bluuebunny/crossref_metadata_2025 huggingface dataset:
Thomson scattering density calibration by Rayleigh and rotational Raman scattering on NSTX
Authors: LeBlanc B. P.
Published: November 2008

The multipoint Thomson scattering diagnostic measures the profiles of the electron temperature Te(R) and density ne(R) on the horizontal midplane of NSTX. Normal operation makes use of Rayleigh scattering in nitrogen or argon to derive the density profile. While the Rayleigh scattering ne(R) calibration has been validated by comparison to other density measurements and through its correlation with plasma phenomena, it does require dedicated detectors at the laser wavelength in this filter polychromator based diagnostic. The presence of dust and/or stray laser light precludes routine use of these dedicated spectral channels for Thomson scattering measurement. Hence it is of interest to investigate the use of Raman scattering in nitrogen for the purpose of density calibration since it could free up detection equipment, which could then be used for the instrumentation of additional radial channels. In this paper the viewing optics “geometrical factor” profiles obtained from Rayleigh and Raman scattering are compared. While both techniques agree nominally, residual effects on the order of 10% remain and will be discussed.
Development of a virtual Zeff diagnostic for the W7-X stellarator
Authors: Krychowiak M. , Dodt D. , Dreier H. , König R. , Wolf R.
Published: November 2008

For the W7-X stellarator a diagnostic system for measurement of local Zeff values from the visible bremsstrahlung continuum is foreseen. The method is based on passive, absolute measurement of the bremsstrahlung intensity along several lines of sight. In order to eliminate the spurious impact of other radiation sources of different spectral intensity distribution than the bremsstrahlung, like the line radiation, it is intended to spectrally resolve the detected radiation, e.g., by use of microspectrometers. The visible bremsstrahlung background can be extracted by making use of the 1∕λ2 dependence of its intensity (expressed in unit W∕m3srnm) in the high temperature plasmas by using Bayesian data analysis techniques. In a second step, the local values of Zeff as a function of the effective plasma radius are derived by inversion, using different regularization methods, of the line-integrated bremsstrahlung signals with the knowledge of the magnetic flux surfaces. Inversion of the full model based on statistical methods allows taking into account all conceivable uncertainties accompanying Zeff measurement and provides uncertainties of the local Zeff values and valuable information on other uncertain parameters of the model. In this paper we show the first steps in developing a virtual Zeff diagnostic for W7-X which allows the optimization of the statistical model as well as of the future diagnostic setup.
Time-resolved voltage measurements of Z-pinch radiation sources with a vacuum voltmeter
Authors: Murphy D. P. , Allen R. J. , Weber B. V. , Commisso R. J. , Apruzese J. P. , Phipps D. G. , Mosher D.
Published: November 2008

A vacuum-voltmeter (VVM) was fielded on the Saturn pulsed power generator during a series of argon gas-puff Z-pinch shots. Time-resolved voltage and separately measured load current are used to determine several dynamic properties as the load implodes, namely, the inductance, L(t), net energy coupled to the load, Ecoupled(t), and the load radius, r(t). The VVM is a two-stage voltage divider, designed to operate at voltages up to 2MV. The VVM is presently being modified to operate at voltages up to 6MV for eventual use on the Z generator.
An original calibration technique for soft x-ray detectors and its use in the Tore Supra tomographic system
Authors: Mazon D. , Pacella D. , Malard P. , Garnier D. , Romano A. , Bouchand C.
Published: November 2008

This paper describes in detail the recent progresses which have been made in Tore Supra for developing a new technique of calibration of the soft x-ray (SXR) detectors in the range 1–30keV. The diode response as a function of the flux of photons resulted accurately linear over almost three orders of magnitude. Apart from a limited number of deficient detectors, promptly replaced with new ones, the spread of the diode responses (84 detectors) is about 20% total of the average value. It allowed the derivation of the calibration factor for each detector of the tomography system. The effect of the environmental temperature, in the range 15°–40°, has also been studied, revealing that up to 35° the linearity of diode response and these calibration factors remain constant. It demonstrates the capability of discriminating slight and localized changes in the two dimensional spatial distribution of the SXR intensity.
Forward modeling of JET polarimetry diagnostic
Authors: Ford Oliver , Svensson J. , Boboc A. , McDonald D. C. ,   JET-EFDA Contributors
Published: November 2008

An analytical Bayesian inversion of the JET interferometry line integrated densities into density profiles and associated uncertainty information, is demonstrated. These are used, with a detailed model of plasma polarimetry, to predict the rotation and ellipticity for the JET polarimeter. This includes the lateral channels, for over 45,000 time points over 1313 JET pulses. Good agreement with measured values is shown for a number of channels. For the remaining channels, the requirement of a more detailed model of the diagnostic is demonstrated. A commonly used approximation for the Cotton-Mouton effect on the lateral channels is also evaluated.
Characterization of cross-section correction to charge exchange recombination spectroscopy rotation measurements using co- and counter-neutral-beam views
Authors: Solomon W. M. , Burrell K. H. , Feder R. , Nagy A. , Gohil P. , Groebner R. J.
Published: November 2008

