G. Renda

Princeton University, Princeton, NJ, United States

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Publications (31)29.18 Total impact

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    ABSTRACT: The two-dimensional structure of edge plasma turbulence has been measured in the National Spherical Torus Experiment (NSTX) by viewing the emission of the D<sub>a</sub> spectral line of deuterium. Images have been made at framing rates of up to 250 000 frames/s using an ultra-high speed charged coupled device camera developed by Princeton Scientific Instruments. A sequence of images showing the transition between L-mode and H-mode states is shown.
    IEEE Transactions on Plasma Science 05/2005; · 0.87 Impact Factor
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    ABSTRACT: Fast, two-dimensional, soft x-ray imaging is a powerful technique for the study of magnetohydrodynamic instabilities in tokamak plasmas. We have constructed an ultra-fast frame rate soft x-ray camera for the national spherical torus experiment (NSTX). It is based on a recently developed 64×64 pixel charge-coupled device (CCD) camera capable of capturing 300 frames at up to 500 000 frames per second. A pinhole aperture images the plasma soft x-ray emission (0.2–10 keV ) onto a P 47 scintillator deposited on a fiber-optic faceplate; the scintillator visible light output is detected and amplified by a demagnifying image intensifier and lens-coupled to the CCD chip. A selection of beryllium foils provides discrimination of low-energy emission. The system is installed on NSTX with a wide-angle tangential view of the plasma. Initial plasma data and an assessment of the system performance are presented.
    Review of Scientific Instruments 11/2004; · 1.60 Impact Factor
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    ABSTRACT: The internal magnetic fields of plasmas can be measured under certain conditions from the integrated v×B deflection of MeV alpha particles emitted by a small radioactive source. The alpha source and large-area alpha particle detector would be located inside the vacuum vessel but outside the plasma. Alphas with a typical energy of 5.5 MeV (<sup>241</sup> Am ) can reach the center of almost all laboratory plasmas and magnetic fusion devices, so this method can potentially determine the q(r) profile of tokamaks or spherical toris (STs). Orbit calculations, background evaluations, and conceptual designs for such α v×B (or “AVB”) detector are described.
    Review of Scientific Instruments 11/2004; · 1.60 Impact Factor
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    ABSTRACT: Design studies and bench tests will be described for a simple new diagnostic to measure the internal magnetic fields of low field, low fusion power plasma devices. A collimated beam of alpha particles from a radioactive source (e.g. the 5.5 MeV alphas from 241Am) is allowed to pass through the plasma, and the integrated vxB deflection is measured by a large-area, position-sensitive alpha detector at the vessel wall. The internal magnetic field can (in principle) be unfolded from simultaneous measurement of a small set of such beams trajectories. Alpha orbit calculations will be described for various cases, including a realistic case for NSTX. This technique is being bench-tested by measuring the distribution of alpha particle impact locations on a scintillator screen using an intensified CCD camera. Possible applications and limitations of this diagnostic will be described.
    11/2004;
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    ABSTRACT: A high speed (250 kHz), 300 frame charge coupled device camera has been used to image turbulence in the Alcator C-Mod Tokamak. The camera system is described and some of its important characteristics are measured, including time response and uniformity over the field-of-view. The diagnostic has been used in two applications. One uses gas-puff imaging to illuminate the turbulence in the edge∕scrape-off-layer region, where D2 gas puffs localize the emission in a plane perpendicular to the magnetic field when viewed by the camera system. The dynamics of the underlying turbulence around and outside the separatrix are detected in this manner. In a second diagnostic application, the light from an injected, ablating, high speed Li pellet is observed radially from the outer midplane, and fast poloidal motion of toroidal striations are seen in the Li+ light well inside the separatrix.
    Review of Scientific Instruments 10/2004; 75(10):4196-4199. · 1.60 Impact Factor
