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  • 1
    Electronic Resource
    Electronic Resource
    Search for the ion temperature gradient mode in a tokamak plasma and comparison with theoretical predictions : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2232-2237 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In recent experiments in DIII-D, comparison of specific turbulence characteristics with linear and nonlinear modeling has identified common features associated with the ion temperature gradient (ITG) mode. A low-frequency turbulence feature is observed in high-density saturated Ohmic confinement discharges, which is absent in low-density linear Ohmic confinement discharges. The feature is in a range of wavelength (k⊥ρs(approximate)0.2–0.5) and the frequency expected for the ITG mode and onset of the feature is coincident with onset of confinement saturation. The density profile is significantly broader in the high-density discharge, a known destabilizing effect on the ITG mode. Gyrokinetic stability calculations of the growth rate of the most unstable drift ballooning mode show the ITG mode to be more unstable in the high-density discharges. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1362537
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  • 2
    Electronic Resource
    Electronic Resource
    Characterization of avalanche-like events in a confined plasma : Review,Tutorial and Invited Papers from the 43rd Annual Meeting of the APS Division of Plasma Physics (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 1962-1969 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One mechanism for transport of energy and particles in a plasma is by discrete, intermittent, uncorrelated events, often called avalanches. This paper reports observations and quantitative characterization of avalanche events in a magnetically confined plasma. The observations are primarily of electron temperature fluctuations. Avalanches are identified by their large spatial scale, up to the system size, by self-similar behavior in the frequency spectrum and the autocorrelation function and by propagation. The two-point cross-correlation function allows determination of a characteristic velocity, which typically varies from several hundred meters per second in the outer part of the plasma to zero or even inward near the axis. This can be interpreted as resulting from the prevalence of negative avalanches (i.e., holes) near the axis. The presence of a long-tailed probability distribution is indicated by a Hurst parameter (H) in the range 0.80 to 0.95, which becomes smaller in the outer quarter of the plasma radius. Density fluctuation spectra from the plasma core also show self-similar behavior. Power transport estimates show that about half of the heat flux is carried by the avalanche events under conditions with no magnetohydrodynamic activity. These observations are qualitatively similar to results of modeling calculations based on drift wave turbulence. It is reasonable to infer that avalanches are the macroscopic manifestation of turbulence which inherently has a small spatial scale and, thus, allow a local, gyro-Bohm scaling process to show global Bohm-type behavior. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1452730
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  • 3
    Electronic Resource
    Electronic Resource
    Energy, particle and impurity transport in quiescent double barrier discharges in DIII-D : Review,Tutorial and Invited Papers from the 43rd Annual Meeting of the APS Division of Plasma Physics (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 1970-1980 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Quiescent double barrier discharges (QDB) on DIII-D [Luxon et al., Fusion Technol. 8, Part 2A, 441 (1985)] exhibit near steady high performance (βNH∼7) with a quiescent H-mode edge, i.e., free of edge localized modes (ELMs), but with effective particle control and strongly peaked density profiles. These QDB discharges exhibit an internal transport barrier with low ion thermal transport despite incomplete turbulence suppression. Very short correlation lengths, which reduce the transport step size, however, characterize the residual turbulence. This observation is consistent with simulations using the GLF23 [Waltz et al., Phys. Plasmas 4, 2482 (1997)] model, which reproduce the core ion temperature profile even in the presence of finite turbulence. Increased retention of high-Z impurities is observed and core nickel concentrations (an intrinsic impurity in DIII-D) are as high as 0.3%. To quantify impurity transport, trace impurity injection has been performed in steady QDB discharges showing a fast influx followed by a slow pump out. The measured decay times of the core concentration of two nonrecycling impurities, F(Z=9) and Ca(Z=22), are 299 and 675 ms, respectively, while the energy confinement time is 111 ms. Time dependent analysis of neon influx yields both diffusivities and inward convection velocities significantly greater than neoclassical predictions in the same region of the plasma where measured ion thermal transport is near neoclassical predictions yet significant turbulence is observed. The boundary of these discharges is characterized by a saturated coherent magnetohydrodynamic mode, the edge harmonic oscillation, which takes the place of ELMs in facilitating particle control by allowing particle transport to the open field lines, where both wall- and cryopumping play a major role in particle exhaust. Hot (∼5 keV) ions observed in the outer scrape-off layer may enhance wall pumping. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1456064
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  • 4
    Electronic Resource
    Electronic Resource
    Rotational and magnetic shear stabilization of magnetohydrodynamic modes and turbulence in DIII-D high performance discharges (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1951-1958 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The confinement and the stability properties of the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high-performance discharges are evaluated in terms of rotational and magnetic shear, with an emphasis on the recent experimental results obtained from the negative central magnetic shear (NCS) experiments. In NCS discharges, a core transport barrier is often observed to form inside the NCS region accompanied by a reduction in core fluctuation amplitudes. Increasing negative magnetic shear contributes to the formation of this core transport barrier, but by itself is not sufficient to fully stabilize the toroidal drift mode (trapped-electron-ηi mode) to explain this formation. Comparison of the Doppler shift shear rate to the growth rate of the ηi mode suggests that the large core E×B flow shear can stabilize this mode and broaden the region of reduced core transport. Ideal and resistive stability analysis indicates the performance of NCS discharges with strongly peaked pressure profiles is limited by the resistive interchange mode to low βN≤2.3. This mode is insensitive to the details of the rotational and the magnetic shear profiles. A new class of discharges, which has a broad region of weak or slightly negative magnetic shear (WNS), is described. The WNS discharges have broader pressure profiles and higher β values than the NCS discharges, together with high confinement and high fusion reactivity. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871991
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  • 5
    Electronic Resource
    Electronic Resource
