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Jun 2010

Volume 81, Issue 6, Articles (06xxxx)

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back to top Optics; Atoms and Molecules; Spectroscopy; Photon Detectors

A novel experimental system of high stability and lifetime for the laser-desorption of biomolecules

Mehran Taherkhani, Mikko Riese, Mohammed BenYezzar, and Klaus Müller-Dethlefs

Rev. Sci. Instrum. 81, 063101 (2010); http://dx.doi.org/10.1063/1.3373977 (6 pages) | Cited 1 time

Online Publication Date: 8 June 2010

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A novel laser desorption system, with improved signal stability and extraordinary long lifetime, is presented for the study of jet-cooled biomolecules in the gas phase using vibrationally resolved photoionization spectroscopy. As a test substance tryptophane is used to characterize this desorption source. A usable lifetime of above 1 month (for a laser desorption repetition rate of 20 Hz) has been observed by optimizing the pellets (graphite/tryptophane, 3 mm diameter and 6 mm length) from which the substance is laser-desorbed. Additionally, the stability and signal-to-noise ratio has been improved by averaging the signal over the entire sample pellet by synchronizing the data acquisition with the rotation of the sample rod. The results demonstrate how a combination of the above helps to produce stable and conclusive spectra of tryptophane using one-color and two-color resonant two-photon ionization studies.
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87.50.W- Optical/infrared radiation effects
87.15.mn Photoionization

A quantum cascade laser cw cavity ringdown spectrometer coupled to a supersonic expansion source

Brian E. Brumfield, Jacob T. Stewart, Susanna L. Widicus Weaver, Matthew D. Escarra, Scott S. Howard, Claire F. Gmachl, and Benjamin J. McCall

Rev. Sci. Instrum. 81, 063102 (2010); http://dx.doi.org/10.1063/1.3427357 (10 pages) | Cited 6 times

Online Publication Date: 8 June 2010

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A new instrument has been constructed that couples a supersonic expansion source to a continuous wave cavity ringdown spectrometer using a Fabry–Perot quantum cascade laser (QCL). The purpose of the instrument is to enable the acquisition of a cold, rotationally resolved gas phase spectrum of buckminsterfullerene (C60). As a first test of the system, high resolution spectra of the ν8 vibrational band of CH2Br2 have been acquired at ∼ 1197 cm−1. To our knowledge, this is the first time that a vibrational band not previously recorded with rotational resolution has been acquired with a QCL-based ringdown spectrometer. 62 transitions of the three isotopologues of CH2Br2 were assigned and fit to effective Hamiltonians with a standard deviation of 14 MHz, which is smaller than the laser frequency step size. The spectra have a noise equivalent absorption coefficient of 1.4×10−8 cm−1. Spectral simulations of the band indicate that the supersonic source produces rotationally cold ( ∼ 7 K) molecules.
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42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy
07.57.Ty Infrared spectrometers, auxiliary equipment, and techniques
42.55.Px Semiconductor lasers; laser diodes

85Rb tunable-interaction Bose–Einstein condensate machine

P. A. Altin, N. P. Robins, D. Döring, J. E. Debs, R. Poldy, C. Figl, and J. D. Close

Rev. Sci. Instrum. 81, 063103 (2010); http://dx.doi.org/10.1063/1.3430538 (9 pages) | Cited 5 times

Online Publication Date: 9 June 2010

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We describe our experimental setup for creating stable Bose–Einstein condensates (BECs) of 85Rb with tunable interparticle interactions. We use sympathetic cooling with 87Rb in two stages, initially in a tight Ioffe–Pritchard magnetic trap and subsequently in a weak, large-volume, crossed optical dipole trap, using the 155 G Feshbach resonance to manipulate the elastic and inelastic scattering properties of the 85Rb atoms. Typical 85Rb condensates contain 4×104 atoms with a scattering length of a = +200a0. Many aspects of the design presented here could be adapted to other dual-species BEC machines, including those involving degenerate Fermi–Bose mixtures. Our minimalist apparatus is well suited to experiments on dual-species and spinor Rb condensates, and has several simplifications over the 85Rb BEC machine at JILA, which we discuss at the end of this article.
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03.75.Kk Dynamic properties of condensates; collective and hydrodynamic excitations, superfluid flow
37.10.Gh Atom traps and guides
05.30.Jp Boson systems

