bluuebunny/crossref_metadata_2025 huggingface dataset:
High-resolution edge Thomson scattering measurements on the Alcator C-Mod tokamak
Authors: Hughes J. W. , Mossessian D. A. , Hubbard A. E. , Marmar E. S. , Johnson D. , Simon D.
Published: February 2004

A high-resolution Thomson scattering diagnostic is in operation on the Alcator C-Mod tokamak, measuring radial profiles of electron temperature and density at the plasma edge. Photons are scattered from a Nd–yttrium–aluminum–garnet laser beam pulsed at 30 Hz (1.3 J, 8 ns pulse), and are measured by a filter polychromator with four spectral channels. The polychromator measures Te in the range of 15–800 eV and ne of 0.3–3×1020 m−3. Twenty scattering volumes are located about the last closed flux surface, spaced for a nominal resolution of 1.3 mm in midplane radial coordinates. High resolution is essential for measuring edge Te and ne profiles on C-Mod, since these quantities exhibit gradient scale lengths as small as 2 mm in H mode. The steep profiles at the H mode edge are fit to a parameterized pedestal function for ease of analysis. Measured profiles are compared with edge profiles from electron cyclotron emission and visible continuum diagnostics.
Design and performance of the Thomson scattering diagnostic on LHD
Authors: Narihara K. , Yamada I. , Hayashi H. , Yamauchi K.
Published: February 2004

This article describes the design and performance of a multi-point (200) high repetition rate (4×50 Hz) Thomson scattering diagnostic installed on the Large Helical Device. A unique feature of this system is its oblique back scattering configuration, which enables us to observe the entire plasma region along a major radius on the midplane under a severely restricted port constraint. High throughput collection optics using a mosaic mirror of 1.5 m×1.8 m area yield high quality data even with 0.5 J pulse energy delivered from 50 Hz repetition rate Nd: yttrium–aluminum–garnet lasers. High repetition and high spatial resolution (2–4 cm) of the system enable us to study island evolution in the plasma.
Plasma turbulence imaging using high-power laser Thomson scattering
Authors: Zweben S. J. , Caird J. , Davis W. , Johnson D. W. , Le Blanc B. P.
Published: February 2004

The two-dimensional (2D) structure of plasma density turbulence in a magnetically confined plasma can potentially be measured using a Thomson scattering system made from components of the Nova laser of Lawrence Livermore National Laboratory. For a plasma such as the National Spherical Torus Experiment at the Princeton Plasma Physics Laboratory, the laser would form an ≈10-cm-wide plane sheet beam passing vertically through the chamber across the magnetic field. The scattered light would be imaged by a charge coupled device camera viewing along the direction of the magnetic field. The laser energy required to make 2D images of density turbulence is in the range 1–3 kJ, which can potentially be obtained from a set of frequency-doubled Nd:glass amplifiers with diameters in the range of 208–315 mm. A laser pulse width of ⩽100 ns would be short enough to capture the highest frequency components of the expected density fluctuations.
A possibility of plasma current density measurements using suprathermal electron Bremsstrahlung emission
Authors: Kantor M. Yu.
Published: February 2004

Soft x-ray Bremsstrahlung emission of suprathermal electrons is commonly used for electron temperature measurements in plasma experiments. A difference between suprathermal and bulk electron distributions results in a discrepancy between the measured and real electron temperatures. This is considered as a diagnostic demerit. On the other hand, this discrepancy carries specific information about factors disturbing the electron distribution. One of them is an electric field driving the electrons along a magnetic field. A way of measuring the longitudinal electric field using suprathermal electron Bremsstrahlung emission has been proposed and analyzed in the article. The plasma current density is found from Ohm’s law.
Hot electron diagnostic in a solid laser target by K-shell lines measurement from ultraintense laser–plasma interactions (3×1020 W/cm2,⩽400 J)
Authors: Yasuike K. , Key M. H. , Hatchett S. P. , Snavely R. A. , Wharton K. B.
Published: February 2004

Characterization of hot electron production from an ultraintense laser–solid target plasma interaction by using a buried molybdenum K-shell fluor layer technique has been reported. Laser energy was typically 400 J and its intensity was from 2×1018 up to 3×1020 W cm−2 at 20 TW to 1 PW laser power by changing pulse duration from 20 ps down to 0.5 ps. X-ray background noise level was significantly greater, i.e., gamma flash, in the shorter pulse experiments. Data analysis procedures for the experiments were developed. The conversion efficiency from the laser energy into the energy, carried by hot electrons, has been estimated to be ∼50% at 3×1020 W cm−2 laser intensity, higher than ∼18% at 1019 W cm−2 and ∼12% at 2×1018 W cm−2 intensity.
Infrared imaging video bolometer for the large helical device
Authors: Peterson B. J. , Osakabe M. , Shoji M. , Ashikawa N.
Published: February 2004

