Index of content:
Volume 87, Issue 2, February 2016
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
87(2016); http://dx.doi.org/10.1063/1.4940935View Description Hide Description
Direct impact Hopkinson pressure bar systems offer many potential advantages over split Hopkinson pressure bars, including access to higher strain rates, higher strains for equivalent striker velocity and system length, lower dispersion, and faster achievement of force equilibrium. Currently, these advantages are gained at the expense of all information about the striker impacted specimen face, preventing the experimental determination of force equilibrium, and requiring approximations to be made on the sample deformation history. In this paper, we discuss an experimental method and complementary data analysis for using photon Doppler velocimetry to measuresurfacevelocities of the striker and output bars in a direct impact bar experiment, allowing similar data to be recorded as in a split bar system. We discuss extracting velocity and force measurements, and the precision of measurements. Results obtained using the technique are compared to equivalent split bar tests, showing improved stress measurements for the lowest and highest strains in fully dense metals, and improvement for all strains in slow and non-equilibrating materials.
87(2016); http://dx.doi.org/10.1063/1.4941336View Description Hide Description
Time-of-flight (TOF) is a standard experimental technique for determining, among others, the speed ratio S(velocity spread) of a molecular beam. The speed ratio is a measure for the monochromaticity of the beam and an accurate determination of S is crucial for various applications, for example, for characterising chromatic aberrations in focussing experiments related to helium microscopy or for precise measurements of surface phonons and surface structures in molecular beam scattering experiments. For both of these applications, it is desirable to have as high a speed ratio as possible. Molecular beam TOF measurements are typically performed by chopping the beam using a rotating chopper with one or more slit openings. The TOF spectra are evaluated using a standard deconvolution method. However, for higher speed ratios, this method is very sensitive to errors related to the determination of the slit width and the beam diameter. The exact sensitivity depends on the beam diameter, the number of slits, the chopper radius, and the chopper rotation frequency. We present a modified method suitable for the evaluation of TOF measurements of high speed ratio beams. The modified method is based on a systematic variation of the chopper convolution parameters so that a set of independent measurements that can be fitted with an appropriate function are obtained. We show that with this modified method, it is possible to reduce the error by typically one order of magnitude compared to the standard method.
87(2016); http://dx.doi.org/10.1063/1.4941433View Description Hide Description
The Relative Infrared Radiometer(RIR) is commonly used to measure the irradiance of the Infrared Target Simulator (ITS), and the calibration of the RIR is central for the measurement accuracy. RIRcalibration is conventionally performed using the Radiance Based (RB) calibration method or Irradiance Based (IB) calibration method, and the relationship between the radiation of standard source and the response of RIR is determined by curve fitting. One limitation existing in the calibration of RIR is the undesirable calibration voltage fluctuation in single measurement or in the reproducibility measurement, which reduces the calibration reproducibility and irradiancemeasurement accuracy. To address this limitation, the Equivalent Blackbody Temperature Based (EBTB) calibration method is proposed for the calibration of RIR. The purpose of this study is to compare the proposed EBTB calibration method with conventional RB and IB calibration methods. The comparison and experiment results have shown that the EBTB calibration method is not only able to provide comparable correlation between radiation and response to other calibration methods (IB and RB) in the irradiancemeasurement but also reduces the influence of calibration voltage fluctuation on the irradiancemeasurement result, which improves the calibration reproducibility and irradiancemeasurement accuracy.
- Nuclear Physics, Fusion and Plasmas
Bayesian modelling of the emission spectrum of the Joint European Torus Lithium Beam Emission Spectroscopy system87(2016); http://dx.doi.org/10.1063/1.4940925View Description Hide Description
A Bayesian model of the emission spectrum of the JET lithiumbeam has been developed to infer the intensity of the Li I (2p-2s) line radiation and associated uncertainties. The detected spectrum for each channel of the lithiumbeam emission spectroscopy system is here modelled by a single Li line modified by an instrumental function, Bremsstrahlung background, instrumental offset, and interference filter curve. Both the instrumental function and the interference filter curve are modelled with non-parametric Gaussian processes. All free parameters of the model, the intensities of the Li line, Bremsstrahlung background, and instrumental offset, are inferred using Bayesian probability theory with a Gaussian likelihood for photon statistics and electronic background noise. The prior distributions of the free parameters are chosen as Gaussians. Given these assumptions, the intensity of the Li line and corresponding uncertainties are analytically available using a Bayesian linear inversion technique. The proposed approach makes it possible to extract the intensity of Li line without doing a separate background subtraction through modulation of the Li beam.