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    ABSTRACT: This paper describes the architecture, process technology, and performance of a family of high burst rate CCDs. These imagers employ high speed, low lag photo-detectors with local storage at each photo-detector to achieve image capture at rates greater than 106 frames per second. One imager has a 64 x 64 pixel array with 12 frames of storage. A second imager has a 80 x 160 array with 28 frames of storage, and the third imager has a 64 x 64 pixel array with 300 frames of storage. Application areas include capture of rapid mechanical motion, optical wavefront sensing, fluid cavitation research, combustion studies, plasma research and wind-tunnel-based gas dynamics research.
    Proc SPIE 02/2004;
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    ABSTRACT: The two-dimensional radial vs poloidal structure and motion of edge turbulence in the National Spherical Torus Experiment (NSTX) were measured using high-speed imaging of the visible light emission from a localized neutral gas puff. Edge turbulence images are shown and analysed for Ohmic, L- and H-mode plasma conditions. The two-dimensional images often show regions of strong localized light emission known as 'blobs', which move both poloidally and radially at a typical speed of ≈105 cm s−1, and sometimes show spatially periodic features.
    Nuclear Fusion 12/2003; 44(1):134. · 2.73 Impact Factor
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    ABSTRACT: Not Available
    Fusion Engineering, 2003. 20th IEEE/NPSS Symposium on; 11/2003
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    ABSTRACT: High-speed 2-D images of edge turbulence in NSTX and Alcator C-Mod have been made using an ultra-fast CCD camera from Princeton Scientific Instruments, Inc. (PSI-4 camera). This camera views fluctuations in the neutral line emission from He or D gas puffs near the outer midplane where roughly 5 eV < Te < 50 eV. The movies show a variety of interesting structures and motion with a resolution of 160 x 80 pixels per frame for 28 frames at up to 1 million frames/sec. Typical movies will be shown, analyzed, and compared with other edge turbulence data from these machines.
    10/2003;
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    ABSTRACT: The gas puff imaging (GPI) diagnostic can be used to study the turbulence present at the edge of magnetically confined plasmas. In this diagnostic the instantaneous two-dimensional (2D) radial vs poloidal structure of the turbulence is measured using fast-gated cameras and discrete fast chords. By imaging a controlled neutral gas puffs of typically helium or deuterium, the brightness and contrast of the turbulent emission fluctuations are increased and the structure can be measured independently of natural gas recycling. In addition, recent advances. in ultrafast framing cameras allow the turbulence to be followed in time. The gas puff itself does not perturb the edge turbulence and the neutral gas does not introduce fluctuations in the emission that could possibly arise from, a nonsmooth (turbulent) neutral gas puff. Results from neutral transport and atomic physics simulations using the DEGAS 2 code are discussed showing that the observed line emission is sensitive to modulations in both the electron density and the electron temperature. The GPI diagnostic implementation in the National Spherical Torus Experiment (NSTX) and Alcator C-Mod, tokamak is presented together with example results from these two experiments. (C) 2003 American Institute of Physics.
    Review of Scientific Instruments 01/2003; 74(3):2020-2026. · 1.60 Impact Factor
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    ABSTRACT: The ultimate performance of any remote sensor is ideally governed by the hardware signal-to-noise capability and allowed signal-averaging time. In real-world scenarios, this may not be realizable and the limiting factors may suggest the need for more advanced capabilities. Moving from passive to active remote sensors offers the advantage of control over the illumination source, the laser. Added capabilities may include polarization discrimination, instantaneous imaging, range resolution, simultaneous multi-spectral measurement, or coherent detection. However, most advanced detection technology has been engineered heavily towards the straightforward passive sensor requirements, measuring an integrated photon flux. The need for focal plane array technology designed specifically for laser sensing has been recognized for some time, but advances have only recently made the engineering possible. This paper will present a few concepts for laser sensing receiver architectures, the driving specifications behind those concepts, and test/modeling results of such designs.
    Proc SPIE 08/2002;