Data processing correction of the irising effect of a fast-gating intensified charge-coupled device on laser-pulse-excited luminescence spectra

L. Ondič, K. Dohnalová, I. Pelant, K. Žídek, and W. D. A. M. de Boer

Rev. Sci. Instrum. 81, 063104 (2010); http://dx.doi.org/10.1063/1.3431536 (5 pages) | Cited 1 time

Online Publication Date: 9 June 2010

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Intensified charge-coupled devices (ICCDs) comprise the advantages of both fast gating detectors and spectrally broad CCDs into one device that enables temporally and spectrally resolved measurements with a few nanosecond resolution. Gating of the measured signal occurs in the image intensifier tube, where a high voltage is applied between the detector photocathode and a microchannel plate electron multiplier. An issue arises in time-resolved luminescence spectroscopy when signal onset characterization is required. In this case, the transient gate closing process that causes the detected signal always arises in the middle of the ICCD chip regardless of the spectral detection window—the so-called irising effect. We demonstrate that in case when the detection gate width is comparable to the opening/closing time and the gate is pretriggered with respect to the signal onset, the irising effect causes the obtained data to be strongly distorted. At the same time, we propose a software procedure that leads to the spectral correction of the irising effect and demonstrate its validity on the distorted data.
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42.79.Pw Imaging detectors and sensors
42.79.Ls Scanners, image intensifiers, and image converters
85.30.Tv Field effect devices
42.30.-d Imaging and optical processing

A 12.5 GHz-spaced optical frequency comb spanning >400 nm for near-infrared astronomical spectrograph calibration

F. Quinlan, G. Ycas, S. Osterman, and S. A. Diddams

Rev. Sci. Instrum. 81, 063105 (2010); http://dx.doi.org/10.1063/1.3436638 (9 pages) | Cited 15 times

Online Publication Date: 10 June 2010

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A 12.5 GHz-spaced optical frequency comb locked to a global positioning system disciplined oscillator for near-infrared (IR) spectrograph calibration is presented. The comb is generated via filtering a 250 MHz-spaced comb. Subsequent nonlinear broadening of the 12.5 GHz comb extends the wavelength range to cover 1380–1820 nm, providing complete coverage over the H-band transmission window of earth’s atmosphere. Finite suppression of spurious sidemodes, optical linewidth, and instability of the comb has been examined to estimate potential wavelength biases in spectrograph calibration. Sidemode suppression varies between 20 and 45 dB, and the optical linewidth is ∼ 350 kHz at 1550 nm. The comb frequency uncertainty is bounded by ±30 kHz (corresponding to a radial velocity of ±5 cm/s), limited by the global positioning system disciplined oscillator reference. These results indicate that this comb can readily support radial velocity measurements below 1 m/s in the near IR.
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95.55.Sh Auxiliary and recording instruments; clocks and frequency standards
06.20.fb Standards and calibration
07.57.Ty Infrared spectrometers, auxiliary equipment, and techniques
95.75.Fg Spectroscopy and spectrophotometry

Generation of tunable narrow bandwidth nanosecond pulses in the deep ultraviolet for efficient optical pumping and high resolution spectroscopy

Luis Velarde, Daniel P. Engelhart, Daniel Matsiev, Jerry LaRue, Daniel J. Auerbach, and Alec M. Wodtke

Rev. Sci. Instrum. 81, 063106 (2010); http://dx.doi.org/10.1063/1.3436973 (10 pages) | Cited 5 times

Online Publication Date: 17 June 2010

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Nanosecond optical pulses with high power and spectral brightness in the deep ultraviolet (UV) region have been produced by sum frequency mixing of nearly transform-limited-bandwidth IR light originating from a home-built injection-seeded ring cavity KTiOPO4 optical parametric oscillator (OPO) and the fourth harmonic beam of an injection-seeded Nd:YAG laser used simultaneously to pump the OPO with the second harmonic. We demonstrate UV output, tunable from 204 to 207 nm, which exhibits pulse energies up to 5 mJ with a bandwidth better than 0.01 cm−1. We describe how the approach shown in this paper can be extended to wavelengths shorter than 185 nm. The injection-seeded OPO provides high conversion efficiency (>40% overall energy conversion) and superior beam quality required for highly efficient downstream mixing where sum frequencies are generated in the UV. The frequency stability of the system is excellent, making it highly suitable for optical pumping. We demonstrate high resolution spectroscopy as well as optical pumping using laser-induced fluorescence and stimulated emission pumping, respectively, in supersonic pulsed molecular beams of nitric oxide.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.65.Lm Parametric down conversion and production of entangled photons