A new type of infrared imaging bolometer, known as the infrared imaging video bolometer (IRVB) has been under development at the National Institute of Fusion Science (NIFS) over the last year. A prototype of this diagnostic has been designed and constructed at NIFS and installed on the large helical device (LHD) and successfully operated. This diagnostic utilizes a 66×90×0.001 mm gold foil mounted in a frame to detect the radiation and neutrals from the plasma which are incident on the foil through a 1 cm diameter pinhole. The resulting temperature distribution on the foil is measured using an AGEMA THV 900 LW infrared camera having 136×272 pixels with a frame rate of 15 Hz and a nominal sensitivity of 80 mK. An image of the foil and surrounding frame consisting of 120×160 pixels is resampled down to 12×16 pixels using a linear interpolation scheme. Using a numerical technique the spatial and temporal derivatives of the temperature distribution on the foil are calculated and the incident power density on the foil is determined using a calibration of the foil obtained by means of a HeNe laser. The resulting 10×14 pixel view of the plasma radiation at a 15 Hz frame rate has a noise equivalent power density of 0.5 mW/cm2. The IRVB was mounted on a LHD tangential port for the 1999 campaign. An image of the plasma radiation during a discharge using the inboard vacuum vessel wall as a limiter shows radiation localized near the limiting surface.
Design considerations for fiber-coupled streaked optical spectroscopy
Authors: Montgomery D. S. , Johnson R. P.
Published: February 2004

Optical fibers are used to couple light into fast time-resolved spectrometers for many applications. Here several issues arise that can seriously limit the temporal resolution, such as the wavelength dependent group delay, and intermodal dispersion. These issues are investigated in 10 m length fibers using either a 400 μm core step index or graded index fiber. A unique broadband (400–700 nm) short pulse (∼1 ps) light source is used to measure the fiber group delay with a time-resolved spectrometer. Intermodal dispersion is also studied in these fibers using a narrow-band 1 ps pulse that is injected into the fiber with various coupling schemes, and a novel technique is employed to record the time-dependent angular mode structure. Finally, a conceptual design using ∼100 μm core graded-index fibers is proposed for multichannel streaked optical spectroscopy on the National Ignition Facility. Designs appropriate for a low spectral resolution instrument (stimulated Raman backscattering) and a high spectral resolution instrument (stimulated Brillouin backscattering) are presented. The fiber dispersion issues are discussed in the design of these diagnostics.
Spectral catalog of Kr optical lines for the development of diagnostics for fusion plasmas
Authors: Chen H. , Beiersdorfer P. , Harris C. L. , Utter S. B. , Wong K. L.
Published: February 2004

We made an inventory of krypton spectra over the wavelength range 3700–6000 Å for the development of fusion plasma diagnostics. The measurements were performed using a prism spectrometer on the Lawrence Livermore National Laboratory low energy electron beam ion trap (EBIT II). With the electron energy from 150 to 17 000 eV, we recorded low ionization stages together with a number of magnetic dipole transitions from higher charge states. In total, we observed over 80 lines, of which about 70% of the lines have not been listed in the literature. This measurement established a baseline for future extension using spectrometers with very high resolution. As an example, we present the Kr spectra from 3770 to 3900 Å measured with a transmission grating spectrometer that has a resolving power of about 15 000. Among the 41 lines observed, only six lines have been listed in the databases.
Linewidth-modulated motional Stark effect measurements of internal field structure in low-field configurations
Authors: Reinecke E. A. , Fonck R. J. , Thorson T. A.
Published: February 2004

Motional Stark effect measurements of internal field structure in low-field magnetic confinement configurations are considered for both magnitude and direction of the local magnetic field. The amplitude and phase delay of an oscillating spectral linewidth driven by a rotating polarizer provides a means of determining the magnitude and direction of the total field simultaneously while avoiding difficulties of neutral beam energy drift. Photon-noise limit estimates for a diagnostic beam on the low-field PEGASUS toroidal experiment indicate sensitivities of roughly 20 G and 0.2° for the magnitude and direction angle. These values are sufficient to provide significant constraints on magnetic equilibrium reconstructions.
Implementation of reflectometry as a standard density profile diagnostic on DIII-D
Authors: Zeng L. , Doyle E. J. , Luce T. C. , Peebles W. A.
Published: February 2004

The profile reflectometer system on the DIII-D tokamak has been significantly upgraded in order to improve time coverage, data quality, and profile availability. The performance of the reflectometer system, which utilizes continuous frequency modulated (FMCW) radar techniques, has been improved as follows: First, a new PC-based data acquisition system has been installed, providing higher data sampling rates and larger memory depth. The higher sampling rate enables use of faster frequency sweeps of the FMCW microwave source, improving time resolution, and increasing profile accuracy. The larger memory depth enables longer data records, so that profiles can now be obtained throughout 5 s discharges at 100 Hz profile measurement rates, while continuous sampling at 10 MHz is available for 1 s for high time resolution physics studies. Second, an initial automated between-shots profile analysis capability is now available. Third, availability of the profiles to end users has been significantly improved.
Alfvén eigenmodes excitation by external loop antennas in the Compact Helical System heliotron/torsatron
Authors: Matsunaga G. , Takechi M. , Toi K. ,   CHS Group
Published: February 2004

Four loop antennas are installed in the Compact Helical System (CHS) heliotron/torsatron in order to externally excite Alfvén eigenmodes (AEs). These antennas are used in two different operation methods: one is as a set of four external loop antennas, and the other two pairs of electrodes inserted near the plasma boundary. For these two operations, magnetic perturbations in the frequency range of 10–250 kHz are generated in the plasma. The plasma response for the applied magnetic perturbations is measured by magnetic probe arrays. Information of AEs can be derived from the analysis of the response. In the excitation experiment of AEs using the “electrode method,” magnetic fluctuations are resonantly enhanced in a certain electron density during the afterglow of a neutral beam heated plasma, where no energetic ions exist. The fluctuations have the toroidal mode number n=1 and the frequency lies just above the lower bound of the toroidicity-induced Alfvén eigenmode (TAE) gap in the plasma core region. The excited fluctuations are thought to be related to TAEs.
Dual mode (ordinary–extraordinary) correlation reflectometry for magnetic field and turbulence measurements (invited)
Authors: Gilmore M. , Peebles W. A. , Nguyen X. V.
Published: February 2004