Measurements of rotation using charge exchange recombination spectroscopy can be affected by the energy dependence of the charge exchange cross section. On DIII-D, the associated correction to the rotation can exceed 100km∕s at high temperatures. In reactor-relevant low rotation conditions, the correction can be several times larger than the actual plasma rotation and therefore must be carefully validated. New chords have been added to the DIII-D CER diagnostic to view the counter-neutral-beam line. The addition of these views allows determination of the toroidal rotation without depending on detailed atomic physics calculations, while also allowing experimental characterization of the atomic physics. A database of rotation comparisons from the two views shows that the calculated cross-section correction can adequately describe the measurements, although there is a tendency for “overcorrection.” In cases where accuracy better than about 15% is desired, relying on calculation of the cross-section correction may be insufficient.
An ion Doppler spectrometer instrument for ion temperature and flow measurements on SSPX
Authors: King J. D. , McLean H. S. , Wood R. D. , Romero-Talamás C. A. , Moller J. M. , Morse E. C.
Published: November 2008

A high-resolution ion Doppler spectrometer (IDS) has been installed on the sustained spheromak plasma experiment to measure ion temperatures and plasma flow. The system is composed of a 1m focal length Czerny–Turner spectrometer with a diffraction grating line density of 2400lines∕mm, which allows for first order spectra between 300 and 600nm. A 16-channel photomultiplier tube detection assembly combined with output coupling optics provides a spectral resolution of 0.0126nm/channel. We calculate in some detail the mapping of curved slit images onto the linear detector array elements. This is important in determining the wavelength resolution and setting the optimum vertical extent of the slit. Also, because of the small wavelength window of the IDS, a miniature fiber-optic survey spectrometer sensitive to a wavelength range 200–1100nm and having a resolution of 0.2nm is used to obtain a time-integrated spectrum for each shot to verify specific impurity line radiation. Several measurements validate the systems operation. Doppler broadening of CIII 464.72nm line in the plasma shows time-resolved ion temperatures up to 250eV for hydrogen discharges, which is consistent with neutral particle energy analyzer measurements. Flow measurements show a sub-Alfvénic plasma flow ranging from 5to45km∕s for helium discharges.
Optimizing a Thomson scattering diagnostic for fast dynamics and high background
Authors: O’Connell R. , Den Hartog D. J. , Borchardt M. T. , Holly D. J. , Reusch J. A. , Stephens H. D.
Published: November 2008

The Madison Symmetric Torus (MST) presents challenging conditions for Thomson scattering (TS) measurements. The MST plasmas are reversed-field pinches (RFPs) with electron density ne<3×1013cm−3, typically 1×1013cm−3. The TS system was designed to measure from 10eVto2keV; however, six polychromators were upgraded from four to eight spectral channels to resolve to 10keV. There is no diverter or vertical field, so wall interaction results in high background light both from ion and neutral bremsstrahlungs and from line radiation. Also during standard plasmas, the RFP exhibits regular reconnection sawteeth events during which the plasma current, density, and temperature profiles are flattened. These events are of interest both due to the reconnection physics and to their contribution to the MST equilibrium and confinement. These events occur over 100μS and exhibit large changes in background light and fast changes in temperature. During improved confinement plasmas, there are no sawteeth; the background is low but the temperature can be over an order of magnitude higher. Data analysis of the system has been developed to accommodate both the large dynamic range of the temperature, the fast dynamics, and the fast changing, high amplitude background. Special attention has been paid to the sources of error, in particular, the contribution of the background. A response-function method reduces the measured uncertainty by a factor of 2. Numerical techniques have been developed which are extremely robust. Two methods are used, a conventional χ2 minimization using a Levenberg–Marquardt algorithm coupled with Monte Carlo modeling for the error bar and a Bayesian statistics method. The Bayesian method computes the probability distribution for the measured photons and electron temperature and this information can be used to ensemble data and will allow future integrated data analysis efforts.
Short wavelength far infrared laser polarimeter with silicon photoelastic modulators
Authors: Akiyama T. , Kawahata K. , Tanaka K. , Okajima S. , Nakayama K.
Published: November 2008

A short wavelength far infrared laser whose wavelength λ is about 50μm is preferable for a polarimeter and an interferometer for high density operations in the Large Helical Device (LHD) and on future large fusion devices such as ITER. This is because the beam bending effect (∝λ2) in a plasma, which causes fringe jump errors, is small and the Faraday and the Cotton–Mouton effects are moderate. We have developed a polarimeter with highly resistive silicon photoelastic modulators (PEMs) for the CH3OD laser (λ=57.2 and 47.7μm). We performed bench tests of the polarimeter with a dual PEM and demonstrated the feasibility for the polarimeter. Good linearity between actual and evaluated polarization angles is achieved with an angular resolution of 0.05° and a temporal resolution of 1ms. The baseline drift of the polarization angle is about 0.1° for 1000s.
A torquing shearing interferometer for cylindrical wire array experiments
Authors: Pikuz S. A. , Schrafel P. C. , Shelkovenko T. A. , Kusse B. R.
Published: November 2008

In standard shearing interferometry, a single probing beam passes through a perturbing medium and is then split into two beams. A linear shift results in an overlap, an interference, and a fringe pattern yielding the perturbing medium density profile. The probing beam usually needs to be larger than the perturbing medium so that part of it passes through a well separated low density region. During early time axial (end-on) views of imploding cylindrical wire arrays low density regions lie in between the high density regions that are near the initial wire positions. In addition, for end-on viewing, the probing beam diameter is limited by electrodes and is comparable to the array diameter. In this case a linear translation will not work but the overlap can be accomplished by an azimuthal rotation of one beam with respect to the other. Such a torquing shearing interferometer has been set up on the COBRA experiment to give time resolved, radial, and azimuthal electron density profiles during early time cylindrical wire array implosions.
Imaging photomultiplier array with integrated amplifiers and high-speed USB interface
Authors: Blacksell M. , Wach J. , Anderson D. , Howard J. , Collis S. M. , Blackwell B. D. , Andruczyk D. , James B. W.
Published: November 2008