Double conical crystal x-ray spectrometer for high resolution ultrafast x-ray absorption near-edge spectroscopy of Al K edge

A. Levy, F. Dorchies, C. Fourment, M. Harmand, S. Hulin, J. J. Santos, D. Descamps, S. Petit, and R. Bouillaud

Rev. Sci. Instrum. 81, 063107 (2010); http://dx.doi.org/10.1063/1.3441983 (5 pages) | Cited 1 time

Online Publication Date: 17 June 2010

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An x-ray spectrometer devoted to dynamical studies of transient systems using the x-ray absorption fine spectroscopy technique is presented in this article. Using an ultrafast laser-induced x-ray source, this optical device based on a set of two potassium acid phthalate conical crystals allows the extraction of x-ray absorption near-edge spectroscopy structures following the Al absorption K edge. The proposed experimental protocol leads to a measurement of the absorption spectra free from any crystal reflectivity defaults and shot-to-shot x-ray spectral fluctuation. According to the detailed analysis of the experimental results, a spectral resolution of 0.7 eV rms and relative fluctuation lower than 1% rms are achieved, demonstrated to be limited by the statistics of photon counting on the x-ray detector.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
29.40.-n Radiation detectors

A new spectroscopic method for resolving the electronic symmetry properties of the highly excited molecules produced in photoexcitation

Takeshi Odagiri, Yoshiaki Kumagai, Takehiko Tanabe, Motoyoshi Nakano, Isao H. Suzuki, Masashi Kitajima, and Noriyuki Kouchi

Rev. Sci. Instrum. 81, 063108 (2010); http://dx.doi.org/10.1063/1.3436653 (6 pages)

Online Publication Date: 18 June 2010

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A novel method of spectroscopy for highly excited states of molecules in the valence excitation range has been established through the detection of metastable hydrogen atoms in the 2s state formed by photoexcitation. The detector for the metastable hydrogen atom is composed of a stack of parallel plate electrodes that creates a localized electric field and triggers the emission of the Lyman-α photon from the atom and a chevron pair of microchannel plates that detects the photon. For linear molecules, the angle-resolved detection of the metastable hydrogen atom enables us to measure cross sections in which electronic symmetries of highly excited molecular states are resolved. Such symmetry-resolved cross section measurements were carried out for doubly excited states of H2.
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33.80.-b Photon interactions with molecules

Nanograting-based compact vacuum ultraviolet spectrometer and beam profiler for in situ characterization of high-order harmonic generation light sources

Oleg Kornilov, Russell Wilcox, and Oliver Gessner

Rev. Sci. Instrum. 81, 063109 (2010); http://dx.doi.org/10.1063/1.3443575 (8 pages) | Cited 1 time

Online Publication Date: 21 June 2010

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A compact, versatile device for vacuum ultraviolet (VUV) beam characterization is presented. It combines the functionalities of a VUV spectrometer and a VUV beam profiler in one unit and is entirely supported by a standard DN200 CF flange. The spectrometer employs a silicon nitride transmission nanograting in combination with a microchannel plate-based imaging detector. This enables the simultaneous recording of wavelengths ranging from 10 to 80 nm with a resolution of 0.25–0.13 nm. Spatial beam profiles with diameters up to 10 mm are imaged with 0.1 mm resolution. The setup is equipped with an in-vacuum translation stage that allows for in situ switching between the spectrometer and beam profiler modes and for moving the setup out of the beam. The simple, robust design of the device is well suited for nonintrusive routine characterization of emerging laboratory- and accelerator-based VUV light sources. Operation of the device is demonstrated by characterizing the output of a femtosecond high-order harmonic generation light source.
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07.60.Rd Visible and ultraviolet spectrometers
42.79.Dj Gratings
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
06.60.Jn High-speed techniques (microsecond to femtosecond)
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Development of a multiplex fast-scan system for ultrafast time-resolved spectroscopy