Dual mode [ordinary–extraordinary (O–X)] correlation reflectometry has the potential advantage of being able to determine magnetic field strength, |B_|, simultaneously with correlation properties of turbulence (e.g., radial correlation length, coherency, power spectra, etc.). Controlled dual mode correlation reflectometry experiments, conducted in the Large Plasma Device at UCLA, are presented. The purpose of these experiments was (1) to demonstrate the proof of principle of |B_| measurement, and (2) to make detailed comparisons between dual mode and single mode (O–O,X–X) reflectometer, and Langmuir probe radial correlation measurements. It is shown that, in these experiments, |B_| can be determined from O–X correlation measurements, interpreted via a one-dimensional numerical model. In addition, good agreement is found between correlation lengths (within 10%–15%), coherency, and power spectra measured by O–X, O–O, and X–X reflectometry, and a Langmuir probe array.
Investigation of x-ray-energy responses of semiconductor detectors under deuterium–tritium fusion-produced neutron irradiation
Authors: Kohagura J. , Cho T. , Hirata M. , Numakura T. , Minami R. , Yoshida M. , Nagashima S. , Watanabe H. , Yatsu K. , Miyoshi S. , Kondoh T. , Nishitani T. , Takeuchi H.
Published: February 2004

For the purpose of investigating fusion-produced neutron effects on semiconductor x-ray detectors, detection characteristics of x-ray tomography detectors used before and after deuterium–tritium (DT) and/or DD fusion-plasma experiments in the Joint European Torus (JET) tokamak are studied using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory. Degradations in the responses after neutron exposure into the detectors are found to have functional dependence on x-ray energy. Changes in the depletion thicknesses of the detectors are investigated by means of impedance analyses. The Fusion Neutronics Source (FNS) facility of the Japan Atomic Energy Research Institute is also employed for well calibrated DT fusion-produced neutron irradiation onto these semiconductor x-ray detectors. Recovery of the response degradation is found due to a method for supplying the operational bias to the degraded detector. Our theory is applied to interpret these detector characteristics under the irradiation of DT fusion-produced neutrons in the JET tokamak and the FNS facility.
NIF total neutron yield diagnostic
Authors: Cooper Gary. W. , Ruiz Carlos L.
Published: February 2004

We have designed a total neutron yield diagnostic for the National Ignition Facility (NIF) which is based on the activation of In and Cu samples. The particular approach that we have chosen is one in which we calibrate the entire counting system and which we call the “F factor” method. In this method, In and/or Cu samples are exposed to known sources of DD and DT neutrons. The activated samples are then counted with an appropriate system: a high purity Ge detector for In and a NaI coincidence system for Cu. We can then calculate a calibration factor, which relates measured activity to total neutron yield. The advantage of this approach is that specific knowledge of such quantities as cross sections and detector efficiencies is not needed. Unless the actual scattering environment of the NIF can be mocked up in the calibration experiment, the F factor will have to be modified using the results of a numerical simulation of the NIF scattering environment. In this article, the calibration factor methodology will be discussed and experimental results for the calibration factors will be presented. Total NIF neutron yields of 109–1019 can be measured with this method assuming a 50 cm stand-off distance can be employed for the lower yields.
Secondary-neutron-yield measurements by current-mode detectors
Authors: Glebov V. Yu. , Meyerhofer D. D. , Stoeckl C. , Zuegel J. D.
Published: February 2004

Secondary deuterium–tritium (DT) neutrons from pure-deuterium inertial confinement fusion targets can be used to diagnose the fuel areal density. Single-hit detectors like LaNSA at Lawrence Livermore National Laboratory or MEDUSA at the Laboratory for Laser Energetics (LLE) saturate for fairly low secondary DT- and primary DD-neutron yields. These detectors are not suitable for the high-yield, direct-drive implosion experiments currently carried out on the 30 kJ, 60 beam OMEGA laser system or for future cryogenic-capsule experiments on OMEGA. The status of several current-mode detectors (e.g., a single scintillator and a photomultiplier tube) now being developed at LLE for secondary-neutron-yield measurements is described.
Neutron time-of-flight and emission time diagnostics for the National Ignition Facility
Authors: Murphy T. J. , Jimerson J. L. , Berggren R. R. , Faulkner J. R. , Oertel J. A. , Walsh P. J.
Published: February 2004

Current plans call for a system of current mode neutron detectors for the National Ignition Facility for extending the range of neutron yields below that of the neutron activation system, for ion-temperature measurements over a wide yield range, and for determining the average neutron emission time. The system will need to operate over a yield range of 106 for the lowest-yield experiments to 1019 for high-yield ignited targets. The requirements will be satisfied using several detectors located at different distances from the target. This article presents a conceptual design for the NIF nToF system.
Gamma-ray-based fusion burn measurements
Authors: Berggren R. R. , Caldwell S. E. , Faulkner J. R. , Lerche R. A. , Mack J. M. , Moy K. J. , Oertel J. A. , Young C. S.
Published: February 2004