87(2016); http://dx.doi.org/10.1063/1.4939672View Description Hide Description
Recent analysis of Gas Puff Imaging (GPI) data from Alcator C-Mod found blob velocities with a modified tracking time delay estimation (TDE). These results disagree with velocityanalysis performed using direct Fourier methods. In this paper, the two analysis methods are compared. The implementations of these methods are explained, and direct comparisons using the same GPI data sets are presented to highlight the discrepancies in measuredvelocities. In order to understand the discrepancies, we present a code that generates synthetic sequences of images that mimic features of the experimental GPI images, with user-specified input values for structure (blob) size and velocity. This allows quantitative comparison of the TDE and Fourier analysis methods, which reveals their strengths and weaknesses. We found that the methods agree for structures of any size as long as all structures move at the same velocity and disagree when there is significant nonlinear dispersion or when structures appear to move in opposite directions. Direct Fourier methods used to extract poloidal velocities give incorrect results when there is a significant radial velocity component and are subject to the barber pole effect. Tracking TDE techniques give incorrect velocity measurements when there are features moving at significantly different speeds or in different directions within the same field of view. Finally, we discuss the limitations and appropriate use of each of methods and applications to the relationship between blob size and velocity.
87(2016); http://dx.doi.org/10.1063/1.4940929View Description Hide Description
A self-standing single-crystal chemical vapor deposited diamond was obtained using lift-off method. It was fabricated into a radiation detector and response function measurements for 14 MeV neutrons were taken at the fusion neutronics source. 1.5% of high energy resolution was obtained by using the 12C(n, α)9Be reaction at an angle of 100° with the deuteron beam line. The intrinsic energy resolution, excluding energy spreading caused by neutron scattering, slowing in the target and circuit noises was 0.79%, which was also the best resolution of the diamonddetector ever reported.
Feedback system for divertor impurity seeding based on real-time measurements of surface heat flux in the Alcator C-Mod tokamak87(2016); http://dx.doi.org/10.1063/1.4941047View Description Hide Description
Mitigation of the intense heat flux to the divertor is one of the outstanding problems in fusion energy. One technique that has shown promise is impurity seeding, i.e., the injection of low-Z gaseous impurities (typically N2 or Ne) to radiate and dissipate the power before it arrives to the divertor target plate. To this end, the Alcator C-Mod team has created a first-of-its-kind feedback system to control the injection of seed gas based on real-time surface heat flux measurements.Surfacethermocouples provide real-time measurements of the surfacetemperature response to the plasma heat flux. The surfacetemperaturemeasurements are inputted into an analog computer that “solves” the 1-D heat transport equation to deliver accurate, real-time signals of the surface heat flux. The surface heat flux signals are sent to the C-Mod digital plasmacontrol system, which uses a proportional–integral–derivative (PID) algorithm to control the duty cycle demand to a pulse width modulated piezo valve, which in turn controls the injection of gas into the private flux region of the C-Mod divertor. This paper presents the design and implementation of this new feedback system as well as initial results using it to control divertor heat flux.
87(2016); http://dx.doi.org/10.1063/1.4941441View Description Hide Description
X-ray phase-contrast techniques can measure electron density gradients in high-energy-density plasmas through refraction induced phase shifts. An 8 keV Talbot-Lau interferometer consisting of free standing ultrathin gratings was deployed at an ultra-short, high-intensity laser system using K-shell emission from a 1-30 J, 8 ps laser pulse focused on thin Cu foil targets. Grating survival was demonstrated for 30 J, 8 ps laser pulses. The first x-ray deflectometry images obtained under laser backlighting showed up to 25% image contrast and thus enabled detection of electron areal density gradients with a maximum value of 8.1 ± 0.5 × 1023 cm−3 in a low-Z millimeter sized sample. An electron density profile was obtained from refraction measurements with an error of <8%. The 50 ± 15 μm spatial resolution achieved across the full field of view was found to be limited by the x-ray source-size, similar to conventional radiography.