Multianode photomultiplier tube (PMT) arrays are finding application as convenient high-speed light sensitive devices for plasma imaging. This paper describes the development of a USB-based “plug-n-play” 16-channel PMT camera with 16bits simultaneous acquisition of 16 signal channels at rates up to 2MS∕s per channel. The preamplifiers and digital hardware are packaged in a compact housing which incorporates magnetic shielding, on-board generation of the high-voltage PMT bias, an optical filter mount and slits, and F-mount lens adaptor. Triggering, timing, and acquisition are handled by four field-programmable gate arrays (FPGAs) under instruction from a master FPGA controlled by a computer with a LABVIEW interface. We present technical design details and specifications and illustrate performance with high-speed images obtained on the H-1 heliac at the ANU.
Poloidal mode analysis of magnetic probe data in a spherical tokamak configuration
Authors: Tojo H. , Ejiri A. , Gryaznevich M. P. , Takase Y. , Adachi Y.
Published: November 2008

A method to determine the poloidal mode number m in a spherical tokamak based on magnetic probe data was developed. Perturbed magnetic fields at Mirnov coils are calculated for distributed helical filamentary currents on rational surfaces assuming the maximum current amplitude, m and n (toroidal mode number), and the toroidal location of the filaments. These free parameters were determined from the best fit to the measured signals. The residual error was reduced by a factor of 2 by introducing helical filaments instead of toroidal filaments. Using this method, m∕n=2∕1 and 3∕2 modes were identified in Mega-Ampere Spherical Tokamak discharges, and the time evolution of the tearing modes was derived.
Compact, accurate description of diagnostic neutral beam propagation and attenuation in a high temperature plasma for charge exchange recombination spectroscopy analysis
Authors: Bespamyatnov Igor O. , Rowan William L. , Granetz Robert S.
Published: November 2008

Charge exchange recombination spectroscopy on Alcator C-Mod relies on the use of the diagnostic neutral beam injector as a source of neutral particles which penetrate deep into the plasma. It employs the emission resulting from the interaction of the beam atoms with fully ionized impurity ions. To interpret the emission from a given point in the plasma as the density of emitting impurity ions, the density of beam atoms must be known. Here, an analysis of beam propagation is described which yields the beam density profile throughout the beam trajectory from the neutral beam injector to the core of the plasma. The analysis includes the effects of beam formation, attenuation in the neutral gas surrounding the plasma, and attenuation in the plasma. In the course of this work, a numerical simulation and an analytical approximation for beam divergence are developed. The description is made sufficiently compact to yield accurate results in a time consistent with between-shot analysis.
Measurement and modeling of pulsed microchannel plate operation (invited)
Authors: Rochau G. A. , Wu M. , Kruschwitz C. , Joseph N. , Moy K. , Bailey J. , Krane M. , Thomas R. , Nielsen D. , Tibbitts A.
Published: November 2008

Microchannel plates (MCPs) are a standard detector for fast-framing x-ray imaging and spectroscopy of high-temperature plasmas. The MCP is coated with conductive striplines that carry short duration voltage pulses to control the timing and amplitude of the signal gain. This gain depends on the voltage to a large exponent so that small reflections or impedance losses along the striplines can have a significant impact on the position-dependent amplitude and pulse width of the gain. Understanding the pulsed gain response therefore requires careful measurements of the position- and time-dependent surface voltage coupled with detailed modeling of the resulting electron cascade. We present measurements and modeling of the time- and space-dependent gain response of MCP detectors designed for use at Sandia National Laboratories’ Z facility. The pulsed gain response is understood through measurements using a high impedence probe to determine the voltage pulse propagating along the stripline surface. Coupling the surface voltage measurements with Monte Carlo calculations of the electron cascade in the MCP provides a prediction of the time- and position-dependent gain that agrees with measurements made on a subpicosecond UV laser source to within the 25% uncertainty in the simulations.
Simultaneous density and magnetic field fluctuation measurements by far-infrared interferometry and polarimetry in MST
Authors: Yates T. F. , Ding W. X. , Carter T. A. , Brower D. L.
Published: November 2008

Fluctuations are expected to play an important role in anomalous particle, momentum, and energy transport for magnetic confinement devices. Magnetic and density fluctuations are simultaneously measured using a high-speed laser-based Faraday rotation-interferometry system with a bandwidth of 500 kHz and 8 cm chord spacing. Density fluctuation and magnetic fluctuation profiles are obtained by using a newly developed fitting procedure.
Ab initio modeling of the motional Stark effect on MAST
Authors: De Bock M. F. M. , Conway N. J. , Walsh M. J. , Carolan P. G. , Hawkes N. C.
Published: November 2008