A gas Cerenkov detector with a 12-MeV threshold for gamma-ray detection has been built for use on the OMEGA laser system to record high-energy gamma rays emitted during DT gas burn. Recording the 16.7-MeV gamma ray while discriminating against the lower energy 14-MeV neutron-induced gammas is an important objective using this detector system. Detector design, sensitivity, and background studies were possible using the Integrated Tiger Series Monte Carlo code modified to include Cerenkov production and full time-history of all particles. The results of this code were iterated with the ASAP optics code to optimize the light collection system, while providing the radiation shielding and stray light baffles to minimize backgrounds. As an initial test of the instrument, 8–20 MeV electrons from the Idaho State University linear accelerator were used in lieu of gamma rays. The primary results of these tests are that electron-produced Cerenkov has been observed and the Cerenkov threshold curve established for this instrument.
Fusion gamma-ray measurements for D–3He experiments at JT-60U
Authors: Nishitani T. , Tobita K. , Kusama Y. , Shibata Y.
Published: February 2004

Fusion gamma rays were measured in D–3He experiments using negative ion-based neutral beam injection (N-NBI) in reverse shear plasmas of the JT-60 tokamak. He3 gas was puffed at plasma initiation and just before N-NB injection. The D–3He reaction produces 3.6 MeV alphas and 14.7 MeV protons, but there is also a small branch which provides Li5 and 16.7 MeV gamma rays. The total D–3He reaction rate can be evaluated from measurement of gamma rays of the He3 (d,γ) 5Li reactions using a 3 in. diam by 3 in. long Bi4Ge3O12 scintillator. The gamma-ray detector was located 17 m below the plasma center and measured the gamma-rays in a vertical line of sight. The detector was mounted inside a heavy collimator with polyethylene and lead shielding. The floor penetration, a 4×8 cm2 hole, was used as a precollimator. Energy calibration of the detector was done with photopeaks for neutron capture gamma rays from the structural materials in D–D discharges. The detection efficiency was calculated with Monte Carlo code MCNP-4B for 16.7 MeV gammas. The pulse height analysis of the gamma rays resulted in the D–3He fusion power of 110±30 kW in this experiment.
Diagnostics for advanced tokamak research (invited)
Authors: Burrell K. H.
Published: February 2004

Advanced tokamak research seeks to find the ultimate potential of the tokamak as a magnetic confinement system. Achieving this potential involves optimizing the plasma cross-sectional shape, current density, and pressure profiles for stability to magnetohydrodynamic (MHD) modes while simultaneously controlling the current density, pressure, and radial electric field profiles to minimize the cross field transport of plasma energy. In its ultimate, steady-state incarnation, the advanced tokamak also requires pressure profiles that have been adjusted to achieve the maximum possible bootstrap current, subject to the constraints of MHD stability. This simultaneous, nonlinear optimization of shape, current, pressure, and electric field profiles to meet multiple goals is a grand challenge to plasma physics. To keep the plasma at peak performance, active feedback control will almost certainly be required. Diagnostic measurements play a crucial role in advanced tokamak research both for developing the scientific understanding underlying the optimization and for serving as sensors for real time feedback control. One outstanding example of this is the way motional Stark effect (MSE) measurements of the internal magnetic field revolutionized work on current profile shaping. Improved diagnostic measurements are essential in testing theories which must be validated in order to apply advanced tokamak results to next step devices.
Diagnostics for liquid lithium experiments in CDX-U
Authors: Kaita R. , Efthimion P. , Hoffman D. , Jones B. , Kugel H. , Majeski R. , Munsat T. , Raftopoulos S. , Taylor G. , Timberlake J. , Soukhanovskii V. , Stutman D. , Iovea M. , Finkenthal M. , Doerner R. , Luckhardt S. , Maingi R. , Causey R.
Published: February 2004

A flowing liquid lithium first wall or divertor target could virtually eliminate the concerns with power density and erosion, tritium retention, and cooling associated with solid walls in fusion reactors. To investigate the interaction of a spherical torus plasma with liquid lithium limiters, large area divertor targets, and walls, discharges will be established in the Current Drive Experiment-Upgrade (CDX-U) where the plasma–wall interactions are dominated by liquid lithium surfaces. Among the unique CDX-U lithium diagnostics is a multilayer mirror (MLM) array, which will monitor the 13.5 nm LiIII line for core lithium concentrations. Additional spectroscopic diagnostics include a grazing incidence extreme ultraviolet (XUV) spectrometer (STRS) and a filterscope system to monitor Dα and various impurity lines local to the lithium limiter. Profile data will be obtained with a multichannel tangential bolometer and a multipoint Thomson scattering system configured to give enhanced edge resolution. Coupons on the inner wall of the CDX-U vacuum vessel will be used for surface analysis. A 10 000 frame per second fast visible camera and an IR camera will also be available.
Digital-image capture system for the IR camera used in Alcator C-Mod
Authors: Maqueda R. J. , Wurden G. A. , Terry J. L. , Gaffke J.
Published: February 2004