- Microscopy and Imaging
87(2016); http://dx.doi.org/10.1063/1.4940443View Description Hide Description
X-ray topographs are taken for a sapphire wafer with the  surface normal, as an example, by forward transmitted synchrotron x-ray beams combined with two-dimensional electronic arrays in the x-ray detector having a spatial resolution of 1 μm. They exhibit no shape deformation and no position shift of the dislocation lines on the topographs. Since the topography is performed under multiple-beam diffraction conditions, the topographic images of a single diffraction (two-wave approximation condition) or plural diffractions (six-wave approximation condition) can be recorded without large specimen position changes. As usual Lang topographs, it is possible to determine the Burgers vector of each dislocation line. Because of high parallelism of the incoming x-rays and linear sensitivity of the electronic arrays to the incident x-rays, the present technique can be used to visualize individual dislocations in single crystals of the dislocation density as high as 1 × 105 cm−2.
Evaluation and optimization of quartz resonant-frequency retuned fork force sensors with high Q factors, and the associated electric circuits, for non-contact atomic force microscopy87(2016); http://dx.doi.org/10.1063/1.4941065View Description Hide Description
High-Q factor retuned fork (RTF) force sensors made from quartztuning forks, and the electric circuits for the sensors, were evaluated and optimized to improve the performance of non-contact atomic force microscopy (nc-AFM) performed under ultrahigh vacuum (UHV) conditions. To exploit the high Q factor of the RTF sensor, the oscillation of the RTF sensor was excited at its resonant frequency, using a stray capacitance compensation circuit to cancel the excitation signal leaked through the stray capacitor of the sensor. To improve the signal-to-noise (S/N) ratio in the detected signal, a small capacitor was inserted before the input of an operational (OP) amplifier placed in an UHV chamber, which reduced the output noise from the amplifier. A low-noise, wideband OP amplifier produced a superior S/N ratio, compared with a precision OP amplifier. The thermal vibrational density spectra of the RTF sensors were evaluated using the circuit. The RTF sensor with an effective spring constant value as low as 1000 N/m provided a lower minimum detection limit for force differentiation. A nc-AFM image of a Si(111)-7 × 7 surface was produced with atomic resolution using the RTF sensor in a constant frequency shift mode; tunneling current and energy dissipation images with atomic resolution were also simultaneously produced. The high-Q factor RTF sensor showed potential for the high sensitivity of energy dissipation as small as 1 meV/cycle and the high-resolution analysis of non-conservative force interactions.
87(2016); http://dx.doi.org/10.1063/1.4941292View Description Hide Description
An advanced scanning magnetoresistive microscopy (SMRM) — a robust magnetic imaging and probing technique — will be presented, which utilizes state-of-the-art recording heads of a hard disk drive as sensors. The spatial resolution of modern tunneling magnetoresistive sensors is nowadays comparable to the more commonly used magnetic force microscopes. Important advantages of SMRM are the ability to detect pure magnetic signals directly proportional to the out-of-plane magnetic stray field, negligible sensor stray fields, and the ability to apply local bipolar magnetic field pulses up to 10 kOe with bandwidths from DC up to 1 GHz. Moreover, the SMRM can be further equipped with a heating stage and external magnetic field units. The performance of this method and corresponding best practices are demonstrated by presenting various examples, including a temperature dependent recording study on hard magnetic L10 FeCuPt thin films,imaging of magnetic vortex states in an in-plane magnetic field, and their controlled manipulation by applying local field pulses.
- Condensed Matter; Materials
Qualification of a sublimation tool applied to the case of metalorganic chemical vapor deposition of In2O3 from In(tmhd)3 as a solid precursor87(2016); http://dx.doi.org/10.1063/1.4940930View Description Hide Description
A solid delivery system consisting of a source canister, a gas management, and temperature controlled enclosure designed and manufactured by Air Liquide Electronics Systems was tested in the context of gas-phase delivery of the In(tmhd)3 solid precursor. The precursor stream was delivered to a thermal metalorganic chemical vapor deposition reactor to quantify deposition yield under various conditions of carrier gas flow and sublimation temperature. The data collected allowed the determination of characteristic parameters such as the maximum precursor flow rate (18.2 mg min−1 in specified conditions) and the critical mass (defined as the minimum amount of precursor able to attain the maximum flow rate) found to be about 2.4 g, as well as an understanding of the influence of powder distribution inside the canister. Furthermore, this qualification enabled the determination of optimal delivery conditions which allowed for stable and reproducible precursor flow rates over long deposition times (equivalent to more than 47 h of experiment). The resulting In2O3 layers was compared with those elaborated via pulsed liquid injection obtained in the same chemical vapor deposition chamber and under the same deposition conditions.