A multichord motional Stark effect (MSE) system has recently been built on the MAST tokamak. In MAST the π and σ lines of the MSE spectrum overlap due to the low magnetic field typical for present day spherical tokamaks. Also, the field curvature results in a large change in the pitch angle over the observation volume. The measured polarization angle does not relate to one local pitch angle but to an integration over all pitch angles in the observation volume. The velocity distribution of the neutral beam further complicates the measurement. To take into account volume effects and velocity distribution, an ab initio code was written that simulates the MSE spectrum on MAST. The code is modular and can easily be adjusted for other tokamaks. The code returns the intensity, polarized fraction, and polarization angle as a function of wavelength. Results of the code are presented, showing the effect on depolarization and wavelength dependence of the polarization angle. The code is used to optimize the design and calibration of the MSE diagnostic.
Diagnostic techniques for measuring suprathermal electron dynamics in plasmas (invited)
Authors: Coda S.
Published: November 2008

Plasmas, both in the laboratory and in space, are often not in thermodynamic equilibrium, and the plasma electron distribution function is accordingly non-Maxwellian. Suprathermal electron tails can be generated by external drives, such as rf waves and electric fields, or internal ones, such as instabilities and magnetic reconnection. The variety and importance of the phenomena in which suprathermal electrons play a significant role explains an enduring interest in diagnostic techniques to investigate their properties and dynamics. X-ray bremsstrahlung emission has been studied in hot magnetized plasmas for well over two decades, flanked progressively by electron-cyclotron emission in geometries favoring the high-energy end of the distribution function (high-field-side, vertical, oblique emission), by electron-cyclotron absorption, by spectroscopic techniques, and at lower temperatures, by Langmuir probes and electrostatic analyzers. Continuous progress in detector technology and in measurement and analysis techniques, increasingly sophisticated layouts (multichannel and tomographic systems, imaging geometries), and highly controlled suprathermal generation methods (e.g., perturbative rf modulation) have all been brought to bear in recent years on an increasingly detailed, although far from complete, understanding of suprathermal electron dynamics.
In-vessel activation monitors in JET: Progress in modeling
Authors: Bonheure Georges , Lengar I. , Syme B. , Wieslander Elisabeth , Hult Mikael , Gasparro Joël , Marissens Gerd , Arnold Dirk , Laubenstein Matthias , Popovichev S. ,   JET-EFDA Contributors
Published: November 2008

Activation studies were performed in JET with new in-vessel activation monitors. Though primarily dedicated to R&D in the challenging issue of lost α diagnostics for ITER, which is being addressed at JET with several techniques, these monitors provide for both neutron and charged particle fluences. A set of samples with different orientation with respect to the magnetic field is transported inside the torus by means of a manipulator arm (in contrast with the conventional JET activation system with pneumatic transport system). In this case, radionuclides with longer half-life were selected and ultralow background gamma-ray measurements were needed. The irradiation was closer to the plasma and this potentially reduces the neutron scattering problem. This approach could also be of interest for ITER, where the calibration methods have yet to be developed. The MCNP neutron transport model for JET was modified to include the activation probe and so provide calculations to help assess the new data. The neutron induced activity on the samples are well reproduced by the calculations.
Detection of high k turbulence using two dimensional phase contrast imaging on LHD
Authors: Michael C. A. , Tanaka K. , Vyacheslavov L. N. , Sanin A. , Kharchev N. K. , Akiyama T. , Kawahata K. , Okajima S.
Published: November 2008

High k turbulence, up to 30cm−1, can be measured using the two dimensional CO2 laser phase contrast imaging system on LHD. Recent hardware improvements and experimental results are presented. Precise control over the lens positions in the detection system is necessary because of the short depth of focus for high k modes. Remote controllable motors to move optical elements were installed, which, combined with measurements of the response to ultrasound injection, allowed experimental verification and shot-to-shot adjustment of the object plane. Strong high k signals are observed within the first 100–200ms after the initial electron cyclotron heating (ECH) breakdown, in agreement with gyrotron scattering. During later times in the discharge, the entire k spectrum shifts to lower values (although the total amplitude does not change significantly), and the weaker high k signals are obscured by leakage of low k components at low frequency, and detector noise, at high frequency.
The motional Stark effect diagnostic on NSTX
Authors: Levinton F. M. , Yuh H.
Published: November 2008

This work describes the implementation and recent results from the motional Stark effect (MSE) collisionally induced fluorescence diagnostic on NSTX. Due to the low magnetic field on NSTX the MSE diagnostic requires a new approach for the viewing optics and spectral filter. This has been accomplished with a novel optical design that reduces the geometric Doppler broadening, and a high throughput, high resolution spectral filter to optimize signal-to-noise ratio. With these improvements the polarization fraction is ∼30%–40% and, combined with the large throughput, a time resolution of ∼5 ms. The MSE diagnostic presently has 16 sight lines operating, providing measurements of the magnetic field line pitch from the plasma center to near the outboard edge of the plasma.
Gamma bang time/reaction history diagnostics for the National Ignition Facility using 90° off-axis parabolic mirrors
Authors: Malone R. M. , Herrmann H. W. , Stoeffl W. , Mack J. M. , Young C. S.
Published: November 2008