An infrared imaging system, based on an Amber Radiance 1 infrared camera, is used at Alcator C-Mod to measure the surface temperatures in the lower divertor region. Due to the supra-linear dependence of the thermal radiation with temperature it is important to make use of the 12-bit digitization of the focal plane array of the Amber camera and not be limited by the 8 bits inherent to the video signal. It is also necessary for the image capture device (i.e., fast computer) to be removed from the high magnetic field environment surrounding the experiment. Finally, the coupling between the digital camera output and the capture device should be nonconductive for isolation purposes (i.e., optical coupling). A digital video remote camera interface (RCI) coupled to a PCI bus fiber optic interface board is used to accomplish this task. Using this PCI-RCI system, the 60 Hz images from the Amber Radiance 1 camera, each composed of 256×256 pixels and 12 bits/pixel, are captured by a Windows NT computer. An electrical trigger signal is given directly to the RCI module to synchronize the image stream with the experiment. The RCI can be programmed from the host computer to work with a variety of digital cameras, including the Amber Radiance 1 camera.
Applications of visible CCD cameras on the Alcator C-Mod tokamak
Authors: Boswell C. J. , Terry J. L. , Lipschultz B. , Stillerman J.
Published: February 2004

Five 7 mm diameter remote-head visible charge-coupled device (CCD) cameras are being used on Alcator C-Mod for several different diagnostic purposes. All of the cameras’ detectors and optics are placed inside a magnetic field of up to 4 T. Images of the cameras are recorded simultaneously using two three-channel color framegrabber cards. Two CCD cameras are used typically to generate two-dimensional emissivity profiles of deuterium line radiation from the divertor. Interference filters are used to select the spectral line to be measured. The local emissivity is obtained by inverting the measured brightnesses assuming toroidal symmetry of the emission. Another use of the cameras is the identification and localization of impurity sources generated by the ion cyclotron radio frequency (ICRF) antennas, which supply the auxiliary heating on Alcator C-Mod. The impurities generated by the antennas are identified by correlating in time the injections seen at the cameras with measurements made with core diagnostics. Fibers whose views aligned with the camera views and whose outputs are coupled to a visible spectrometer are also used to identify the species of the impurities injected.
Rutherford scattering diagnostic for the Madison symmetric torus reversed-field pinch
Authors: Reardon J. C. , Fiksel G. , Forest C. B. , Abdrashitov A. F. , Davydenko V. I. , Ivanov A. A. , Korepanov S. A. , Murachtin S. V. , Shulzhenko G. I.
Published: February 2004

The technique of Rutherford scattering (RS) has been used (for the first time on a reversed-field pinch) to measure the bulk majority ion temperature on the Madison symmetric torus (MST). RS has been in routine operation on MST since December 1999. The neutral beam source and electrostatic energy analyzers which comprise the RS diagnostic were built by the Budker Institute of Nuclear Physics (Novosibrsk, Russia). The source is described in another paper (Abdrashitov et al., these proceedings) and the analyzers are described. A data analysis routine has been developed which yields accurate fits to the data, and estimates of errors in the fit parameters. Typical results are shown.
Nonperturbing field profile measurements of a sustained spheromak
Authors: Holcomb C. T. , Jarboe T. R. , Mattick A. T. , Hill D. N. , McLean H. , Wood R. D. , Cellamare V. , Bulmer R. H. , Hooper E. B.
Published: February 2004

In this article we discuss the measurement of the field profile in the sustained spheromak physics experiment (SSPX). We have built a transient internal probe (TIP) diagnostic to measure the internal field profile in a SSPX plasma sustained by dc coaxial helicity injection. TIP is a diagnostic that makes a spatially resolved (i.e., not chord averaged) measurement of the local magnetic field using Faraday rotation. A 1-cm by 4-mm-diameter verdet probe is fired through the plasma at about 2 km/s by a two-stage light gas gun. The probe is illuminated by an argon ion laser throughout the traverse of the plasma—the retro-reflected light is then analyzed with an ellipsometer to determine the field at each location. The speed, small size of the probe, and the probe cladding make this measurement possible even in hot plasmas (100 s of eV). The measurement is accurate enough (1 MHz, ±7 G, 1-cm spatial resolution) to map out magneto hydrodynamic (MHD) mode amplitudes from the edge to the magnetic axis.
Faraday rotation densitometry for Large Helical Device
Authors: Akiyama T. , Sato E. , Nozawa T. , Tsuji-Iio S. , Shimada R. , Murayama H. , Nakayama K. , Okajima S. , Tanaka K. , Watanabe K. , Tokuzawa T. , Kawahata K.
Published: February 2004

A CO2 laser polarimeter to measure the Faraday rotation in the large helical device (LHD) was developed and installed on a LHD. A CO2 laser whose wavelength is 10.6 μm was selected to avoid effects of the refraction. The Faraday rotation angle along toroidally tangential chords is of the order of one degree. The polarimeter utilizes the frequency-shift heterodyne technique with the use of acousto-optic modulators for high resolution. This polarimeter can be operated with a maximum of three channels. The accuracy of the Faraday rotation angle with a time resolution of 16 ms is about 0.01 deg by digital complex demodulation. We succeeded in performing preliminary measurements of the Faraday rotation angle from LHD plasmas and the values were 10%–30% larger than those estimated with the electron density profiles from interferometry.
Infrared laser polarimetry for electron density measurement in tokamak plasmas
Authors: Kawano Yasunori , Chiba Shin-ichi , Inoue Akira
Published: February 2004

