bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray spectropolarimetry of high-temperature plasmas
Authors: Shlyaptseva A. S. , Hansen S. B. , Kantsyrev V. L. , Bauer B. S. , Fedin D. A. , Ouart N. , Kazantsev S. A. , Petrashen A. G. , Safronova U. I.
Published: February 2004

X-ray spectropolarimetry is a powerful new tool for investigating the anisotropy of high-temperature plasmas. It is sensitive to the energetic electron distribution function and magnetic field, which, in general, have not been measured adequately. This diagnostic is based on the theoretical modeling of x-ray line polarization-dependent spectra measured simultaneously by spectrometers with different sensitivities to polarization. The increasing sophistication of spectroscopic monitoring of high-temperature plasmas has led to the necessity to account for both non-Maxwellian electron distribution functions and polarization properties of lines. This diagnostic can be applied to a broad range of plasmas, from low-density astrophysical plasmas to dense z-pinch plasmas. This work shows how x-ray spectropolarimetry complements the usual spectroscopic monitoring of hot plasmas and demonstrates the importance of accounting for x-ray line polarization in plasma diagnostics.
Soft x-ray detector array system on the Large Helical Device
Authors: Ohdachi S. , Toi K. ,   LHD Experimental Group, Takechi M. , Yamamoto S.
Published: February 2004

Soft x-ray (SX) detector array systems are installed on the Large Helical Device (LHD). Two types of systems are in operation: An 80 ch array for detailed profile measurement and two sets of 40 ch array installed inside the vacuum vessel suitable for fluctuation studies. Recent results of the profile and fluctuation measurement with this system are discussed.
Development and energy calibration of Si-FNA for LHD fast ion measurement
Authors: Osakabe M. , Yamamoto T. , Takeiri Y. , Mutoh T. , Asano E. , Ikeda K. , Tsumori K. , Kaneko O. , Kawahata K. , Ohyabu N. , Motojima O. ,   LHD Experimental Group
Published: February 2004

A Si-FNA (silicon-diode-based fast neutral analyzer) was newly developed at NIFS, which uses an electrically cooled silicon-diode as an energy analyzer. Si-FNA is suitable for energy-resolved profile measurements of fast neutrals since it is compact and inexpensive. Two Si-FNAs were recently designed and installed on the Large Helical Device (LHD) during the experimental campaign of 1999. The basic performance of a Si-FNA was examined by using x/γ-ray sources of Co57 and Am241. An energy resolution of about 3.5 keV was achieved for optimum shaping times and a resolution of about 6 keV was achieved for shorter shaping times which allowed for the higher counting rates required for LHD. The energy calibration for fast neutrals was performed by injecting neutral beams (NB) into the LHD vessel without target plasmas present. An energy loss of about 37 keV and an energy broadening of about 4.5 keV were observed at the calibration. Excellent slowing-down energy spectra are obtained for NB-injected fast particles without any influences of x/γ-rays.
Wavelet-based time-delay estimation for time-resolved turbulent flow analysis
Authors: Jakubowski M. , Fonck R. J. , Fenzi C. , McKee G. R.
Published: February 2004

A wavelet-transform-based spectral analysis is examined for application to beam emission spectroscopy (BES) data to extract poloidal rotation velocity fluctuations from the density turbulence data. Frequency transfer functions for a wavelet cross-phase extraction method are calculated. Numerical noise is reduced by shifting the data to give an average zero time delay, and the applicable frequency range is extended by numerical oversampling of the measured density fluctuations. This approach offers potential for direct measurements of turbulent transport and detection of zonal flows in tokamak plasma turbulence.
VISAR for measuring equation of state and shock propagation in liquid deuterium (abstract)
Authors: Celliers P. M. , Collins G. W. , Bradley D. K. , Moon S. J. , Munro D. H. , Cauble R. , Gold D. M. , Da Silva L. B. , Weber F. A. , Wallace R. J. , Hammel B. A. , Hsing W. W.
Published: February 2004

Initial laser-driven equation of state (EOS) experiments on liquid deuterium employed x-ray radiography to track the shock and particle speeds in the shock compressed sample. With the high pressures available with laser drivers we found that it is also possible to track the shock front directly with a velocity interferometer system for any reflector (VISAR) because the shock front reflects light across the visible spectrum with reflectance around 50% for shocks stronger than 50 GPa in liquid deuterium. We have observed similar reflectances in other dielectric samples, such as diamond, LiF, and water. The pressure required to produce a reflecting shock varies with each material. This phenomenon allows us to design impedance-matched EOS experiments using velocity interferometry to measure the propagation speed in the transparent shocked materials, and step breakout measurements to determine the speed in the pusher. In a different kind of experiment we have observed double shock compression in liquid deuterium by impacting a shock in liquid deuterium at a LiF anvil placed in the liquid sample. VISAR can be used to track the shock in the deuterium as well as the motion of the deuterium–LiF interface subsequent to impact. This allows us to diagnose double-shock states using the VISAR technique. As a final example VISAR can be used to track shock overtake events such as produced by shaped pulse compression or shock reverberation effects in the accelerating pusher. This capability is directly applicable to shock timing experiments needed to tune the drive pulse for inertial confinement fusion capsules on the National Ignition Facility.
Status of far infrared tangential interferometry/polarimetry system on NSTX (abstract)
Authors: Park H. , Deng B. , Domier C. W. , Johnson M. , Luhmann N. C.
Published: February 2004