Gas Cherenkov detectors (GCDs) have been used to convert fusion gamma into photons to achieve gamma bang time and reaction history measurements. The GCDs designed for OMEGA used Cassegrain reflector optics in order to fit inside a 10in. manipulator. A novel design for the National Ignition Facility using 90° off-axis parabolic mirrors will increase light collection efficiency from fusion gammas and achieve minimum time dispersion. The broadband Cherenkov light (from 200to800nm) is relayed into a high-speed detector using three parabolic mirrors. Because light is collected from many source planes throughout the CO2 gas volume, the detector is positioned at the stop position rather than at an image position. The stop diameter and its position are independent of the light-generation location along the gas cell. The current design collects light from a 100mm diameter by 500mm long gas volume. Optical ray tracings demonstrate how light can be collected from different angled trajectories of the Compton electrons as they fly through the CO2 gas volume. A cluster of four channels will allow for increased dynamic range as well as for different gamma energy threshold sensitivities.
NIF-scale re-emission sphere measurements of early-time Tr=100eV hohlraum symmetry (invited)
Authors: Dewald E. L. , Thomas C. , Milovich J. , Edwards J. , Sorce C. , Kirkwood R. , Meeker D. , Jones O. , Izumi N. , Landen O. L.
Published: November 2008

The indirect-drive National Ignition Campaign (NIC) proposes to set the first 2ns of hohlraum radiation symmetry by observing the instantaneous soft x-ray re-emission pattern from a high-Z sphere in place of the ignition capsule. To assess this technique under NIC conditions, we used the Omega Laser Facility to image the re-emission of Bi coated spheres with 200ps temporal, 50–100μm spatial, and 30% spectral resolution. The sphere is driven by 70% NIC-scale vacuum Au hohlraums heated to Tr=100eV using two cones per side laser beam illumination. The experiments have demonstrated the required accuracies of <3%P2∕P0 and <3%P4∕P0 Legendre mode flux asymmetry at both 900 and 1200eV re-emission photon energies. The re-emission patterns at 900 and 1200eV are also consistent with each other and their relative dependence on radiation temperature. We measured the P2∕P0 and P4∕P0 dependence to laser cone power ratio. View factor calculations are in agreement with the experimentally measured radiation flux and re-emit images when assuming 55% inner beam and 100 % outer beam coupling into x rays at the hohlraum wall.
Magnetic measurements using array of integrated Hall sensors on the CASTOR tokamak
Authors: Ďuran Ivan , Hronová Olena , Stöckel Jan , Sentkerestiová Jana , Havlicek Josef
Published: November 2008

We have performed the first tests of “integrated” Hall sensors (IHSs) in a tokamak in-vessel environment. IHS combines the sensing element together with the complex electronic circuitry on a single small chip. The on-chip integrated circuits provide stabilization of the supply voltage, output amplification, noise suppression, and elimination of temperature dependencies. Eight IHSs of A1322LUA type produced by Allegro MicroSystems, Inc. were mounted on a stainless steel ring symmetrically encircling the CASTOR plasmas in poloidal direction 10 mm outside the limiter radius. IHSs were oriented such that they measure the horizontal and vertical magnetic fields. We found out that these sensors qualify for in-vessel use of small to middle sized fusion devices where the radiation is not an issue and the temperature below 150 °C can be guaranteed. The main advantages over the traditional pickup coils are the smaller size and more straightforward interpretation of output without the need of rather cumbersome integration and drift removal procedure associated with the use of inductive loops. We successfully exploited the sensors for determination of vertical plasma displacement on CASTOR. This new diagnostic helped us to shed more light into long term observed discrepancy on CASTOR between vertical plasma displacement as deduced by standard magnetic and by nonmagnetic diagnostics (Langmuir probes, bolometers).
Design of a new Nd:YAG Thomson scattering system for MAST
Authors: Scannell R. , Walsh M. J. , Carolan P. G. , Darke A. C. , Dunstan M. R. , Huxford R. B. , McArdle G. , Morgan D. , Naylor G. , O’Gorman T. , Shibaev S. , Barratt N. , Gibson K. J. , Tallents G. J. , Wilson H. R.
Published: November 2008

A new infrared Thomson scattering system has been designed for the MAST tokamak. The system will measure at 120 spatial points with ≈10 mm resolution across the plasma. Eight 30 Hz 1.6 J Nd:YAG lasers will be combined to produce a sampling rate of 240 Hz. The lasers will follow separate parallel beam paths to the MAST vessel. Scattered light will be collected at approximately f/6 over scattering angles ranging from 80° to 120°. The laser energy and lens size, relative to an existing 1.2 J f/12 system, greatly increases the number of scattered photons collected per unit length of laser beam. This is the third generation of this polychromator to be built and a number of modifications have been made to facilitate mass production and to improve performance. Detected scattered signals will be digitized at a rate of 1 GS/s by 8 bit analog to digital converters (ADCs.) Data may be read out from the ADCs between laser pulses to allow for real-time analysis.
Characterization of radiochromic film scanning techniques used in short-pulse-laser ion acceleration
Authors: Cowan Joseph S. , Flippo Kirk A. , Gaillard Sandrine A.
Published: November 2008