The infrared CO2 laser polarimetry for electron density measurement based on the tangential Faraday rotation has been progressed in JT-60U tokamak. The reliability of polarimetry in a pellet-injected plasma was confirmed. The Faraday rotation was measured with good angle resolution of ∼0.01° under the sufficiently fast temporal resolution of 4 ms. Electron density evaluated from the tangential Faraday rotation measured by polarimetry shows good agreement with that measured by interferometry for an identical probing laser beam. The long-time continuous measurement was successful with good stability and accuracy. The basic feasibility of the infrared CO2 laser polarimetry for electron density measurement in long-time and steady-state operation in large fusion devices was demonstrated.
Multichannel far-infrared polarimeter-interferometer system on the MST reversed field pinch
Authors: Brower D. L. , Jiang Y. , Ding W. X. , Terry S. D. , Lanier N. E. , Anderson J. K. , Forest C. B. , Holly D.
Published: February 2004

The multichannel far-infrared (FIR) heterodyne polarimeter-interferometer system on the Madison Symmetric Torus (MST) is now operational. The combined system consists of 11 channels with variable radial and toroidal spacing. Poloidal magnetic field is determined by measuring the Faraday rotation of the FIR laser beam after propagation through the plasma by use of a phase technique. The polarimeter has 3 mrad rms noise level and 1 ms temporal resolution while the interferometer resolution is nedl=1×1012 cm−2 with time response of 1 μs. Absolute calibration of the polarimeter system is achieved by use of a rotating quartz half-wave plate. The first 11-channel polarimeter measurements from MST indicate a Faraday rotation profile in good agreement with expectations from the MSTFIT equilibrium code. Future plans to reduce the polarimeter time response from 1 ms to 10 μs will allow direct measurement of magnetic fluctuations associated with global resistive tearing modes on MST. The effect of these modes on density is already clearly resolved and provides insight into the dynamics of these structures. Improving the time response will also result in lower phase noise for both the polarimeter and interferometer.
Spatially scanned two-color mid-infrared interferometer for FTU
Authors: Canton A. , Innocente P. , Martini S. , Tasinato L. , Tudisco O.
Published: February 2004

The design of a scanning beam two-color mid-infrared (MIR) interferometer is presented. The diagnostic is being developed for the Frascati Tokamak Upgrade (FTU) which calls for a new interferometer to perform detailed study of advanced confinement regimes in D-shaped plasmas. After performing a feasibility study and a prototype test, we designed a scanning interferometer based on a resonant tilting mirror providing 40 chords of ≈1 cm diameter and a full profile every 62 μs. Such a high number of chords is obtained with a very simple optical scheme, resulting in a system which is compact, low cost, and easy to align. An important feature of the interferometer is its higher immunity to fringe jumps compared to conventional far infrared (FIR) systems. Three main factors contribute to that: the high critical density associated to MIR beams, the large bandwidth provided by 40 MHz heterodyne detection, and the fact that each scan provides a “self-consistent” profile.
A simple air wedge shearing interferometer for studying exploding wires
Authors: Pikuz S. A. , Romanova V. M. , Baryshnikov N. V. , Hu Min , Kusse B. R. , Sinars D. B. , Shelkovenko T. A. , Hammer D. A.
Published: February 2004

A new type of shearing interferometer using an air wedge is described. This interferometer is based on a beam splitter constructed using two 90-degree prisms. A small air gap, which varies in spacing from top-to-bottom, separates the second prism from the first and forms the air wedge. The single incident laser beam is focused near the gap, and the two primary reflections from the long sides of each prism form the two coherent virtual sources necessary for interferometry. The shift between the two images of the object at the detector, as well as the orientation and frequency of the fringes, can be independently adjusted by altering the air gap thickness and angle, as well as the position of the laser focus in the gap. This interferometry scheme is inexpensive and easily aligned, and has been successfully and reliably used in exploding wire experiments.
Improvements in the evaluation of Thomson scattering data on ASDEX upgrade
Authors: Kurzan B. , Murmann H. , Salzmann H. ,   ASDEX Upgrade Team
Published: February 2004

The avalanche diodes used as light detectors in the Thomson scattering diagnostic on the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak show a reduced signal-to-noise ratio during plasma discharges with high neutron rates and during plasma edge measurements. This often makes it impossible to determine the electron temperature from the ratios of the scattering signals of the different spectral channels analyzing the Thomson scattering spectrum. With the use of the maximum likelihood method combined with linearized electron temperature characteristics the electron temperature and density are determined directly from the scattering signals without calculating ratios between the signals. With this method the scattering signals for nearly all timepoints can be evaluated and the statistical errors of the electron temperature and density are up to factors of 2 and 1.4, respectively, smaller than achieved with the previous method.
Upgraded calibrations of the Thomson system at DIII-D
Authors: Bray B. , Hsieh C. , Carlstrom T. N. , Makariou C. C.
Published: February 2004

The DIII-D Thomson system measures electron density (ne) and temperature (Te) with eight pulsed ND:YAG lasers along three paths through the plasma vessel. The components of the Thomson system are absolutely calibrated so the measurements can be combined into a single profile from a normalized plasma radius (ρ) of about 0.1 to the edge of the plasma. A monochromator calibration and opto-electronic calibration measure the detectors’ absolute sensitivity to background and pulsed light. A Rayleigh scattering calibration and transmission calibrations measure the transmission of light to the detectors. The calibration systems are being upgraded to reduce the effect of systematic errors on the temperature and density measurements. The observed systematic errors can be estimated by comparing fits to averaged profiles to the average profiles. The contributions of the systematic uncertainties relative to the statistical uncertainties of the measurement are discussed through simulations and experimental data.
Measurements of the electron temperature by the Thomson scattering system on the Hanbit magnetic mirror device
Authors: Lee H. G. , Lee S. G. , Kim B. C. , Hong J. , Kim W. C. , Choh K. K. , Choi J. H. , Yang J. G. , Na H. K. , Doh C. J. , Hwang S. M. , Kwon M. , Won Y. H.
Published: February 2004