A load-lock compatible system for in situ electrical resistivity measurements during thin film growth87(2016); http://dx.doi.org/10.1063/1.4940933View Description Hide Description
An experimental setup designed for in situelectrical resistance measurement during thin film growth is described. The custom-built sample holder with a four-point probe arrangement can be loaded into a high-vacuum magnetron sputter-deposition chamber through a load-lock transfer system, allowing measurements on series of samples without venting the main chamber. Electrical contact is ensured with circular copper tracks inserted in a Teflon plate on a mounting holder station inside the deposition chamber. This configuration creates the possibility to measure thickness-dependent electrical resistance changes with sub-monolayer resolution and is compatible with use of sample rotation during growth. Examples are presented for metallic films with high adatom mobility growing in a Volmer-Weber mode (Ag and Pd) as well as for refractory metal(Mo) with low adatom mobility. Evidence for an amorphous-to-crystalline phase transition at a film thickness of 2.6 nm is reported during growth of Mo on an amorphous Si underlayer, supporting previous findings based on in situ wafer curvature measurements.
Mapping the process dependent conductivity of carbon nanotube thin-films using a non-invasive contact probing system87(2016); http://dx.doi.org/10.1063/1.4941294View Description Hide Description
We report on a non-invasive contact probing (NICP) system for measuring the distribution of local surface conductivity of macroscopic thin-films of carbon nanotubes. Using the NICP system, we were able to obtain the local sheet resistance of the conducting thin-films continuously at ∼10 μm resolution over few centimeters which would not have been possible using conventional contact probing methods. Measurements performed on carbon nanotube thin-films with various nanotube densities, physical, and chemical treatments revealed that the local variation in electrical characteristics was not reflected in global conductance measurements. This demonstrated the usefulness of the NICP system for evaluating the effect of processing on the electrical uniformity of conducting thin-films made using nanomaterials.
- Electronics; Electromagnetic Technology; Microwaves
87(2016); http://dx.doi.org/10.1063/1.4940725View Description Hide Description
The design and characterization of a novel toroidal split-ring resonator (SRR) are described in detail. In conventional cylindrical SRRs, there is a large magnetic flux within the bore of the resonator. However, there also exists a non-negligible magnetic flux in the free space surrounding the resonator. The energy losses associated with this radiated power diminish the resonator’s quality factor. In the toroidal SRR, on the other hand, the magnetic field lines are strongly confined within the bore of the resonator resulting in high intrinsic quality factors and stable resonance frequencies without requiring additional electromagnetic shielding. This paper describes the design and construction of a toroidal SRR as well as an experimental investigation of its cw response in the frequency-domain and its time-domain response to a rf pulse. Additionally, the dependence of the toroidal SRR’s resonant frequency and quality factor on the strength of inductive coupling to external circuits is investigated both theoretically and experimentally.
- Thermometry; Thermal Diffusivity; Acoustics; Photothermal and Photoacoustic
A comparison between finite element modeling and various thermographic non-destructive testing techniques for the quantification of the thermal integrity of macro-brush plasma facing components used in a tokamak87(2016); http://dx.doi.org/10.1063/1.4940728View Description Hide Description
The plasma facing components (PFCs) inside a tokamak are typically exposed to extremely high heat flux of the order of MW/m2. The brazing quality between the plasma facing materials (PFMs) and the heat sink will determine the structural integrity and hence the effective service life of these PFCs. Suitable non-destructive testing (NDT) techniques for the pre-qualification of these components are thus essential to evaluate their structural integrity at various stages of their service life. Macro-brush type mockups of prototype PFCs with graphite as PFM have been inspected for their brazing quality using different active Infrared (IR)–thermographic NDT techniques. The results obtained from these techniques are compared and discussed. The brazing quality was quantified by establishing a comparison between the experimental results and the results from Finite Element Analysis (FEA). The percentage of contact between the PFM and the substrate was varied in FEA.FEA results when compared with experiments shows that tiles have different amounts of contact with the substrate ranging between 10% and 80%.
