beam fields will scatter more power into higher order modes from beam line sources such as collimators, masks and tapers. This power penetrates into sensitive components such as a bellows, causing undesirable heating and limits machine performance. To overcome this limitation we propose incorporating ceramic absorbers in the vicinity of the bellows to damp beam induced modes while preserving a matched impedance to the beam. This is accomplished with an absorber configuration which damps TE dipole and quadrupole traveling waves while preserving TM monopole propagation. A scattering parameter analysis is presented utilizing properties of commercial grade ceramics and indicates a feasible solution

Synchrotron Radiation Photoemission Spectroscopic Study of Band Offsets and Interface Self-cleaning by Atomic Layer Deposited HfO2 on In0.53Ga0.47As and In0.52Al0.48As

The Synchrotron Radiation Photoemission Spectroscopic (SRPES) study was conducted to (a) investigate the surface chemistry of In{sub 0.53}Ga{sub 0.47}As and In{sub 0.52}Al{sub 0.48}As post chemical and thermal treatments, (b) construct band diagram and (c) investigate the interface property of HfO{sub 2}/In{sub 0.53}Ga{sub 0.47}As and HfO{sub 2}/In{sub 0.52}Al{sub 0.48}As. Dilute HCl and HF etch remove native oxides on In{sub 0.53}Ga{sub 0.47}As and In{sub 0.52}Al{sub 0.47}As, whereas in-situ vacuum annealing removes surface arsenic pile-up. After the atomic layer deposition of HfO{sub 2}, native oxides were considerably reduced compared to that in as-received epi-layers, strongly suggesting the self-clean mechanism. Valence and conduction band offsets are measured to be 3.37 {+-} 0.1eV, 1.80 {+-} 0.3eV for In{sub 0.53}Ga{sub 0.47}As and 3.00 {+-} 0.1eV, 1.47 {+-} 0.3eV for In{sub 0.52}Al{sub 0.47}As, respectively

Ultrafast Coherent Control and Characterization of Surface Reactions using FELs

The microscopic understanding of reactions at surfaces requires an in-depth knowledge of the dynamics of elementary processes on an ultrafast timescale. This can be accomplished using an ultrafast excitation to initiate a chemical reaction and then probe the progression of the reaction with an ultrashort x-ray pulse from the FEL. There is a great potential to use atom-specific spectroscopy involving core levels to probe the chemical nature, structure and bonding of species on surfaces. The ultrashort electron pulse obtained in the linear accelerator to feed the X-ray FEL can also be used for generation of coherent synchrotron radiation in the low energy THz regime to be used as a pump. This radiation has an energy close to the thermal excitations of low-energy vibrational modes of molecules on surfaces and phonons in substrates. The coherent THz radiation will be an electric field pulse with a certain direction that can collectively manipulate atoms or molecules on surfaces. In this respect a chemical reaction can be initiated by collective atomic motion along a specific reaction coordinate. If the coherent THz radiation is generated from the same source as the X-ray FEL radiation, full-time synchronization for pump-probe experiments will be possible. The combination of THz and X-ray spectroscopy could be a unique opportunity for FEL facilities to conduct ultrafast chemistry studies at surfaces

CsBr Photocathode at 257nm: A Rugged High Current Density Electron Source

There is a continuing need for high intensity electron sources that will operate in demountable vacuum and can be externally modulated. Materials with wide bandgap, e.g. diamond, are rugged but need photon energies exceeding the bandgap to emit efficiently and this rules out the use of CW lasers. We have found that a photocathode of CsBr is both adequately intense(>150A/cm{sup 2}) and rugged and can be excited with photons of energy of 4.8eV(257nm). This is below the energy gap of CsBr(7.3eV) but such operation can be explained by the presence of intraband states about 4eV below the conduction band minimum

Chemical Bonding, Interfaces and Defects in Hafnium Oxide/Germanium Oxynitride Gate Stacks on Ge (100)

Correlations among interface properties and chemical bonding characteristics in HfO{sub 2}/GeO{sub x}N{sub y}/Ge MIS stacks were investigated using in-situ remote nitridation of the Ge (100) surface prior to HfO{sub 2} atomic layer deposition (ALD). Ultra thin ({approx}1.1 nm), thermally stable and aqueous etch-resistant GeO{sub x}N{sub y} interfaces layers that exhibited Ge core level photoelectron spectra (PES) similar to stoichiometric Ge{sub 3}N{sub 4} were synthesized. To evaluate GeO{sub x}N{sub y}/Ge interface defects, the density of interface states (D{sub it}) was extracted by the conductance method across the band gap. Forming gas annealed (FGA) samples exhibited substantially lower D{sub it} ({approx} 1 x 10{sup 12} cm{sup -2} eV{sup -1}) than did high vacuum annealed (HVA) and inert gas anneal (IGA) samples ({approx} 1x 10{sup 13} cm{sup -2} eV{sup -1}). Germanium core level photoelectron spectra from similar FGA-treated samples detected out-diffusion of germanium oxide to the HfO{sub 2} film surface and apparent modification of chemical bonding at the GeO{sub x}N{sub y}/Ge interface, which is related to the reduced D{sub it}

Ultrafast X-ray Studies of Structural Dynamics at SLAC

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements demonstrate that the initial stage of crystal disordering results from inertial motion on a laser softened potential energy surface. These inertial dynamics dominate for the first half picosecond following laser excitation, indicating that interatomic forces minimally influence atomic excursions from the equilibrium lattice positions, even for motions in excess of an {angstrom}. This also indicates that the atoms disorder initially without losing memory of their lattice reference