A Thomson scattering system on the Hanbit magnetic mirror device has been installed to measure the electron temperature and density of the plasma in the central cell. The configuration is based on a standard 90° scattering scheme. The optical system consists of a Q-switched Nd:YAG laser, input optics, collection optics, spectrograph optics, detectors, and a data acquisition system. Although the laser beam path is about 50 m long and the background emissions are not low, the electron temperature measurements have been made at a single point on a shot-by-shot basis, in which the stray light was considerably suppressed by using a beam dump, a viewing dump, and baffles. The measured electron temperature is about 50–70 eV in experiments for plasma production and heating by ICRF of 200-kW-rf power using a slot antenna. A description of the installed system and the experimental results are presented.
Active control of laser beam direction for LHD YAG Thomson scattering
Authors: Yamada I. , Narihara K. , Yamauchi K. , Hayashi H.
Published: February 2004

We have developed a YAG Thomson scattering (TS) system for the measurements of electron temperature and density profiles on the large helical device (LHD). The LHD-TS has four YAG lasers, and flexible operational modes are possible by using them. For example, (1) high-energy mode: The pulse energy can be increased up to four times by firing the four lasers simultaneously. In this mode, the data quality can be improved for low-density plasmas. (2) High repetition mode: When firing the lasers at intervals of 5 ms, the lasers work as a 200 Hz laser. The laser beams are guided to the LHD by seven steering mirrors. The first mirror is real-time feedback controlled for precise beam transport. The beam pointing stability is improved successfully from 200 μrad to below 4 μrad with the feedback-control system. We describe the details of the laser system for the LHD-TS.
APD detector electronics for the NSTX Thomson scattering system
Authors: Johnson D. W. , LeBlanc B. P. , Long D. L. , Renda G.
Published: February 2004

An electronics system has been installed and tested for the readout of avalanche photodiode (APD) detectors for the National Spherical Torus Experiment (NSTX) Thomson scattering system. Similar to previous designs, it features preamps with a fast and a slow output. The fast output uses pulse shaping to optimize sensitivity for the 8 ns scattered light pulse while rejecting noise in the intrinsic plasma background. A low readout noise of ∼25 photoelectrons is achieved at an APD gain of 75. The design incorporates a number of features to provide flexibility for various modes of calibration.
The new RFX Thomson scattering system
Authors: Pasqualotto R. , Nielsen P. , Giudicotti L.
Published: February 2004

The present Thomson scattering system at the reversed field experiment routinely determines electron temperature and density profiles in various modes of operation but only provides a single profile of each discharge. The new system will use a Nd:YLF laser providing measurements every 20 ms. At the same time we wish to improve the accuracy of the measurement: to this end a new detection system based on filter spectrometers and avalanche photodiodes has been built. The higher throughput of the spectrometers and the higher quantum efficiency of the detectors give an improvement of signal to noise levels of almost two orders. To improve dynamic range the output is ac coupled. Noise figure of detectors has been investigated in detail. The system is novel in the way data are recorded using transient recorders with multiplexing of two channels on each input. The effective accuracy and dynamic range of this system is compared to charge integrating systems. With a look to the future we have also investigated the possible use of multi-element avalanche photodiodes.
Collective Thomson scattering using a pulsed CO2 laser in JT-60U
Authors: Kondoh T. , Lee S. , Hutchinson D. P. , Richards R. K.
Published: February 2004

A collective Thomson scattering (CTS) diagnostic system using a carbon dioxide (CO2) laser has been developed for the purpose of establishing measurement technique of ion temperature and fast alpha particle in a fusion reaction plasma. A pulsed CO2 laser (15 J, 1 μs at 10.6 μm) and a heterodyne receiver with a stray light notch filter has been successfully installed. The noise equivalent power of the heterodyne receiver is below 9×10−19 W/Hz up to a frequency of 8 GHz. A six-channel filter bank analyzes the spectrum of the scattered light in the frequency range from 0.4 to 4.5 GHz to measure ion temperature and to detect fast ions generated by negative-ion source neutral beam injection of the JT-60U (Japan Atomic Energy Research Institute Tokamak 60-Upgrade) plasmas [S. Ishida, JT-60U Team, Nucl. Fusion 39, 1211 (1999)]. Test of the CTS system by injecting the CO2 laser into the vacuum vessel of JT-60U has started. Stray light signal around the JT-60U vacuum vessel was detected and optimization of the optical alignment has proceeded.
Simultaneous observations of temporally and spatially resolved electron temperatures of both circular central-cell and elliptical anchor-region plasmas in GAMMA 10
Authors: Minami R. , Cho T. , Kohagura J. , Hirata M. , Numakura T. , Yoshida M. , Watanabe H. , Yatsu K. , Miyoshi S.
Published: February 2004