Measurement of the core BT(r,t) value is essential in National Spherical Torus Experiment (NSTX), since effects of paramagnetism and diamagnetism in the Spherical Torus are expected to be considerably greater than that in higher aspect ratio tokamaks. Therefore, plasma parameters dependent upon BT such as the q-profile and local q value cannot be evaluated without independent measurement of BT(r,t). The multichord tangential far infrared interferometer/polarimeter (FIReTIP) system currently under development for NSTX will provide temporally and radially resolved toroidal field profile [BT(r,t)] and two-dimensional (2D) electron density profile [ne(r,t)] data. The seven channel system is based on three FIR lasers including the Stark tuned FIR laser operating at 119 μm which is an optimum wavelength for NSTX plasma parameters. The optical system is configured as a Michelson system using retroreflectors. The final system design is completed and initial two channels will be installed this fiscal year to support NSTX operation. The channel spacing of FIReTIP system is optimized via data analysis with various expected electron density and magnetic field profile shapes. In simulation study, the solution of self-consistent magnetohydrodynamics equilibrium was used to project a diamagnetic effect on the toroidal magnetic field. A detailed inversion process based on assumed profiles of the electron density and self-consistent toroidal field is exercised to estimate the source of errors for both local density and local magnetic field. The outcome of the proposed system is crucial to the study of confinement, heating and stability of NSTX plasmas.
X-ray interferometry with spherically bent crystals (abstract)
Authors: Koch Jeffrey A.
Published: February 2004

Recent progress in manufacturing high-quality spherically bent crystals allows highly monochromatic x-ray beams to be produced, and allows efficient x-ray imaging with μm-scale resolution. This article explores some of the constraints for x-ray interferometry utilizing spherically bent crystals and laser-produced plasma sources, and discusses several shearing interferometer concepts that might be experimentally investigated
Status of the new multipoint, multipulse Thomson scattering diagnostic for the MST RFP (abstract)
Authors: Den Hartog D. J. , Andrew P. L. , Holly D. J.
Published: February 2004

We are building a new Thomson scattering diagnostic system to measure electron temperature and density on the Madison Symmetric Torus (MST) reversed-field pinch experiment. This system has been designed to produce accurate single-shot measurements for 10 eV<Te<2 keV at electron densities ⩾1018 m−3. Scattered light will be simultaneously recorded from 20 radial locations across the 50 cm minor radius of the plasma. Multipulse capability will be provided by two identical Nd:YAG pulsed lasers whose trigger timing can be independently varied. This will allow several combinations of input energy and pulse timing during an MST discharge, ranging from one 4 J pulse for increased accuracy during low density operation to 1 J pulses at 100 Hz for temporal evolution measurements. Scattered light will be collected by a custom deep-focus lens and coupled by optical fiber to 20 identical filter polychromators. These polychromators are being manufactured by General Atomics and use silicon avalanche photodiode detectors. Each polychromator contains three wavelength channels to allow determination of Te, plus one channel at the laser wavelength to allow calibration using Rayleigh scattering for measurement of ne. System control and data acquisition will be done with dedicated personal computers.
X-ray observations and analysis with the Chandra X-ray observatory (abstract)
Authors: Smith Randall
Published: February 2004

The Chandra X-ray telescope has four separate detectors, two charge coupled devices, and two microchannel plates, behind a set of four highly polished, nested mirrors. The on-orbit performance of the imaging system is <1 arcsec, a dramatic improvement from the 5–8 arcsec previously available. In addition, two transmission gratings can be placed in the system, which for point sources can reach resolutions of 300–1000+ over the range λ∼1.5–160 Å. The entire system was calibrated prelaunch at NASA/MSFC in Huntsville, Alabama. The results have been dramatic: the first light image of the supernova remnant Cas A showed a never-before-seen point source near the center of the remnant that may be the neutron star left over after the explosion. Even the calibration image of a distant quasar (taken for focusing purposes) showed an x-ray emitting jet extending out from the nucleus for more than 20 kpc, which challenges current theories about jet propagation. The grating observations done by Chandra take x-ray astrophysics out of the photometry era into the spectroscopic age. Astrophysical plasmas reach conditions far out of the reach of any terrestrial laboratory, and astronomers are just beginning to understand the diagnostics of these plasmas; Chandra observations may even someday provide the best measurements of certain atomic rates. However, before that point is reached the astronomical community must gain a much better understanding of the existing and ongoing work in x-ray diagnostics from other fields. The general state of x-ray diagnostics for astronomical plasmas will also be discussed.
Hard x-ray spectrometers for NIF (abstract)
Authors: Seely John , Holland Glenn , Brown Charles , Deslattes Richard , Hudson Lawrence , Bell Perry , Miller Michael , Back Christina
Published: February 2004