Atsushi Yabushita, Yu-Hsien Lee, and Takayoshi Kobayashi

Rev. Sci. Instrum. 81, 063110 (2010); http://dx.doi.org/10.1063/1.3455809 (10 pages) | Cited 3 times

Online Publication Date: 24 June 2010

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A fast-scan method was developed to obtain time-resolved signals with femtosecond resolution over a picosecond range on the fly and in real time. Traditional fast-scan methods collect data at each probe wavelength one by one, which is time consuming and thus not possible for the study of photofragile materials. In this work, we have developed a system that performs fast scans with multiplex detection. Ultrafast time-resolved spectroscopy was demonstrated using the newly developed system. Femtosecond laser pulses have been used for pump-probe studies of ultrafast processes in various materials, and both electronic relaxation and vibrational dynamics have been studied. However, experiments have been limited in sensitivity and reliability because they are affected by the long-term instability of the ultrashort laser pulses and by the fragility of the samples. The instability of the sources hinders precise determination of electronic decay dynamics and introduces systematic errors. The fragility of the samples reduces their amount or concentration, and can lead to contamination of the materials even if they were pure before the measurement. These effects make it difficult to obtain reproducible and reliable experimental data. In the present work, we have developed a fast-scan pump-probe spectroscopic system that can complete a set of measurements in less than 2 min. Quantitative estimates of the signal reproducibility demonstrate that these measurements provide higher reproducibility and reliability than conventional measurements.
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42.65.Re Ultrafast processes; optical pulse generation and pulse compression
Author Select

Sample cells for probing solid/liquid interfaces with broadband sum-frequency-generation spectroscopy

Dominique Verreault, Volker Kurz, Caitlin Howell, and Patrick Koelsch

Rev. Sci. Instrum. 81, 063111 (2010); http://dx.doi.org/10.1063/1.3443096 (10 pages) | Cited 7 times

Online Publication Date: 24 June 2010

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Two sample cells designed specifically for sum-frequency-generation (SFG) measurements at the solid/liquid interface were developed: one thin-layer analysis cell allowing measurement of films on reflective metallic surfaces through a micrometer layer of solution and one spectroelectrochemical cell allowing investigation of processes at the indium tin oxide/solution interface. Both sample cells are described in detail and data illustrating the capabilities of each are shown. To further improve measurements at solid/liquid interfaces, the broadband SFG system was modified to include a reference beam which can be measured simultaneously with the sample signal, permitting background correction of SFG spectra in real time. Sensitivity tests of this system yielded a signal-to-noise ratio of 100 at a surface coverage of 0.2 molecules/nm2. Details on data analysis routines, pulse shaping methods of the visible beam, as well as the design of a purging chamber and sample stage setup are presented. These descriptions will be useful to those planning to set up a SFG spectrometer or seeking to optimize their own SFG systems for measurements of solid/liquid interfaces.
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78.30.-j Infrared and Raman spectra
78.40.-q Absorption and reflection spectra: visible and ultraviolet

A rotatable electron spectrometer for multicoincidence experiments

D. Céolin, J.-O. Forsell, B. Wannberg, S. Legendre, J. Palaudoux, G. Öhrwall, S. Svensson, and M. N. Piancastelli

Rev. Sci. Instrum. 81, 063112 (2010); http://dx.doi.org/10.1063/1.3449333 (8 pages)

Online Publication Date: 29 June 2010

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We have developed a rotatable hemispherical spectrometer with good energy and angular resolution, which can be positioned with the lens axis arbitrarily within a solid angle of 1 π. The collection angle of the emitted electrons with respect to the polarization axis of the light is set by means of a three-axes goniometer, operating under vacuum. An important requirement for this setup was the possibility to perform coincidences between the electron analyzed by the spectrometer and one or several other particles, such as ions, electrons, or photons. The lens system and the hemispheres have been designed to accommodate such experimental demands, regarding parameters such as the resolving power, the acceptance angle, or the width of the kinetic energy window which can be recorded for a given pass energy. We have chosen to detect the impact position of the electron at the focal plane of the hemispherical analyzer with a delay line detector and a time-to-digital converter as acquisition card rather than using a conventional charge-coupled device camera.
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07.81.+a Electron and ion spectrometers
back to top Particle Sources, Optics and Acceleration; Particle Detectors