Radiochromic film (RCF) is increasingly being used as a detector for proton beams from short-pulse laser-matter interaction experiments using the RCF imaging spectroscope technique. The community has traditionally used inexpensive flatbed scanners to digitize and analyze the data, as opposed to more expensive and time-consuming microdensitometers (MicroDs). Often, the RCF densities in some regions exceed an optical density (OD) of 3. Flatbed scanners are generally limited to a maximum OD of ∼3. Because of the high exposure density, flatbed scanners may yield data that are not reliable due to light scatter and light diffusion from areas of low density to areas of high density. This happens even when the OD is slightly above 1. We will demonstrate the limitations of using flatbed scanners for this type of radiographic media and characterize them compared to measurements made using a MicroD. A technique for cross characterizing both systems using a diffuse densitometer with a NIST wedge will also be presented.
Pellet charge exchange helium measurement using neutral particle analyzer in large helical device
Authors: Ozaki T. , Goncharov P. , Veshchev E. , Tamura N. , Sudo S. , Seki T. , Kasahara H. , Takase Y. , Ohsako T.
Published: November 2008

It is very important to investigate the confinement of α particles, which will be produced by nuclear reactions in ITER and fusion reactors. The pellet charge exchange (PCX) measurement is one of the most powerful methods because it can directly provide the profile of the α particle energy spectra in a plasma. In the large helical device, PCX using tracer encapsulated solid pellet (TESPEL) has been tried in many hydrogen and helium plasmas, including helium accelerated by using the cyclotron resonance heating. In the PCX, we use the compact neutral particle analyzer without simultaneous mass separation ability. The helium particle measurement can be achieved by the application of voltage in the condenser plate. The scattering of hydrogen particle is carefully considered during the estimation of the helium amount. The radial helium profiles can also be obtained by comparing four TESPEL injection shots with/without higher harmonic fast wave heating and at applied plate voltages for He or H, respectively.
Observation of asymmetrically imploded core plasmas with a two-dimensional sampling image x-ray streak camera
Authors: Shiraga Hiroyuki , Lee Myongdok , Mahigashi Norimitsu , Fujioka Shinsuke , Azechi Hiroshi
Published: November 2008

A shell target with a cone for guiding the heating beam has been proposed for the fast ignition scheme. Implosion of such target is no longer symmetric because of the cone. A fast two-dimensional x-ray imaging technique, two-dimensional (2D) sampling image x-ray streak camera was applied for the first time to observation of the dynamics of implosion and core plasma. X-ray emission image of the plasma was sampled with two-dimensionally distributed image sampling points, streaked with the tube, and the recorded signals were reconstructed as sequential 2D frame images. Shape and movement of the core plasma were clearly observed.
Long duration backlighter experiments at Omega
Authors: Reighard A. B. , Glendinning S. G. , Young P. E. , Hsing W. W. , Foord M. , Schneider M. , Lu K. , Dittrich T. , Wallace R. , Sorce C.
Published: November 2008

We have successfully demonstrated a 7.5 ns duration pinhole-apertured backlighter at the Omega laser facility. Pinhole-apertured point-projection backlighting for 8 ns will be useful for imaging evolving features in experiments at the National Ignition Facility. The backlighter consisted of a 20 μm diameter pinhole in a 75 μm thick Ta substrate separated from a Zn emitter (9 keV) by a 400 μm thick high-density carbon piece. The carbon prevented the shock from the laser-driven surface from reaching the substrate before 8 ns and helped minimize x-ray ablation of the pinhole substrate. Grid wires in x-ray framing camera images of a gold grid have a source-limited resolution significantly smaller than the pinhole diameter due to the high aspect ratio of the pinhole, but do not become much smaller at late times.
A versatile high-resolution x-ray imager (HRXI) for laser-plasma experiments on OMEGA
Authors: Bourgade J. L. , Troussel P. , Casner A. , Huser G. , Sangster T. C. , Pien G. , Marshall F. J. , Fariaud J. , Remond C. , Gontier D. , Chollet C. , Zuber C. , Reverdin C. , Richard A. , Jaanimagi P. A. , Keck R. L. , Bahr R. E. , Armstrong W. J. , Dewandel J. , Maroni R. , Aubard F. , Angelier B. , Cote C. Y. , Magnan S.
Published: November 2008

A high-resolution x-ray imager (HRXI) devoted to laser-plasma experiments combines two state-of-the-art technologies developed in France: a high-resolution x-ray microscope and a high-speed x-ray streak camera. The resulting streaked imager achieves spatial and temporal resolutions of ∼5 μm and ∼10 ps, respectively. The HXRI has recorded enhanced spatial and temporal resolution radiographs of indirectly driven targets on OMEGA. This paper describes the main features of the instrument and details the activation process on OMEGA (particularly the alignment). Recent results obtained on joint CEA/LLE radiographic OMEGA experiments will also be presented.
Line integration effects on ion temperatures in tokamak plasmas measured with neutron emission spectroscopy
Authors: Ognissanto F. , Gorini G. , Tardocchi M. , Albergante M. , Ballabio L. , Conroy S. , Källne J.
Published: November 2008

The line integrated line emission measured by neutron spectrometers at JET along sight lines in the vertical and horizontal planes has been simulated in Monte Carlo calculations to determine the relationship between the measured (effective) ion temperature (Teff) relative to the peak value of the profile (T0). The general sight line dependence of (Teff) was expressed analytically for circular plasmas which was used to explain the simulated results for the actual JET sight lines. The analytical model with parametrization of sight lines and plasma profiles is described and the results are used to discuss dual sight line measurements that can be tested at JET and its forward implications for study burning (nearly thermal) plasmas of ITER.
Commissioning activities and first results from the collective Thomson scattering diagnostic on ASDEX Upgrade (invited)
Authors: Meo F. , Bindslev H. , Korsholm S. B. , Furtula V. , Leuterer F. , Leipold F. , Michelsen P. K. , Nielsen S. K. , Salewski M. , Stober J. , Wagner D. , Woskov P.
Published: November 2008