A National Ignition Facility (NIF) core diagnostic instrument has been designed and will be fabricated to record x-ray spectra in the 1.2–20 keV energy range. The high-energy electronic x-ray instrument has four reflection crystals with overlapping coverage of 1.2–10.9 keV and one transmission crystal covering 8.6–20 keV. The spectral resolving power varies from approximately 1000 at low energies to 315 at 20 keV. The spectrum produced by each crystal is recorded by a modified commercial dental x-ray charge coupled device (CCD) detector. The scintillators on the CCD detectors are optimized for the energy ranges. A one-channel x-ray spectrometer, using one transmission crystal covering 12–60 keV, will be fabricated for the OMEGA laser facility. The transmission crystal spectrometers are based on instruments originally designed at National Institute for Standards and Technology for the purpose of characterizing the x-ray flux from medical radiography sources. Utilizing one of those instruments and a commercial dental x-ray CCD detector, x-ray images were recorded using a single pulse from a laboratory x-ray source with a peak charging voltage of 200 kV. A resolving power of 300 was demonstrated by recording on film the Kα1 and Kα2 characteristic x-ray lines near 17 keV from a molybdenum anode. The continuum radiation from a tungsten anode was recorded in the 20–50 keV energy range. The transmission crystal spectrometer has sufficient spectral resolution and sensitivity to record the line and continuum radiation from high-Z targets irradiated by the NIF laser and the OMEGA laser.
Soft x-ray characterization of a silicon p–n photodiode using a laser produced plasma source (abstract)
Authors: Eagleton R. T. , Ruggles L. E.
Published: February 2004

Silicon p–n diodes have proved to be excellent soft x-ray detectors due to their high sensitivity, nominally flat response, and long term stability. Advances in fabrication techniques have overcome many of the limitations of older silicon diodes by minimizing the thickness of the surface dead layer which would otherwise absorb low energy x-ray photons. Silicon photodiodes with extremely thin (80 Å) surface dead layers are now available.1 One of these diodes has been characterized for spectral sensitivity at x-ray photon energies of 163 eV and 1.4 keV using a laser produced plasma soft x-ray source. Measurements have also been made to characterize the impulse response using fourth harmonic laser light from a short pulse (80 ps full width half maximum) NdYAG laser.
Characterization of axially directed x rays generated from a target within a high-power z-pinch (abstract)
Authors: Sanford T. W. L. , Bailey J. E. , Chandler G. A. , Cuneo M. E. , Fehl D. L. , Hebron D. E. , Leeper R. J. , Lemke R. W. , Mock R. C. , Olson R. E. , Nash T. J. , Porter J. L. , Ruggles L. E. , Ruiz C. L. , Simpson W. W. , Struve K. W. , Stygar W. A. , Bowers R. L. , Chrien R. E. , Idzorek G. C. , Matuska W. , Peterson D. L. , Watt R. G.
Published: February 2004

X-ray powers on the order of 10 TW over an area of 4.5 mm2 are produced in the axial direction from the compression of a low-density foam target centered within a z-pinch on the Z generator.1 The x rays from this source are used for high-energy–density physics experiments, including the heating of hohlraums for inertial confinements fusion studies.2 In this article, detailed characteristics of this radiation source measured using an upgraded axial-radiation-diagnostic suite3 together with other on- and off-axis diagnostics are summarized and discussed in terms of Eulerian and Lagrangian radiation–magnetohydrodynamic code simulations. The source, characterized here, employs a nested array of 10-mm-long tungsten wires, at radii of 20 and 10 mm, having a total masses of 2 and 1 mg, and wire numbers of 240 and 120, respectively. The target is a 14 mg/cc CH2 foam cylinder of 5 mm diameter. The codes take into account the development of the Rayleigh–Taylor instability in the r–z plane, and provide integrated calculations of the implosion together with the x-ray generation. The radiation exiting the imploding target through the 4.5 mm2 aperture is measured primarily by the axial diagnostic suite that now includes diagnostics at an angle of ∼30° to the z axis. The near on-axis diagnostics include: (1) a seven-element filtered silicon-diode array,4 (2) a five-element filtered x-ray diffraction (XRD) array,5 (3) a six-element filtered PCD array,6 (4) a three-element bolometer,7 (5) time-resolved and time-integrating crystal spectrometers, and (6) two fast-framing x-ray pinhole cameras having 11 frames each. The filtered silicon diodes, XRDs, and PCDs are sensitive to 1–200, 140–2300, and 1000–4000 eV x rays, respectively. They (1) establish the magnitude of the prepluse generated during the run in of the imploding wire arrays, (2) measure the Planckian nature of the dominant thermal, and (3) nonthermal component of the emission. The bolometers and XRDs mounted on the near-normal and 30° LOS (line-of-sight) measure the total power and check the Lambertian nature of the emission. Additionally, a suite of filtered fast-framing x-ray pinhole cameras and silicon-diode arrays behind a transmission grating, mounted on LOSs nearly normal to the z axis, quantify the plasma plume exiting the aperture. The hard bremsstrahlung generated is estimated with both on- and off-axis shielded scintillator photomultiplier diagnostics.
An imaging diode array soft x-ray diagnostic for Z (abstract)
Authors: Simpson W. W. , Porter J. L. , Ruggles L. E. , Wenger D. F.
Published: February 2004