- Sensors and Actuators/MEMS/NEMS
An integrated temperature-compensated flexible shear-stress sensor microarray with concentrated leading-wire87(2016); http://dx.doi.org/10.1063/1.4941067View Description Hide Description
Flexible shear stress sensor is quite important for characterizing curved surfaceflows. In this work, a novel integrated shear stress sensormicroarray is designed with twenty parallel channels, which share the concentrated leading-wire to transmit the ground signal. Electrical pads in rows are easily connected to the circuits with two separate Wheatstone bridges and constant-temperature-difference mode operation is provided for the hot-wires. Temperature crosstalk between adjacent hot-wires is prevented well and the effectiveness of the temperature compensated circuits is verified. Relatively large output response is obtained as the shear stress varies and the sensitivity of the sensors is measured about 0.086 V2/Pa1/3 with nonlinearity lower than 1%, revealing high performance characteristic of the sensors.
- General Instruments
Development of a new direct liquid injection system for nanoparticle deposition by chemical vapor deposition using nanoparticle solutions87(2016); http://dx.doi.org/10.1063/1.4940937View Description Hide Description
Nanoparticles of different materials are already in use for many applications. In some applications, these nanoparticles need to be deposited on a substrate in a fast and reproducible way. We have developed a new direct liquid injection system for nanoparticledeposition by chemical vapor deposition using a liquid nanoparticle precursor. The system was designed to depositnanoparticles in a controlled and reproducible way by using two direct liquid injectors to deliver nanoparticles to the system. The nanoparticle solution is first evaporated and then the nanoparticles flow onto a substrate inside the vacuum chamber. To allow injection and evaporation of the liquid, a direct liquid injection and vaporization system are mounted on top of the process chamber. The deposition of the nanoparticles is controlled by parameters such as deposition temperature, partial pressure of the gases, and flow rate of the nanoparticle suspension. The concentration of the depositednanoparticles can be varied simply by changing the flow rate and deposition time. We demonstrate the capabilities of this system using gold nanoparticles. The selected suspension flow rates were varied between 0.25 and 1 g/min. AFM analysis of the deposited samples showed that the aggregation of gold nanoparticles is well controlled by the flow and deposition parameters.
87(2016); http://dx.doi.org/10.1063/1.4941066View Description Hide Description
A high precision five rotation-axes polarimeter using transmission multilayers as polarizers and reflection multilayers as analyzers has been designed and manufactured. To cover the extreme ultraviolet regime, Mo/Si, Cr/C, Sc/Cr, and W/B4C multilayers for transmission and reflection have also been designed and produced. The polarimeter mechanics is supported on a hexapod to simplify the alignment relative to photon beam. The instrument is designed so that it can be easily transferred between different beamlines.
A Mo-anode-based in-house source for small-angle X-ray scattering measurements of biological macromolecules87(2016); http://dx.doi.org/10.1063/1.4940936View Description Hide Description
We demonstrate the use of a molybdenum-anode-based in-house small-angle X-ray scattering (SAXS) setup to study biological macromolecules in solution. Our system consists of a microfocus X-ray tube delivering a highly collimated flux of 2.5 × 106 photons/s at a beam size of 1.2 × 1.2 mm2 at the collimation path exit and a maximum beam divergence of 0.16 mrad. The resulting observable scattering vectors q are in the range of 0.38 Å−1 down to 0.009 Å−1 in SAXS configuration and of 0.26 Å−1 up to 5.7 Å−1 in wide-angle X-ray scattering (WAXS) mode. To determine the capabilities of the instrument, we collected SAXS data on weakly scattering biological macromolecules including proteins and a nucleic acid sample with molecular weights varying from ∼12 to 69 kDa and concentrations of 1.5–24 mg/ml. The measuredscattering data display a high signal-to-noise ratio up to q-values of ∼0.2 Å−1 allowing for an accurate structural characterization of the samples. Moreover, the in-house source data are of sufficient quality to perform ab initio 3D structure reconstructions that are in excellent agreement with the available crystallographic structures. In addition, measurements for the detergent decyl-maltoside show that the setup can be used to determine the size, shape, and interactions (as characterized by the second virial coefficient) of detergent micelles. This demonstrates that the use of a Mo-anode based in-house source is sufficient to determine basic geometric parameters and 3D shapes of biomolecules and presents a viable alternative to valuable beam time at third generation synchrotron sources.