Absolute detection efficiencies of a microchannel plate detector for 0.5–5 keV neutrals

S. Hosokawa, N. Takahashi, M. Saito, and Y. Haruyama

Rev. Sci. Instrum. 81, 063301 (2010); http://dx.doi.org/10.1063/1.3442514 (4 pages) | Cited 4 times

Online Publication Date: 17 June 2010

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The absolute detection efficiencies of a microchannel plate detector for neutral atoms were measured using the coincidence method for neutralized incident ions and ionized target atoms in electron capture collisions. This method does not require knowledge of the absolute electron-capture rates for determination of the detection efficiencies. Results for Ne, Ar, and Kr atoms at energies of 0.5–5 keV are reported. The detection efficiencies for all atomic species increase concomitantly with increasing impact energy and plateau at the efficiency of about 50%. For low impact energies, the efficiency decreases with increasing mass of the impact atom at a given energy.
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42.79.Ls Scanners, image intensifiers, and image converters
85.60.Ha Photomultipliers; phototubes and photocathodes
34.70.+e Charge transfer

Pulsed ion extraction diagnostics in a quadrupole ion trap linear time-of-flight mass spectrometer

Beni B. Dangi, Nicholas A. Sassin, and Kent M. Ervin

Rev. Sci. Instrum. 81, 063302 (2010); http://dx.doi.org/10.1063/1.3436659 (10 pages) | Cited 3 times

Online Publication Date: 18 June 2010

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Pulsed extraction techniques are investigated for a quadrupole ion trap (QIT) interfaced to a linear time-of-flight (TOF) mass analyzer. A nonfocusing short-pulse mode of operation is developed and characterized. The short-pulse mode creates a near-monoenergetic ion packet, which is useful for reaction kinetics experiments and for making diagnostic measurements of the ion cloud size in the trap. Monopolar and bipolar pulsing modes, with the voltage pulses applied to one or both QIT endcaps to extract the ions into the TOF region, are compared. Ion TOF peak distributions are characterized experimentally and by ion trajectory simulations. Also, first-order spatial (Wiley–McLaren) focusing of ions is characterized for the conventional long-pulse extraction mode. The nonparallel fields in the QIT, which serves as the first acceleration region in the linear-TOF mass spectrometer, are shown to degrade spatial focusing and mass resolution.
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82.80.Rt Time of flight mass spectrometry

Study of the reflectivity of neutron supermirrors influenced by surface oil layers

Tamás Veres and László Cser

Rev. Sci. Instrum. 81, 063303 (2010); http://dx.doi.org/10.1063/1.3443319 (6 pages)

Online Publication Date: 24 June 2010

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Neutron guides made of supermirror-coated glass are important components of most neutron scattering instruments, thus their quality and possible deterioration due to various deleterious effects (e.g., surface contamination or defects) deserve careful examination. The modification of the reflectivity of supermirrors and the transmission of neutron guides due to surface contamination with hydrocarbon oil has been investigated using neutron reflectometry together with model calculations. A significant loss in the neutron reflectivity was observed for supermirrors covered with thin hydrocarbon oil films, which were confirmed in model calculations. Simulations carried out for several typical arrangements show drastic decreases in the transmitted neutron flux of neutron guides. These simulations show that determining the distortion of the beam profile (using a slit or a pin hole) enables the detection of oil contamination even in an operating neutron guide.
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42.79.Bh Lenses, prisms and mirrors

A simple double-focusing electrostatic ion beam deflector

H. Kreckel, H. Bruhns, K. A. Miller, E. Wåhlin, A. Davis, S. Höckh, and D. W. Savin

Rev. Sci. Instrum. 81, 063304 (2010); http://dx.doi.org/10.1063/1.3433485 (5 pages) | Cited 1 time