Measurements of the hohlraum wall temperature in Z-pinch driven hohlraum experiments require looking through small (2–4 mm diameter) diagnostic holes that undergo some degree of hole closure. The existing soft x-ray diagnostics on Z measure the total flux exiting this diagnostic hole and are therefore affected by this hole closure. To avoid having to measure the effective diagnostic hole area we have designed and constructed an imaging diode array (IDA) that incorporates pinhole imaging and an array of filtered silicon diodes to measure the absolute x-ray intensity from a spatially resolved region of a target. The instrument uses silicon diodes with subnanosecond time response that are sensitive to soft x rays in the range 100–3000 eV. An image of the target area is projected onto the silicon diodes using pinholes. Between each pinhole and it’s respective diode is a soft x-ray filter. The material and thickness of the filter are selected to allow unfolding of spectral information in the 100–3000 eV spectral region. We plan to insert a set of grazing-incidence mirrors between each of the filter/diode pairs in a future version of this instrument to better define the spectral bandpass of each diode channel. Radiation from the target region is monitored by a gated microchannel-plate-intensified image recording device that is located immediately behind the diode array. A small shadow in the recorded image corresponds to the specific area of the target that is imaged onto each silicon diode. We are presently fielding this instrument in experiments on the Z facility located at Sandia National Laboratories in Albuquerque, NM. The instrument is located on the same line-of-sight and measures the same spatial region as a filtered fast-framing x-ray pinhole camera and a transmission grating spectrometer. This article describes the design of the IDA diagnostic and presents the results of measurements obtained in hohlraum experiments conducted on Z.
X-ray imaging characteristics from a large spherically bent mica crystal (abstract)
Authors: Lee S. G. , Bak J. G. , Lee H. G. , Kwon M. , Bitter M. , Moon M. K.
Published: February 2004

A high vacuum x-ray crystal spectrometer was fabricated utilizing a large spherically bent mica crystal (2 cm×7 cm) and a x-ray sensitive charge coupled device camera to observe heliumlike neon spectra in the wavelength range from 13.4474 to 13.6980 A and heliumlike spectra of other low-Z elements, such as Mg and Al. The spectrometer was specifically designed for use at a low x-ray intensity magnetic mirror machine. The x-ray imaging characteristics of the large spherically bent mica crystal have been tested in the laboratory with an x-ray source, using the bremsstrahlung continuum. The results of these calibration measurements will be presented.
A spectroscopy diagnostic of plasma gradients in inertial confinement fusion imploded cores (abstract)
Authors: Golovkin I. , Mancini R. , Louis S. , Ochi Y. , Fujita K. , Niki I. , Nishimura H. , Uschmann I. , Butzbach R. , Förster E. , Back C. , Lee R. , Klein L. , Delettrez J.
Published: February 2004

X-ray spectroscopy of laser-driven imploded inertial confinement fusion (ICF) cores has proven to be a powerful diagnostic of spatially averaged temperature and density plasma conditions at the collapse of ICF implosion experiments. Temperature and density time histories can be extracted from the analysis of time-resolved x-ray line spectra using the temperature and density sensitivity of line intensities and Stark-broadened line shapes. The next step in the spectroscopy of imploded cores is the bracketing of core plasma gradients as a function of time. To this end, we discuss a spectroscopy diagnostic which is based on the self-consistent and simultaneous simulation and analysis of time-resolved x-ray line spectra and x-ray monochromatic images. Abel inversion of x-ray monochromatic images provide line emissivity spatial profiles; this information is critical for the determination of gradients in the core. We apply this technique to the analysis of data recorded in Ar-doped ICF implosion experiments driven with the GECCO XII laser system at Osaka University. In these experiments, time-resolved x-ray line spectra and x-ray monochromatic images were simultaneously recorded for the Ar He β and Ly β spectral features. From the analysis of the data we can extract the time history of temperature and density gradients in the core through the collapse of the implosion.
SIRINC: A code for assessing and optimizing the neutron imaging diagnostic capabilities in inertial confinement fusion experiments
Authors: Delage O. , Lerche R. A. , Sangster T. C. , Arsenault H. H.
Published: February 2004

Neutron penumbral imaging technique has been demonstrated at the Lawrence Livermore National Laboratory and has been used at CEA (France) to image inertial confinement fusion cores. The information contained in a penumbral image is degraded by the aperture and the camera. In addition, getting an estimate of the source requires some special numerical reconstruction techniques. One of the major steps to achieving a few μm spatial resolution required on the National Ignition Facility/LMJ project is careful analysis of all the information regarding the degradation processes. The first prototype version of SIRINC is presented. The degradation models will be discussed. Particular attention will be given to refinements to the reconstruction technique.
Spectroscopy diagnostics of a low temperature laser ablated LiAg plasma plume
Authors: Sherrill Manolo E. , Mancini Roberto C. , Bailey James E. , Filuk Alex , Clark Brian , Lake Patrick , Abdallah Joseph
Published: February 2004

In experiments performed at Sandia National Laboratories, laser-generated LiAg plasma plumes were produced by irradiation of solid targets using a 10 ns pulse duration, 1×108 W/cm2 intensity Nd YAG laser. Time- and spatially resolved (along a direction normal to the target’s surface) optical spectra were recorded with a framing spectrograph. The observed spectra consist of optical line emission in Li and Ag atoms. Evidence of ions in the plume is suggested by the presence of a forbidden line and Stark-broadened line shapes. A spectroscopic model based on time-dependent collisional-radiative atomic kinetics that self-consistently calculate the Li and Ag level populations in conjunction with detailed line shapes and radiation transport is used to interpret the data. From this analysis, temperature, density, and ionization in the plume as a function of time and position along the normal to the target surface are extracted.
Charge exchange recombination spectroscopy on Alcator C-Mod
Authors: Eisner E. C. , Rowan William L.
Published: February 2004