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    ABSTRACT: The two-dimensional (2D) radial vs poloidal structure of edge turbulence in the Alcator C-Mod tokamak [I. H. Hutchinson, R. Boivin, P. T. Bonoli , Nucl. Fusion 41, 1391 (2001)] was measured using fast cameras and compared with three-dimensional numerical simulations of edge plasma turbulence. The main diagnostic is gas puff imaging, in which the visible D-alpha emission from a localized D-2 gas puff is viewed along a local magnetic field line. The observed D-alpha fluctuations have a typical radial and poloidal scale of approximate to1 cm, and often have strong local maxima ("blobs") in the scrape-off layer. The motion of this 2D structure motion has also been measured using an ultrafast framing camera with 12 frames taken at 250 000 frames/s. Numerical simulations produce turbulent structures with roughly similar spatial and temporal scales and transport levels as that observed in the experiment; however, some differences are also noted, perhaps requiring diagnostic improvement and/or additional physics in the numerical model. (C) 2002 American Institute of Physics.
    Physics of Plasmas 01/2002; · 2.38 Impact Factor
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    ABSTRACT: 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. © 2001 American Institute of Physics.
    Review of Scientific Instruments 12/2000; 72(1):1129-1132. · 1.60 Impact Factor
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    ABSTRACT: We have designed a wide-field, objective grating spectrograph based on a Schmidt optical system, similar to one flown in sounding rocket investigations by the Naval Research Laboratory. The instrument covers a field of view 10 degrees in diameter, has a spectral range of 130 - 300 nanometers, and utilizes a solar blind image intensifier tube coupled to a CCD array camera. The instrument could be flown as a Shuttle Hitchhiker payload, in a Spartan mission, or (eventually) as a Space Station attached payload. Another possible application of the instrument would be to observe any UV emission from lightning `sprites', which would be prominent if these extend to high enough altitudes to be above a significant portion of the ozone layer.
    Proc SPIE 09/1999;
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    ABSTRACT: Intense fluxes of 14‐MeV neutrons from deuterium–tritium (DT) fusion reaction in TFTR resulted in significantly enhanced background noise levels and reduced quality of data from the shielded UV (SPRED) and visible (VIPS) grating spectrometers and the x‐ray imaging system (XIS) camera. Both enhanced background levels, attributed to gamma rays and small angle neutron scattering, and large spikes, attributed to nuclear reactions in the silicon detectors, were observed. Both the enhanced background and the frequency of spikes were higher, on a per neutron basis, and the spike amplitudes were higher for DT than for DD operation. The VIPS shield reduced noise by 1/100 for DD radiation; the noise per DT neutron was 4 times higher than per DD neutron. The SPRED detector shield reduction factor was 1/12 in DD; extension of the shield around the vacuum chamber resulted in another factor of 1/5.5 reduction for DT plasmas. Spikes with amplitude up to 10 MeV were observed in the XIS detectors. The shielding effectiveness agrees with predictions. The spike heights are consistent with (n,p) and (n,α) reactions in the silicon detectors. © 1995 American Institute of Physics.
    Review of Scientific Instruments 02/1995; · 1.60 Impact Factor
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    ABSTRACT: The final hardware modifications for tritium operation have been completed for the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. These activities include preparation of the tritium gas handling system, installation of additional neutron shielding, conversion of the toroidal field coil cooling system from water to a Fluorinert(TM) system, modification of the vacuum system to handle tritium, preparation, and testing of the neutral beam system for tritium operation and a final deuterium-deuterium (D-D) run to simulate expected deuterium-tritium (D-T) operation. Testing of the tritium system with low concentration tritium has successfully begun. Simulation of trace and high power D-T experiments using D-D have been performed. The physics objectives of D-T operation are production of almost-equal-to 10 MW of fusion power, evaluation of confinement, and heating in deuterium-tritium plasmas, evaluation of a-particle heating of electrons, and collective effects driven by alpha particles and testing of diagnostics for confined a particles. Experimental results and theoretical modeling in support of the D-T experiments are reviewed.
    Physics of Plasmas 01/1994; 1(5):1560-1567. · 2.38 Impact Factor