The collective Thomson scattering (CTS) diagnostic installed on ASDEX Upgrade uses millimeter waves generated by the newly installed 1 MW dual frequency gyrotron as probing radiation at 105 GHz. It measures backscattered radiation with a heterodyne receiver having 50 channels (between 100 and 110 GHz) to resolve the one-dimensional velocity distribution of the confined fast ions. The steerable antennas will allow different scattering geometries to fully explore the anisotropic fast ion distributions at different spatial locations. This paper covers the capabilities and operational limits of the diagnostic. It then describes the commissioning activities carried out to date. These activities include gyrotron studies, transmission line alignment, and beam pattern measurements in the vacuum vessel. Overlap experiments in near perpendicular and near parallel have confirmed the successful alignment of the system. First results in near perpendicular of scattered spectra in a neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) plasma (minority hydrogen) on ASDEX Upgrade have shown evidence of ICRH heating phase of hydrogen.
Measurement and calculation of charge deposition in a silicon diode irradiated by 30 MeV protons
Authors: Rocheman Simon , Wrobel Frédéric , Saigné Frédéric , Vaillé Jean-Roch , Weulersse Cécile , Buard Nadine , Miller Florent , Carrière Thierry
Published: November 2008

The aim of this work is to validate the results of the MC-RED nuclear physics code used to determine the deposited energy in a silicon volume taking into account the probabilistic approach of the physical phenomenon. A silicon diode has been irradiated by a quasimonoenergetic beam at the CYCLONE facility at Catholic University of Louvain. Experimental results were compared with the ones obtained by Monte Carlo simulations. When all the possible contributions (direct ionizations and nuclear reactions) of protons to the total deposited charge are taken into account, experiments and simulations are shown to be in good agreement in the field of interest for Static Random Acces Memory (SRAM) soft error rate evaluation.
Proposal of in situ density calibration for Thomson scattering measurement by microwave reflectometry
Authors: Minami Takashi , Funaba Hisamichi , Narihara Kazumichi , Yamada Ichihiro , Hayashi Hiroshi , Kohmoto Toshikazu
Published: November 2008

A density calibration using microwave reflectometry for a Thomson scattering measurement is proposed. The calibration can be performed in parallel with a plasma experiment. A compact and simple FM-type reflectometer is preferable due to the small space around the Thomson scattering device. A Bayesian estimation method is also proposed for the derivation of the calibration factor. The Bayesian method requires fewer data from the reflectometer than an average method for an accurate density calibration factor, which is confirmed by a Monte Carlo simulation.
Reproducible growth of p-type ZnO:N using a modified atomic layer deposition process combined with dark annealing
Authors: Dunlop L. , Kursumovic A. , MacManus-Driscoll J. L.
Published: November 2008

Nitrogen doped ZnO (ZnO:N) films were deposited by atmospheric atomic layer deposition (ALD) between 100 and 300°C. Postannealing was required to remove compensating defects. After a low temperature dark annealing, originally n-type films became p-type. Films deposited at low temperatures (⩽150°C) have low hole mobilities (μ) of 0.2–0.4cm2V−1s−1 and moderate hole concentrations (np) of around 1×1015cm−3. Higher temperature deposited films (⩾200°C) have higher μ values (6cm2V−1s−1) but np values <1×1013cm−3. This crossover in transport properties can be explained by the opposing effects of deposition temperature on nitrogen doping level and distribution, and film crystallinity.
Electromagnetically induced localized ignition in secondary high explosives
Authors: Perry W. Lee , Sewell Thomas D. , Glover Brian B. , Dattelbaum Dana M.
Published: November 2008

A model for electromagnetically induced hot spots is developed from the well-established theories of dielectric mixtures, microwave absorption, heat transfer, and thermal ignition. This mathematical model is used to elucidate the interplay among these theories for a microwave heated system of secondary high explosive within which isolated electromagnetically lossy spheres are randomly distributed. Results are shown in this article for the specific case of pentaerythritol tetranitrate with embedded spheres of varying diameter and conductivity illuminated by a uniform time harmonic electromagnetic field of 1, 8, and 15 GHz. It is shown that for a given frequency and electric field strength internal to a particle embedded in a secondary high explosive, there exists a range of values for the particle’s diameter and conductivity for which electromagnetically induced hot-spot ignition of the high explosive is possible. By providing an accurate estimate for the range of necessary geometric and electrical properties of the system, this idealized model can be used to guide potentially complex and otherwise less tractable computational and experimental studies of microwave-induced hot spots. We found that the condition of relatively large particles, with semiconductorlike conductivity, exposed to the highest possible frequency is most favorable for hot-spot ignition.
Reynolds number effects on the Reynolds-stress budgets in turbulent channels
Authors: Hoyas Sergio , Jiménez Javier
Published: November 2008