The optical systems for charge-exchange recombination spectroscopy (CXRS) on Alcator C-Mod have been developed and installed. They will provide measurements of Ti, νθ, and νφ. With the addition of the motional Stark effect diagnostic to determine Bp, Er can be inferred from the ion pressure balance equation, Er=(Zenz)−1∇rpz+νφzBθ−νθzBφ. The optical systems are simple and have high throughput. In particular, the toroidally viewing systems must be designed for invessel installation close to the plasma where they are subject to large forces and are inaccessible between vacuum vents. Two optical systems, located invessel, provide 20 channels of Ti and νφ data from 67.3 cm<R<88.8 cm. A third system, located in air and viewing the plasma through a window, will provide ten channels of Ti and νθ data from 60 cm<R<95 cm. An extension of the CXRS system for measurement of ion temperature modulation in transient transport experiments, and perhaps for ion temperature fluctuations, is also discussed.
Full aperture backscatter diagnostic for the NIF laser facility (abstract)
Authors: Sewall Noel , Lewis Izzy , Kirkwood Robert , Moody John , Celeste John
Published: February 2004

The current schemes for achieving ignition on the National Ignition Facility require efficient coupling of energy from 192 laser beams to the deuterium–tritium fuel capsule. Each laser beam must propagate through a long scalelength plasma region before being converted to x rays (indirect drive) or being absorbed on the capsule (direct drive). Laser-plasma instabilities such as stimulated Brillouin and stimulated Raman scattering (SBS and SRS) will scatter a fraction of the incident laser energy out of the target leading to an overall reduction in the coupling efficiency. It is important to measure the character of this scattered light in order to understand it and to develop methods for reducing it to acceptable levels. We are designing a system called the full aperature backscatter diagnostic with the capability to measure the time-dependent amplitude and spectral content of the light which is backscattered through the incident beam focusing optic. The backscattered light will be collected over about 85% of the full beam aperture and separated into the SBS wavelength band (348–354 nm) and the SRS wavelength band (400–700 nm). Spectrometers coupled to streak cameras will provide time-resolved spectra for both scattered light components. The scattered light amplitude will be measured with fast and slow diodes. The entire system will be routinely calibrated. Analysis of the data will provide important information for reducing scattered power, achieving power balance, and finally achieving ignition.
A fast phosphor imaging diagnostic for two-dimensional plasma fluctuation measurements
Authors: Liebscher A. , Luckhardt S. C. , Antar G. , Zweben S.
Published: February 2004

A plasma imaging diagnostic is being developed using a fast time response ZnO:Zn phosphor disk to image plasma density fluctuations in a two-dimensional (2D) region. The plasma sensor consists of an 8.9 cm diameter phosphor coated aluminum disk that is inserted into the plasma and is excited by incident electrons resulting in a cathodoluminescent emission image. The phosphor light distribution is then interpreted as plasma density fluctuations using sheath theory and the phosphor response function. The local luminance S of fluctuating phosphor light is dependent on the incident electron energy Ee and current density through the equation S(r,t)=e∫R(Ee)(v⋅n̂)fe(r,v,t)d3v, where R(Ee) is the energy dependent response function of the phosphor. The phosphor persistence time of 1–10 μs (emulsion dependent) combined with fast intensified charge coupled device camera shutter speeds enables the imaging of plasma fluctuations on microsecond range time scales. Broadband turbulent phosphor light fluctuations (f⩽500 kHz) have been measured with photodiode detectors and compared with Langmuir probe data. The 2D phosphor images show density profiles consistent with radial Langmuir probe measurements. Temporal and spatial resolution of the phosphor diagnostic enables 2D imaging of plasma turbulence and coherent modes.
Development of short-wavelength far-infrared laser for high density plasma diagnostics
Authors: Okajima S. , Nakayama K. , Tazawa H. , Kawahata K. , Tanaka K. , Tokuzawa T. , Ito Y. , Mizuno K.
Published: February 2004

For high density operation of the large helical device and for a future large plasma machine such as ITER, a powerful 57 μm CH3OD laser pumped by a continuous wave 9R(8) CO2 laser has been developed. The 57 μm (5.2 THz) laser light has been successfully detected by using a GaAs Schottky barrier diode detector with a corner reflector. For optical windows of the plasma vessel and the far-infrared laser crystal quartz etalons have been designed under a concept of a two-wavelength etalon for 119 and 57 μm lights.
Ab initio analytical potential energy surface and quasiclassical trajectory study of the O+(4S)+H2(X 1Σg+)→OH+(X 3Σ−)+H(2S) reaction and isotopic variants
Authors: Martı́nez Rodrigo , Millán Judith , González Miguel
Published: February 2004