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    ABSTRACT: The beam emission spectroscopy optical fluctuation diagnostic requires the highest possible quantum efficiency detector at 656 nm to minimize the photon statistical baseline limit to the detectable fluctuation level. A photoconductive photodiode detector with an extremely low‐noise preamplifier and a reactive feedback circuit provides quantum efficiencies up to 70%–80% for a useful frequency range of at least 0–150 kHz with incident powers of ∼10 nW. The diodes are chosen for negligible leakage current and hence do not require active cooling. These detectors have provided increase in the sensitivity to plasma fluctuation amplitude by a factor of ∼14 over photomultipliers and a factor of 4 over large area avalanche photodiodes.
    Review of Scientific Instruments 11/1992; · 1.60 Impact Factor
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    ABSTRACT: In D‐T plasmas, the understanding of the physics of confined α particles is extremely valuable for the future fusion plasma device. Among the various proposed α diagnostics, an X‐mode collective Thomson scattering system employing a high‐power gyrotron source (P≂200 kW, f=60 GHz, pulse length ≂0.5 s, and modulation frequency=10–25 kHz) is being designed for TFTR. The detailed description of the gyrotron source, transmission lines, optical designs, beam and viewing dump design, and receiver system will be presented in this paper. In particular, the test results of the beam and viewing dump indicate that the stray light can be reduced by 60 dB. The background emission level (∼20 eV) near 60‐GHz range during high Q discharge may also be reduced with beam and viewing dump further. The optical system is designed to measure the radial profile of α particles and to orient the incident wavevector (k0) to test the electromagnetic effects of the scattered spectrum. Prior to the study of α physics in D‐T plasmas, this scattering system will be used to measure not only a bulk ion temperature but also the scattered spectrum due to fast ions produced by NB and ICRF heating in TFTR. This work was supported by the U.S. Department of Energy Contract No. DE‐AC02‐76‐CHO‐3073.
    Review of Scientific Instruments 09/1992; 63(10):4647-4647. · 1.60 Impact Factor
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    ABSTRACT: Tangential imaging is being pursued on TFTR as a method to study instabilities with high poloidal mode number m. These instabilities cannot be seen with conventional perpendicular viewing soft x‐ray diode arrays because the emission is integrated along a line of sight. Computer simulations of tangential imaging indicate that excellent spatial resolution can be obtained. Prerequisites are (a) that the line of sight is nearly parallel to a magnetic field line at the point of closest approach to the center of the plasma, and (b) that there exists a steep gradient of the plasma radiation that is used for the measurements. The TFTR plans for a tangential, two‐dimensional imaging, soft x‐ray diode array will be outlined.
    Review of Scientific Instruments 11/1990; · 1.60 Impact Factor
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    ABSTRACT: Although the region of the Tokamak Fusion Test Reactor (TFTR) vacuum vessel wall which is susceptible to damage by neutral beam strike is armored with a mosaic of TiC‐clad POCO graphite tiles, at power deposition levels above 2.5 kW/cm<sup>2</sup> the armor surface temperature exceeds 1200 °C within 250 ms, and itself becomes susceptible to damage. In order to protect the wall armor, a neutral beam interlock system based on infrared pyrometry measurement of the armor surface temperature was installed on TFTR. For each beamline, a three‐fiber‐optic telescope views three areas of ∼30 cm diameter centered on the armor hot spots for the three ion sources. Each signal is fiber‐optic coupled to a remote 900‐nm pyrometer which feeds analog signals to the neutral beam interrupt circuits. The pyrometer interlock system is designed to interrupt each of the 12 ion sources independently within 10 ms of the temperature exceeding a threshold which can be set in the range of 500–2300 °C. A description of the pyrometer interlock system and its performance will be presented.
    Review of Scientific Instruments 09/1986; · 1.60 Impact Factor

Publication Stats

395 Citations
29.18 Total Impact Points

Institutions

  • 1985–2005
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, NJ, United States
  • 2000–2004
    • Princeton Instruments
      Jersey City, New Jersey, United States
  • 1992
    • University of Wisconsin, Madison
      • Department of Nuclear Engineering
      Madison, MS, United States