    Fundamental investigation of reflectometry as a density fluctuation diagnostic (invited) : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4599-4604 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Reflectometry is currently used to monitor density fluctuations and turbulent correlation lengths in fusion plasmas. Various models have been used to interpret the experimental data and to determine the regimes of validity of the reflectometer fluctuation measurements. Heretofore, these models have not been validated by direct comparison with experiment. In this paper the first comparison between a controlled laboratory experiment and a one-dimensional numerical model is presented. It is found that the model is unable to predict the observed high degree of spatial localization and dependence on perturbation wave number. The implications of these disagreements are discussed, together with suggestions for their resolution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143681
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  • 6
    Electronic Resource
    Electronic Resource
    The UCLA frequency tunable correlation reflectometer system on DIII-D : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4661-4664 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A broadband tunable (∼50–75 GHz) correlation reflectometer system is installed and operational on the DIII-D tokamak. The same launch and receive horns are utilized for the two probing frequencies ensuring that they sample the same path within the plasma. Very wide band IF electronics allow the separation frequency to be continuously tuned over the range 100 MHz→10 GHz corresponding to a maximum spatial separation of ∼7 cm. Sweeping the separation frequency during the plasma shot produces multiple spatial points ((approximately-greater-than)10) on a single discharge reducing shot to shot reproducibility problems and increasing the spatial resolution. A description of the microwave system, laboratory tests, and representative data from a tokamak discharge are presented. Difficulties with the method and some possible resolutions are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143655
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  • 7
    Electronic Resource
    Electronic Resource
    New reflectometer systems for the DIII-D tokamak (abstract) : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4665-4665 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: During a machine vent in December 1991, two new reflectometer systems were successfully installed and tested on the DIII-D tokamak. The first is an X-mode broadband system primarily intended for density profile measurements, utilizing BWO sources and covering Q and V frequency bands (33–50 and 50–75 GHz). The second system is an adaptation of a pre-existing inside launch (high field side) ECRH waveguide to provide an inside launch reflectometer capability at the same frequencies and polarization as an outside launch fixed frequency O-mode system. The new systems will have a dual role in both directly supporting the DIII-D physics program, and also acting as flexible and adaptable test beds for the development of reactor relevant reflectometer systems, such as required for ITER. Specific examples of planned measurements include investigation of possible in/out plasma asymmetries at the L–H transition and ELMs, and demonstration of routine and reliable density profile measurements. It is expected that preliminary data from the inside launch system will be available by the time of the conference. This work is supported by the U. S. Department of Energy under Grant No. DE-FG03-86-ER53225 and General Atomics subcontract SC120536 under DOE Contract No. DE-AC03-89ER51114.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143656
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  • 8
    Electronic Resource
    Electronic Resource
    Development of technology and techniques for reflectometry : Proceedings of the 9th topical conference on high temperature plasma diagnotics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4666-4668 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Development efforts are directed at advanced reflectometry systems for next generation tokamaks such as ITER. Laboratory tests have been successfully completed for a pulsed radar reflectometer (τ(approximately-equal-to)200 ps) to be tested on DIII-D. In addition, there exists a need for sources and components beyond those commercially available. Monolithic quasi-optical spatial power combined arrays of solid state devices will provide continuous, watt level swept source coverage up to 200 GHz as well as electronically controlled beam steering, focusing, and modulation. Studies are also underway to address the feasibility of instantaneous profile coverage using monolithic impulse generators (τ(approximately-equal-to)1–3 ps) comprised of spatially combined nonlinear transmission lines, employing novel quantum barrier varactors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143657
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  • 9
    Electronic Resource
    Electronic Resource
    Reflectometry as a fluctuation diagnostic: A one-dimensional simulation : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4669-4671 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Reflectometry is currently employed to characterize turbulence in fusion plasmas worldwide and is expected to be a major diagnostic on the next generation of machines (e.g., ITER). Until recently, little was known about the response of a reflectometer to fluctuations (degree of localization of the signal, sensitivity to fluctuation wave number, dependence on density scale length, etc.). To elucidate these properties, we have been modeling reflectometer behavior with a code based on solution of a one-dimensional full wave equation. The code models an infinite plane plasma with density gradient in the x direction and solves the full wave equation to find the electric field of the reflectometer's electromagnetic wave. It can simulate stationary and moving density perturbations with arbitrary waveforms and wave numbers in plasmas with arbitrary density profiles. We present results of test cases comparing computational results to known analytic solutions for linear and 1−α2/x2 plasma density profiles, which show very good agreement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143658
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  • 10
    Electronic Resource
    Electronic Resource
    Fluctuation measurements in the DIII-D and TEXT tokamaks via collective scattering and reflectometry (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 3509-3519 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Anomalous transport in fusion plasmas remains an enigma requiring explanation. A predictive capability is highly desirable if confinement enhancement regimes such as H mode or super shots are to be extrapolated to the next phase in development of the International Fusion program, epitomized, for example in ITER. Therefore, identification of the role that electrostatic turbulence plays in confinement is a critical issue requiring detailed experimental data capable of testing and challenging existing theoretical models. This article presents microturbulence measurements obtained on the DIII-D and TEXT tokamaks utilizing heterodyne, far-infrared collective scattering, and reflectometry techniques. The experimental systems are described on both machines and emphasis placed on results obtained during the L-H transition, ELM activity, and saturated ohmic operation where ion temperature gradient driven (ITGD) turbulence is theoretically predicted to exist.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141560
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