An analytical potential energy surface (PES) representation of the O+(4S)+H2(X 1Σg+) system was developed by fitting around 600 CCSD(T)/cc-pVQZ ab initio points. Rate constant calculations for this reaction and its isotopic variants (D2 and HD) were performed using the quasiclassical trajectory (QCT) method, obtaining a good agreement with experimental data. Calculations conducted to determine the cross section of the title reaction, considering collision energies (ET) below 0.3 eV, also led to good accord with experiments. This PES appears to be suitable for kinetics and dynamics studies. Moreover, the QCT results show that, although the hypotheses of a widely used capture model are not satisfied, the resulting expression for the cross section can be applied within a suitable ET interval, due to errors cancellation. This could be a general situation regarding the application of this simple model to ion–molecule processes.
Effects of equilibrium H-bond distance and angle changes on Raman intensities from water
Authors: Walrafen G. E.
Published: February 2004

The H-bond energy dispersion over the inhomogeneously broadened OD stretching contour from dilute HDO in H2O was determined from absolute Raman intensities; it displays a large minimum near ω=2440 cm−1 from short, strong H bonds (in agreement with the peak ω from lda ice) and a large maximum near 2650–2675 cm−1 due to extremely weak or broken H bonds (in agreement with the peak ω from dense, supercritical HDO in H2O, 0.9 g/cm3, 673 K). The difference between extrema is the maximum H-bond ΔE, 5100±500 cal/mol, in excellent agreement with Pauling’s limiting value. A pressure of 1500 bars yields an additional maximum and shoulder between the two dispersion extrema from pure water; saturated NaCl in water shows the additional maximum. The maxima near 3350 cm−1 (1500 bar) or near 3360 cm−1 (NaCl–H2O) arise from bent H bonds; 3350 cm−1 (1500 bar) corresponding to an angle of ≈170° in the joint frequency/bend, probability of Lawrence and Skinner [J. Chem. Phys. 118, 264 (2003)]. Rising ω refers to a higher probability of larger O–O distances, bent H bonds, and H-bond weakening and breakage. A ≈50–80 cm−1 difference between the 2727 cm−1 OD peak from HDO in steam, and the 2650–2675 cm−1 dispersion maximum is explained via the very broad ≈60 cm−1 liquid peak observed at 342 °C and 2000 bar.
Efficient techniques for the computer simulation of magnetic recording in complex layered materials
Authors: Newns Dennis , Donath Wilm , Martyna Glenn , Schabes Manfred , Lengsfield Byron
Published: February 2004

Recent advances in magnetic recording technology are increasing the relevance of simulation to magnetic media design. In particular, the difficulties inherent in developing perpendicular recording technology require the write process to be modeled at an integrated level via the simulation of a nanoscale machine consisting of the media, the soft underlayer, and the moving head. These simulations need to be very efficient in order to permit extensive testing of both materials and drive specification. Thus, significant methodological improvements that increase the accuracy and speed of the micromagnetic modeling are required. In this article, a method for calculating the magnetic fields in a complex layered material with grained morphologies whose computational cost scales linearly with system size is presented. The speed, accuracy, and parallel efficiency of the method is demonstrated on both supercomputers and PC clusters using our Almaden-Yorktown micromagnetic simulator (AYM). The method and AYM software are then used to perform an example simulation of perpendicular magnetic recording, writing a “tribit” in a grained data layer.
Topology and solid-state physics (Review)
Authors: Kosevich A. M.
Published: February 2004

The topological aspect of the dynamics of electrons in a crystal (band electrons) and of crystal lattice vibrations (phonons) is discussed. The main features of the dynamics of conduction electrons in metals are connected with the shape of their Fermi surface, which is different from that for free electrons. It is demonstrated that the behavior of band electrons under the influence of external electric and magnetic fields depends strongly on the topology of the Fermi surface. Various examples of such a dependence (calculation of the periods of quantum oscillations, magnetic breakdown, features of the magnetoresistance, Bloch oscillations) are adduced and discussed. The features of the dynamics of phonons are manifested in singularities of the density of vibrational states (van Hove singularities), which are directly related to a change in the topology of the constant-frequency surfaces. The presence of a topological invariant that changes by a jump upon a change in topology of the constant-frequency surface is pointed out. The origin of the so-called phase transition of order two and a half is discussed.
Orientational order parameter in the ordered phase of solid deuterium from neutron-diffraction data
Authors: Danchuk V. V. , Galtsov N. N. , Strzhemechny M. A. , Prokhvatilov A. I.
Published: February 2004

The method of direct reconstruction of the orientational order parameter η from diffraction intensities is applied to the case of the orientationally ordered phase of the quantum crystal para-D2 using the neutron elastic scattering data of Yarnell, Mills, and Schuch. The average value obtained, η=0.368, is close but somewhat lower than the value 0.4 corresponding to the theoretical value at T=0 in the molecular field approximation. The nature of this decrease is assessed theoretically with allowance for three factors that influence the absolute value of η: thermal excitation of librons, rotational anharmonicities, and rotational polarization. It is shown that the total theoretical renormalization (−8.8%) is close to the value obtained in the reconstruction (−8.0%).
Coercive properties of a Gd/Si/Co multilayer film with a compensation point
Authors: Merenkov D. N. , Gnatchenko S. L. , Chizhik A. B. , Shakhaeva Yu. A. , Baran M. , Szymczak R. , Vas’kovskiy V. O. , Svalov A. V.
Published: February 2004