Online Publication Date: 28 June 2010

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We have developed an electrostatic, double-focusing 90° deflector for fast ion beams consisting of concentric cylindrical plates of differing heights. In contrast to standard cylindrical deflectors, our design allows for focusing of an incoming parallel beam not only in the plane of deflection but also in the orthogonal direction. The optical properties of our design resemble those of a spherical capacitor deflector while it is much easier and more cost effective to manufacture.
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41.85.Lc Particle beam focusing and bending magnets, wiggler magnets, and quadrupoles
41.85.Ct Particle beam shaping, beam splitting
41.85.Ew Particle beam profile, beam intensity
41.85.Si Particle beam collimators, monochromators
back to top Nuclear Physics, Fusion and Plasmas

First results obtained from the soft x-ray pulse height analyzer on experimental advanced superconducting tokamak

P. Xu, S. Y. Lin, L. Q. Hu, Y. M. Duan, J. Z. Zhang, K. Y. Chen, and G. Q. Zhong

Rev. Sci. Instrum. 81, 063501 (2010); http://dx.doi.org/10.1063/1.3443572 (5 pages) | Cited 2 times

Online Publication Date: 18 June 2010

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An assembly of soft x-ray pulse height analyzer system, based on silicon drift detector (SDD), has been successfully established on the experimental advanced superconducting tokamak (EAST) to measure the spectrum of soft x-ray emission (E = 1–20 keV). The system, including one 15-channel SDD linear array, is installed on EAST horizontal port C. The time-resolved radial profiles of electron temperature and Kα intensities of metallic impurities have been obtained with a spatial resolution of around 7 cm during a single discharge. It was found that the electron temperatures derived from the system are in good agreement with the values from Thomson scattering measurements. The system can also be applied to the measurement of the long pulse discharge for EAST. The diagnostic system is introduced and some typical experimental results obtained from the system are also presented.
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52.70.La X-ray and γ-ray measurements
52.80.-s Electric discharges
52.25.-b Plasma properties
52.55.Fa Tokamaks, spherical tokamaks

Diagnostics for first plasma and development plan on KSTAR

J. H. Lee, H. K. Na, S. G. Lee, J. G. Bak, D. C. Seo, S. H. Seo, S. T. Oh, W. H. Ko, J. Chung, Y. U. Nam, K. D. Lee, E. M. Ka, Y. K. Oh, M. Kwon, and S. H. Jeong

Rev. Sci. Instrum. 81, 063502 (2010); http://dx.doi.org/10.1063/1.3429942 (6 pages) | Cited 2 times

Online Publication Date: 21 June 2010

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The first plasma with target values of the plasma current and the pulse duration was finally achieved on June 13, 2008 in the Korea Superconducting Tokamak Advanced Research (KSTAR). The diagnostic systems played an important role in achieving successful first plasma operation for the KSTAR tokamak. The employed plasma diagnostic systems for the KSTAR first plasma including the magnetic diagnostics, millimeter-wave interferometer, inspection illuminator, Hα, visible spectrometer, filterscope, and electron cyclotron emission (ECE) radiometer have provided the main plasma parameters, which are essential for the plasma generation, control, and physics understanding. Improvements to the first diagnostic systems and additional diagnostics including an x-ray imaging crystal spectrometer, reflectometer, ECE radiometer, resistive bolometer, and soft x-ray array are scheduled to be added for the next KSTAR experimental campaign in 2009.
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52.55.Fa Tokamaks, spherical tokamaks
52.70.-m Plasma diagnostic techniques and instrumentation

Electrostatic diagnostics of nanosecond pulsed electron beams in a Malmberg–Penning trap

B. Paroli, G. Bettega, G. Maero, M. Romé, M. Norgia, A. Pesatori, and C. Svelto

Rev. Sci. Instrum. 81, 063503 (2010); http://dx.doi.org/10.1063/1.3455200 (5 pages) | Cited 2 times