Budgets for the nonzero components of the Reynolds-stress tensor are presented for numerical channels with Reynolds numbers in the range Reτ=180–2000. The scaling of the different terms is discussed, both above and within the buffer and viscous layers. Above x2+≈150, most budget components scale reasonably well with uτ3/h, but the scaling with uτ4/ν is generally poor below that level. That is especially true for the dissipations and for the pressure-related terms. The former is traced to the effect of the wall-parallel large-scale motions, and the latter to the scaling of the pressure itself. It is also found that the pressure terms scale better near the wall when they are not separated into their diffusion and deviatoric components, but mostly only because the two terms tend to cancel each other in the viscous sublayer. The budgets, together with their statistical uncertainties, are available electronically from http://torroja.dmt.upm.es/channels.
An accurate determination of the electronic transitions of InAs/InGaAs/InP quantum dots for midinfrared lasers using simultaneous complementary spectroscopic techniques
Authors: Sharma T. K. , Hosea T. J. C. , Sweeney S. J. , Tang X.
Published: November 2008

InAs/InGaAs/InP quantum dots (QDs) emitting at ∼2 μm for midinfrared laser applications are studied using the complementary spectroscopic techniques of photoluminescence (PL), photoreflectance (PR), and surface photovoltage spectroscopy (SPS). We use a procedure that ensures that the same sample spot is studied virtually simultaneously by these three different spectroscopic techniques under almost identical conditions. We are able to measure the ground and excited states transitions of the InAs QDs without any ambiguity, thus providing a complete and clear understanding of the electronic transitions. Temperature dependent PL, SPS, and PR measurements provide a systematic thermal evolution of the ground and excited states. However, the QD transitions are not all seen together at any given temperature in either the PL or SPS measurements. By contrast, the PR technique can measure the complete set of ground and two excited state QD transitions and also the InGaAs barrier energy at all studied temperatures.
Phonon-engineered mobility enhancement in the acoustically mismatched silicon/diamond transistor channels
Authors: Nika Denis L. , Pokatilov Evghenii P. , Balandin Alexander A.
Published: November 2008

The authors have shown that the low-field electron drift mobility in the ultrathin silicon films can be enhanced if they are embedded within acoustically hard materials such as diamond. The increase results from phonon spectrum modification in the acoustically mismatched silicon/diamond heterostructure and suppression of the deformation-potential electron-phonon scattering. The room temperature mobility in silicon films with 2 nm thickness can be increased by a factor of 2–3 depending on the hardness and thickness of the barrier layers. The obtained results suggest a new phonon-engineering approach for increasing the speed and drive current of downscaled electronic devices.
Biocompatible poly(vinylidene fluoride)/cyanoacrylate composite coatings with tunable hydrophobicity and bonding strength
Authors: Bayer I. S. , Tiwari M. K. , Megaridis C. M.
Published: November 2008

Biocompatible composite coatings are produced from solution processable poly(vinylidene fluoride)/cyanoacrylate blends prepared in the presence of rosin and ZnO particle fillers, the latter for control of coating surface microstructure. Dispersions are spray coated and cured at 100 °C onto aluminum foils and fiberglass cloths suitable for tissue engineering applications. The elastic modulus of the composite films matches or exceeds that of polyethylene-based orthopedic implant materials. Contact angle measurements on coated fiberglass cloths reveal that wettability of hydrophobic coatings is maintained under strain for applied mechanical stress levels up to ∼15 kN/m2, whereas ultrahydrophobic coatings fail at ∼5 kN/m2.
An electron microscopic investigation of structural variation of V2O5 fibers after working as ethanol sensors
Authors: Ding Ning , Liu Shuhua , Chen Chunhua , Lieberwirth Ingo
Published: November 2008

This is a study of V2O5 fibers as ethanol sensors. The variation in the V2O5 electronic state and its structure after reaction with ethanol are investigated by electron energy-loss spectroscopy and electron diffraction. Results show that after reaction the oxidation state of vanadium in V2O5 is reduced to +4, and the crystallinity of V2O5 is completely destroyed, forming amorphous VO2 nanoparticles. High-resolution transmission electron microscopy shows that the diameter of amorphous VO2 nanoparticles is about 4 nm.
The effect of KrF laser annealing within an ultrashort time on metal-alumina-nitride-oxide-silicon-type flash memory devices
Authors: Heo Sungho , Chang Man , Ju Yongkyu , Jung Seungjae , Hwang Hyunsang
Published: November 2008

The effect of laser annealing (LA) within an ultrashort time on metal-alumina-nitride-oxide-silicon-type flash memory device is investigated. Conventional high-temperature annealing leads to the degrading quality of high-k blocking oxide due to the long annealing time. On the contrary, an optimized LA at an energy of 500 mJ/cm2 can effectively improve the blocking efficiency due to the reduction in leakage current density. Using the results of x-ray photoemission spectroscopy and high-resolution cross-sectional transmission electron microscopy, this improvement was attributed to the minimized decomposition of the Al2O3 layer and the interfacial layer growth between Al2O3 and Si3N4.
Surface-emitting photonic crystal terahertz quantum cascade lasers
Authors: Marshall Owen P. , Apostolopoulos Vasilis , Freeman Joshua R. , Rungsawang Rakchanok , Beere Harvey E. , Ritchie David A.
Published: November 2008

Surface-emitting terahertz quantum cascade lasers based on double-metal waveguides incorporating photonic crystal structures have been demonstrated. Far-field emission patterns are dominated by lobes close to the surface normal. In addition to modified emission profiles, enhanced output powers are also displayed in comparison to standard ridge waveguides, with over 2 mW peak power observed at a heat sink temperature of 10 K.