The temperature dependence of the coercive field Hc is determined for a multilayer [Gd75 Å/Si5 Å/Co30 Å/Si5 Å]20 film with ferrimagnetic ordering of the magnetic moments of the cobalt and gadolinium layers. The maximum value of Hc is observed at a temperature of around 118 K, which corresponds to the compensation point of the film. It is established that in the entire interval of temperatures studied, the variation of the coercive field is due to the variation of the spontaneous magnetization.
Electron paramagnetic resonance in powder samples of metalorganic copper compounds
Authors: Kravchyna O. V. , Kaplienko A. I. , Anders A. G. , Orendáč M. , Orendáčova A. , Kajňaková M. , Feher A.
Published: February 2004

The EPR spectra of powder samples of a number of metalorganic compounds containing Cu2+ ions are investigated. These compounds are characterized by the presence of identical chains of octahedra forming the local environment of Cu2+, but they differ strongly in their interchain ligand structure. The resonance absorption bands permit determination of the components of the effective g factor and linewidth of individual powder particles and also their temperature dependence. It is found that the orbital singlet |x2−y2〉 is the ground state of the copper ion in all the compounds, and the exchange interaction parameters in them are estimated. The maximum exchange is observed in the system with the simplest geometry of the interchain structure in the series of compounds studied.
Anomalous acoustic transparency under conditions of the tilt effect
Authors: Burma N. G. , Petrishin A. I. , Ryabukha N. A. , Fil’ V. D.
Published: February 2004

An anomalous acoustic transparency phenomenon is discovered and investigated in high-purity Ga samples under conditions of the tilt effect. This phenomenon is due to resonance coupling of the high-frequency elastic waves with weakly damped electromagnetic eigenmodes of the metal.
Local exciton states at isoelectronic centers in superlattices
Authors: Bondar’ N. V.
Published: February 2004

The optical spectrum of strained type-II ZnSe/ZnTe superlattices, both freshly grown and stored for some time, are analyzed with allowance for the formation of ZnSe1−xTex mixed layers, having a cluster structure, at their heterointerfaces. The dependence of the hole localization energy on the radius of the clusters of Te atoms is calculated and found to agree well with the experimental data. In the samples stored for some time there is a change in the shape of the photoluminescence band and a monotonic shift of this band to shorter wavelengths. It is shown that the shift is caused by a change in the internal structure of the mixed layer over time and by the formation of local states induced by isoelectronic impurities (Te atoms).
Binding energies of small lithium clusters (Lin) and hydrogenated lithium clusters (LinH)
Authors: Wheeler Steven E. , Sattelmeyer Kurt W. , Schleyer Paul v. R. , Schaefer Henry F.
Published: February 2004

Large coupled cluster computations utilizing the Dunning weighted correlation-consistent polarized core-valence (cc-pwCVXZ) hierarchy of basis sets have been conducted, resulting in a panoply of internally consistent geometries and atomization energies for small Lin and LinH (n=1–4) clusters. In contrast to previous ab initio results, we predict a monotonic increase in atomization energies per atom with increasing cluster size for lithium clusters, in accordance with the historical Knudsen-effusion measurements of Wu. For hydrogenated lithium clusters, our results support previous theoretical work concerning the relatively low atomization energy per atom for Li2H compared to LiH and Li3H. The CCSD(T)/cc-pwCVQZ atomization energies for LiH, Li2H, Li3H, and the most stable isomer of Li4H, including zero-point energy corrections, are 55.7, 79.6, 113.0, and 130.6 kcal/mol, respectively. The latter results are not consistent with the most recent experiments of Wu.
Calibration of the n-electron valence state perturbation theory approach
Authors: Havenith Remco W. A. , Taylor Peter R. , Angeli Celestino , Cimiraglia Renzo , Ruud Kenneth
Published: February 2004

Extensive tests have been performed to benchmark and to compare with second-order perturbation theory based on a complete active space self-consistent field reference function (CASPT2), the recently developed n-electron valence state perturbation theory at second order (NEVPT2). Test calculations included the group fifteen diatomic molecules X2 (X=N, P, As, and Sb) and the S4/2D and S4/2P splittings for the corresponding atoms, the A11–3B1 splittings for CH2 and SiH2, and the absorption spectra of pyrrole and of Cu(Imidazole)2(SH)(SH2)+, which is a model for plastocyanin. Comparisons with full configuration-interaction calculations and experimental data show that the accuracy of NEVPT2 is in most cases even better than CASPT2. Where intruder states hamper the CASPT2 calculations, NEVPT2 performs significantly better. Care is needed in the choice of active orbitals, for example in the calculation of the S4/2D and S4/2P splittings for the group fifteen atoms. This is due to the different treatment of orbitals belonging to the inactive or active spaces, making the NEVPT2 not invariant for the choice of active space, even in cases where the multiconfiguration self-consistent field energy is invariant.
Competition between strain-induced and temperature-controlled nucleation of InAs/GaAs quantum dots
Authors: Howe P. , Le Ru E. C. , Clarke E. , Abbey B. , Murray R. , Jones T. S.
Published: February 2004