The first results of simultaneous observations of temporally and spatially resolved electron temperatures (Te) in both circularly shaped central-cell and elliptically shaped anchor regions are reported in the GAMMA 10 tandem mirror. A data set is provided using a novel matrix-type x-ray semiconductor detector. The detector has seven “matrix columns” for the measurement of plasma x-ray profiles along with six “matrix rows” for simultaneous analyses of six different x-ray-energy ranges by the fabrication of six different thicknesses of SiO2 semiconductor surface layers from 1 to 495 nm as ultrathin and unbreakable “x-ray absorption filters.” Such a matrix idea enables us to analyze x-ray tomography data in the Te region down to a few tens of eV. Simultaneous application of the x-ray detectors in the central-cell and the anchor region gives information on not only detailed electron behavior in each region but also the mutually communicating relation between the two regions: The role of the anchor-region plasmas in magnetohydrodynamic plasma stabilization in the central-cell plasmas is confirmed by the data from these detailed simultaneous electron observations.
Spectral response calibrations of x-ray diode photocathodes in the 50–5900 eV photon energy region
Authors: Bentley C. D. , Simmons A. C.
Published: February 2004

X-ray diode photocathodes are employed in diagnostic instruments on the Helen laser at the Atomic Weapons Establishment (AWE) Aldermaston, UK. The photocathodes are mainly used in the Dante fast diode array and flat response diodes. These diagnostics enable the soft x-ray spectral emissions of laser irradiated targets to be determined. To derive quantitative spectral information, the quantum efficiency of the photocathodes must be known over the range of x-ray energies of interest. The photocathodes were manufactured in 1982, and were initially calibrated at that time. Since then further measurements have been performed in 1988 and 1999. The photocathodes have been exposed to a wide range of conditions during their lives, ranging from use in experiments to storage in a dry nitrogen environment. Reported here are the results of calibrations performed in 1999 at the soft x-ray calibration facility EXCALIBUR at AWE, Aldermaston, and at the National Synchrotron Light Source in Brookhaven NY. An assessment of their current condition and an evaluation of the change in their response over time, and the possible reasons for these changes, are made.
Time-of-flight neutron spectrometer for JT-60U
Authors: Shibata Y. , Iguchi T. , Hoek M. , Nishitani T. , Morioka A.
Published: February 2004

The authors have developed a 2.45 MeV neutron double crystal time-of-flight (DC–TOF) spectrometer for deuterium plasmas in JT-60U. The DC–TOF neutron spectrometer consists of two fast plastic scintillators (BC-408 made by Bicron, 50 cm2 and 1800 cm2, thickness 2 cm), located on constant TOF spheres. The constant TOF spheres have a radius of 1 m which gives a neutron flight length of 1.64 m and a TOF of 92 ns for 2.45 MeV neutrons. The calculated spectrometer efficiency and resolution are 2.8×10−2 cm2 and 100 keV, respectively. The energy resolution corresponds to a time resolution of 2.0 ns. The spectrometer has been installed in the basement of JT-60U, 10 m away from the plasma center with vertical line-of-sight. Neutron energy spectra have been obtained when the neutron yield from the plasma is ∼1015 neutrons/s.
A Lyman-alpha-based (VUV) plasma density fluctuation diagnostic design
Authors: McKee G. R. , Fonck R. J. , Fenzi C. , Leslie B. P.
Published: February 2004

The benchmark design for a high spatial resolution, high sensitivity, long-wavelength density fluctuation diagnostic that observes collisionally induced Lyman-alpha emissions (λ0≈121.6 nm) from neutral beam atoms is presented. The sensitivity to plasma fluctuations at Lyα is estimated to be about 50 times higher than at Dα. This dramatically increases the photon-noise-limited signal-to-noise ratio. Design criteria include a spectral resolution of Δλ≈0.5 nm at Lyα, to isolate the Doppler-shifted beam emission from the bright edge Lα emission; high quantum efficiency detectors; high optical throughput; multichannel capability in the radial and poloidal directions; spatial resolution near 1 cm in the radial and poloidal directions to measure fluctuations in the wave number range 0.1⩽kr, kθ⩽3 cm−1. A system design based on a Czerny–Turner monochromator is considered.
Assessment of density profile automatic evaluation from broadband reflectometry data
Authors: Varela P. , Manso M. , Vergamota S. , Grossmann V. , Santos J. ,   ASDEX Upgrade Team
Published: February 2004

Automatic evaluation of electron density profiles from frequency modulated continuous wave (FM–CW) broadband microwave reflectometry on ASDEX Upgrade has been achieved in all plasma regimes combining a high performance system and advanced analysis tools. The results presented demonstrate the capability of the system to resolve the abrupt gradient changes occurring in the edge region during the L–H transition and in the presence of edge localized modes (ELMs). The temporal evolution of the density gradient could be obtained extending from the ohmic and L phases to the ELMy H phase. A new data analysis method that uses sets of consecutive sweeps to extract the group delay is also presented and its use versus single sweep analysis is discussed. It is shown that local perturbations may appear and disappear during the sweep time reinforcing the need to obtain redundant data in very short time intervals and apply rejection/averaging techniques.
Newly developed double neural network concept for reliable fast plasma position control
Authors: Jeon Young-Mu , Na Yong-Su , Kim Myung-Rak , Hwang Y. S.
Published: February 2004

Neural network is considered as a parameter estimation tool in plasma controls for next generation tokamak such as ITER. The neural network has been reported to be so accurate and fast for plasma equilibrium identification that it may be applied to the control of complex tokamak plasmas. For this application, the reliability of the conventional neural network needs to be improved. In this study, a new idea of double neural network is developed to achieve this. The new idea has been applied to simple plasma position identification of KSTAR tokamak for feasibility test. Characteristics of the concept show higher reliability and fault tolerance even in severe faulty conditions, which may make neural network applicable to plasma control reliably and widely in future tokamaks.