Online Publication Date: 24 June 2010

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A fast electrostatic diagnostic and analysis scheme on nanosecond pulsed beams in the keV energy range has been developed in the Malmberg–Penning trap ELTRAP. Low-noise electronics has been used for the detection of small induced current signals on the trap electrodes. A discrete wavelet-based procedure has been implemented for data postprocessing. The development of an effective electrostatic diagnostics together with proper data analysis techniques is of general interest in view of deducing the beam properties through comparison of the postprocessed data with the theoretically computed signal shape, which contains beam radius, length, and average density as fit parameters.
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52.70.Ds Electric and magnetic measurements
07.77.Ka Charged-particle beam sources and detectors
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.55.Lf Field-reversed configurations, rotamaks, astrons, ion rings, magnetized target fusion, and cusps

Evaluation of ion collection area in Faraday probes

Daniel L. Brown and Alec D. Gallimore

Rev. Sci. Instrum. 81, 063504 (2010); http://dx.doi.org/10.1063/1.3449541 (11 pages) | Cited 4 times

Online Publication Date: 25 June 2010

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A Faraday probe with three concentric rings was designed and fabricated to assess the effect of gap width and collector diameter in a systematic study of the diagnostic ion collection area. The nested Faraday probe consisted of two concentric collector rings and an outer guard ring, which enabled simultaneous current density measurements on the inner and outer collectors. Two versions of the outer collector were fabricated to create gaps of 0.5 and 1.5 mm between the rings. Distribution of current density in the plume of a low-power Hall thruster ion source was measured in azimuthal sweeps at constant radius from 8 to 20 thruster diameters downstream of the exit plane with variation in facility background pressure. A new analytical technique is proposed to account for ions collected in the gap between the Faraday probe collector and guard ring. This method is shown to exhibit excellent agreement between all nested Faraday probe configurations, and to reduce the magnitude of integrated ion beam current to levels consistent with Hall thruster performance analyses. The technique is further studied by varying the guard ring bias potential with a fixed collector bias potential, thereby controlling ion collection in the gap. Results are in agreement with predictions based on the proposed analytical technique. The method is applied to a past study comparing the measured ion current density profiles of two Faraday probe designs. These findings provide new insight into the nature of ion collection in Faraday probe diagnostics, and lead to improved accuracy with a significant reduction in measurement uncertainty.
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52.70.Ds Electric and magnetic measurements

Spectroscopic interpretation and velocimetry analysis of fluctuations in a cylindrical plasma recorded by a fast camera

S. Oldenbürger, C. Brandt, F. Brochard, N. Lemoine, and G. Bonhomme

Rev. Sci. Instrum. 81, 063505 (2010); http://dx.doi.org/10.1063/1.3442029 (7 pages) | Cited 3 times

Online Publication Date: 30 June 2010

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multimedia

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Fast visible imaging is used on a cylindrical magnetized argon plasma produced by thermionic discharge in the Mirabelle device. To link the information collected with the camera to a physical quantity, fast camera movies of plasma structures are compared to Langmuir probe measurements. High correlation is found between light fluctuations and plasma density fluctuations. Contributions from neutral argon and ionized argon to the overall light intensity are separated by using interference filters and a light intensifier. Light emitting transitions are shown to involve a metastable neutral argon state that can be excited by thermal plasma electrons, thus explaining the good correlation between light and density fluctuations. The propagation velocity of plasma structures is calculated by adapting velocimetry methods to the fast camera movies. The resulting estimates of instantaneous propagation velocity are in agreement with former experiments. The computation of mean velocities is discussed.
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52.25.Gj Fluctuation and chaos phenomena
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.70.Ds Electric and magnetic measurements
52.25.Xz Magnetized plasmas
52.25.Ya Neutrals in plasmas
back to top Microscopy and Imaging

Imaging dislocations in gallium nitride across broad areas using atomic force microscopy

S. E. Bennett, D. Holec, M. J. Kappers, C. J. Humphreys, and R. A. Oliver

Rev. Sci. Instrum. 81, 063701 (2010); http://dx.doi.org/10.1063/1.3430539 (7 pages) | Cited 1 time

Online Publication Date: 4 June 2010

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We have employed an atomic force microscope with a high sampling rate to image GaN samples grown using an epitaxial layer overgrowth technique and treated with silane and ammonia to enlarge the surface pits associated with threading dislocations (TDs). This allows TDs to be identified in high pixel density images tens of microns in size providing detailed information about the spatial distribution of the TDs. An automated software tool has been developed, which identifies the coordinates of the TDs in the image. Additionally, we have imaged the same sample using Kelvin probe force microscopy, again at high pixel density, providing data about the local changes in surface potential associated with hundreds of dislocations.
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68.37.Ps Atomic force microscopy (AFM)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