Atomic force microscopy and photoluminescence spectroscopy (PL) have been used to study asymmetric bilayer InAs quantum dot (QD) structures grown by molecular-beam epitaxy on GaAs(001) substrates. The two QD layers were separated by a GaAs spacer layer (SL) of varying thickness and were grown at different substrate temperatures. Grown independently, these two layers would exhibit a widely different QD number density, and this technique therefore enables us to assess the influence of the strain fields created by the dots in the first layer on the second-layer QD nucleation and characteristics. For very large SLs (>40 nm), total strain relief causes the QD nucleation to be controlled exclusively by the substrate temperature, which influences the migration of In adatoms. In this case, the optical and morphological properties of the second QD layer are identical to a structure with a single QD layer grown at the same temperature. In structures with a much smaller SL, strain effects dominate over the effect of temperature in controlling the nucleation of the QDs, thereby fixing the second-layer QD number density to that of the first (templating effect). There is also evidence that strain relaxation is present in the QDs of the second layer and that this is crucial for extending their emission wavelength. The optimum SL thickness is shown to be 11 nm, for which low-temperature PL emission peaks at 1.26 μm, with a full width at half-maximum of only 15 meV. Intermediate SL thicknesses exhibit broad QD size distributions, with strain effects only partly influencing the QD growth in the second layer.
Three regimes of growth of a Gaussian ripple on a uniform plane electromagnetic wave front in a plasma
Authors: Sharma Ashutosh , Verma M. P. , Prakash Gyan , Sodha M. S.
Published: February 2004

Starting with the scalar wave equation and relevant expressions for nonlinear dielectric constant of plasma, the propagation of a Gaussian ripple on a plane wave front of uniform intensity has been studied in the paraxial approximation. In the plane of the ripple width (along the y axis) and the ratio of the electric field intensities of the ripple and the main beam at r=0, z=0, (along the x axis) three distinct regions (for a given intensity of the main beam) can be identified which correspond to steady divergence, oscillatory divergence, and self-focusing of the ripple. The variation of the ripple width with distance of propagation has also been obtained for typical points in the three regions.
Vibrational spectra of germanium–carbon clusters. II. GeC7 and GeC9
Authors: Robbins D. L. , Chen K.-C. , Rittby C. M. L. , Graham W. R. M.
Published: February 2004

Experimental and theoretical studies of a novel family of germanium–carbon clusters (GenCm) that were initiated with our earlier identification of the GeC3Ge cluster have now been extended to the GeC7 and GeC9 chains. The new clusters, which were formed by laser ablation and trapped in solid Ar at ∼10 K, have been identified using Fourier-transform infrared (FTIR) measurements coupled with density-functional theory (DFT) calculations. The ν1(σ) vibrational fundamental of linear GeC7 has been identified at 2063.6 cm−1, and an absorption at 1928.3 cm−1 has been assigned to the ν4(σ) fundamental of linear GeC9. FTIR measurements of the isotopic shifts for the assignments are in good agreement with the DFT predictions.
Dynamic pathway model for the formation of C60
Authors: Lee In-Ho , Kim Hanchul , Lee Jooyoung
Published: February 2004

We present a dynamic pathway model for the formation of C60 using the action-derived molecular dynamics simulations. We propose candidate precursors for dynamic pathway models in which carbons spontaneously aggregate due to favorable energetics and kinetics. Various planar polycyclic models are in a disadvantageous state where they cannot be trapped in the forward reaction due to their high excess internal energies. Our simulation results show that precursors either in the shape of tangled polycyclics or in the shape of open cages are kinetically favored over precursors in the shape of planar hexagonal graphite fragments. Calculated activation energies for the probable precursor models are in good agreement with experiment. Existence of chains in the models of tangled polycyclics and open cages is beneficially for the formation of C60 molecule. Chains attached to the precursor model are energetically favorable and display lithe movements along the dynamic pathway.
Hybridization dynamics of surface immobilized DNA
Authors: Hagan Michael F. , Chakraborty Arup K.
Published: February 2004

We model the hybridization kinetics of surface attached DNA oligomers with solubilized targets. Using both master equation and rate equation formalisms, we show that, for surface coverages at which the surface immobilized molecules interact, barriers to penetration create a distribution of target molecule concentrations within the adsorbed layer. By approximately enumerating probe and target conformations, we estimate the probability of overlap between complementary probe and target regions as a function of probe density and chain length. In agreement with experiments, we find that as probe molecules interact more strongly, fewer nucleation sites become accessible and binding rates are diminished relative to those in solution. Nucleation sites near the grafted end of the probes are least accessible; thus targets which preferentially bind to this region show more drastic rate reductions than those that bind near the free end of the probe. The implications of these results for DNA-based biosensors are discussed.
Potential-energy surfaces for excited states in extended systems
Authors: Hellman A. , Razaznejad B. , Lundqvist B. I.
Published: February 2004

With a simple and physically intuitive method, first-principles calculations of potential-energy surfaces are performed for excited states in a number of illustrative systems, including dimers (H2 and NaCl) and gas–surface systems [Cl–Na(100) and Cl2–Na(100)]. It is based on density-functional theory and is a generalization of the Δ self-consistent field (ΔSCF) method, where electron–hole pairs are introduced in order to model excited states, corresponding to internal electron transfers in the considered system. The desired excitations are identified by analysis of calculated electron orbitals, local densities of states, and charge densities. For extended systems, where reliable first-principles methods to account for electronically excited states have so far been scarce, our method is very promising. Calculated results, such as the chemiluminescence of halogen molecules impinging on a alkali–metal surface, and the vertical (5σ→2π*) excitation within the adsorbed CO molecule on the Pd(111) surface, are in working agreement with those of other studies and experiments.