A compact, sample-in-atmospheric-pressure soft x-ray microscope developed at Pohang Light Source

Jun Lim, Hyun-Joon Shin, Keun Hwa Chae, Chan-Cuk Hwang, Han-Na Hwang, and Chung Ki Hong

Rev. Sci. Instrum. 81, 063702 (2010); http://dx.doi.org/10.1063/1.3432000 (3 pages) | Cited 1 time

Online Publication Date: 9 June 2010

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A full-field transmission soft x-ray microscope (TXM) was developed at the Pohang Light Source. With a 2 mm diameter condenser zone plate and a 40 nm outermost-zone-width objective zone plate, the TXM’s achieved spatial resolution is better than 50 nm at the photon energy of 500 eV (wavelength: 2.49 nm). The TXM is portable and mounted in tandem with a 7B1 spectroscopy end station. The sample position is outside the vacuum, allowing for quick sample changes and enhanced in situ experimental capability. In addition, the TXM is pinhole-free and easy to align, having commercial mounts located outside the vacuum components.
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07.85.-m X- and γ-ray instruments
29.30.Kv X- and γ-ray spectroscopy
07.85.Tt X-ray microscopes

Correction of the viscous drag induced errors in macromolecular manipulation experiments using atomic force microscope

Runcong Liu, Marisa Roman, and Guoliang Yang

Rev. Sci. Instrum. 81, 063703 (2010); http://dx.doi.org/10.1063/1.3436646 (5 pages) | Cited 1 time

Online Publication Date: 9 June 2010

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We describe a method to correct the errors induced by viscous drag on the cantilever in macromolecular manipulation experiments using the atomic force microscope. The cantilever experiences a viscous drag force in these experiments because of its motion relative to the surrounding liquid. This viscous force superimposes onto the force generated by the macromolecule under study, causing ambiguity in the experimental data. To remove this artifact, we analyzed the motions of the cantilever and the liquid in macromolecular manipulation experiments, and developed a novel model to treat the viscous drag on the cantilever as the superposition of the viscous force on a static cantilever in a moving liquid and that on a bending cantilever in a static liquid. The viscous force was measured under both conditions and the results were used to correct the viscous drag induced errors from the experimental data. The method will be useful for many other cantilever based techniques, especially when high viscosity and high cantilever speed are involved.
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66.20.-d Viscosity of liquids; diffusive momentum transport
36.20.-r Macromolecules and polymer molecules

Error analysis of the residence time of bistable Poisson states obtained by periodic measurements

Jinwoo Lee and In-Whan Lyo

Rev. Sci. Instrum. 81, 063704 (2010); http://dx.doi.org/10.1063/1.3429946 (5 pages)

Online Publication Date: 9 June 2010

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We performed error analysis on the periodic measurement schemes to obtain the residence time of bistable Poisson states. Experimental data were obtained by periodical level-sensitive samplings of oxygen-induced states on Si(111)-7×7 that stochastically switches between two metastable states. Simulated data sequences were created by the Monte Carlo numerical method. The residence times were extracted from the experimental and simulation data sequences by averaging and exponential-fitting methods. The averaging method yields the residence time via the summation of the detected temporal width of each state weighed by the normalized frequency of the state and the exponential fitting via fitting a single exponential function to the frequency histogram of the data. It is found that the averaging method produces consistently more accurate results with no arbitrariness, when compared to the exponential fitting method. For further understanding, data modeling using the first-order approximation was performed; the enhanced accuracy in the averaging method is due to the mutual cancellation of errors associated with detection of zero-width states and long-tail states. We investigated a multi-interval detection scheme as well. Similar analysis shows that the dual-interval scheme produces larger error compared to the single interval one, and has narrower optimum region.
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02.50.Fz Stochastic analysis
02.70.Uu Applications of Monte Carlo methods
02.50.Ey Stochastic processes
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
82.37.Gk STM and AFM manipulations of a single molecule
07.79.Cz Scanning tunneling microscopes
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