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  • 1
    Electronic Resource
    Electronic Resource
    Response to "Comment on ‘The Hurst exponent and long time correlation' " [Phys. Plasmas 7, 5267 (2000)] (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 5269-5271 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1319337
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  • 2
    Electronic Resource
    Electronic Resource
    Visible light scattering to measure small scale turbulence (2000)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 71 (2000), S. 113-117 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A laser scattering optical bench using ionized argon laser was set up to study the small scales of turbulence. Scales between 200 and 10 μm are resolved. The signal being proportional to the Fourier transform of the density fluctuations, heterodyne detection is used to recover its real and imaginary parts. The optical and electronic facilities are described in detail. The minimum density fluctuations for a signal-to-noise ratio greater than 1 is assessed. Using an acoustic wave, the spatial bandpass filter of the Fourier transform is demonstrated. A sample of recorded signals on a turbulent air jet is shown and the dependence of their intensity on the analyzing wave vector is discussed. The frequency spectra are shown to reflect the temperature and the pressure fluctuations. One can thus measure the flow and sound speed. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1150171
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  • 3
    Electronic Resource
    Electronic Resource
    Turbulence reduction and poloidal shear steepening in reversed shear plasmas investigated by light scattering on Tore Supra (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 186-192 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Infrared light scattering is used to characterize turbulence during reversed shear scenario obtained by a fast current ramp-up on Tore Supra [Hoang et al., Phys. Rev. Lett. 84, 4593 (2000)]. The preformed hollow profile is freezed using ion cyclotron resonance minority heating (ICRH). Turbulence intensity is recorded to dramatically decrease during the ramp-up and to recover smoothly as the discharge is ended. Simultaneously, a confinement improvement of about 40% is achieved for about 2 s. Although the diagnostic performs a line average and the measurement is dominated by turbulence at the edge, we show that the core behavior can be deduced. Core turbulence decrease is in agreement with the effective heat diffusivity behavior. With the same optical device, the radial electric shear is deduced from the frequency spectra Doppler shift. Turbulence reduction is thus demonstrated to be correlated with the E×B shear steepening. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1330202
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  • 4
    Electronic Resource
    Electronic Resource
    Turbulence intermittency and burst properties in tokamak scrape-off layer (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 1612-1624 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Density fluctuation measured by a reciprocating Langmuir probe on the Tore Supra tokamak [Garbet et al., Nucl. Fusion 32, 2147 (1992)] is investigated. The purpose of this article is to give a rather comprehensive analysis of intermittency by using several data analyses to compare the bursts properties to that of coherent structures and avalanches. The probability distribution of the density fluctuations is found positively skewed, while a Gaussian shape for the negative values is recorded. It is shown that the fluctuation spectra possess one scaling region with a power law close to the one predicted by a Kolmogorov–Kraichnan model in the inverse cascade subrange. However, a net deviation from this law at higher moment orders is demonstrated. The deviation from the mono-fractal model is investigated by the multifractal analysis that reveals the variety of the dissipative structures similar to what is found in fully developed fluid turbulence. The spectra are found asymmetric, indicating the presence of structures not generated by a multiplicative process. Using conditional analysis, a detailed study of the intermittent bursts independently of the background is performed. The typical form of the intermittent structures is asymmetric. Furthermore, they do not conserve mass for only positive density fluctuations are recorded. Their poloidal velocity is estimated to be 70% greater than the background turbulence, suggesting that they may not result from a diffusive process. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1363663
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  • 5
    Electronic Resource
    Electronic Resource
    Transport by intermittent convection in the boundary of the DIII-D tokamak (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4826-4833 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Intermittent plasma objects (IPOs) featuring higher pressure than the surrounding plasma, and responsible for ∼50% of the E×BT radial transport, are observed in the scrape off layer (SOL) and edge of the DIII-D tokamak [J. Watkins et al., Rev. Sci. Instrum. 63, 4728 (1992)]. Conditional averaging reveals that the IPOs, produced at a rate of ∼3×103 s−1, are positively charged and also polarized, featuring poloidal electric fields of up to 4000 V/m. The IPOs move poloidally at speeds of up to 5000 m/s and radially with E×BT/B2 velocities of ∼2600 m/s near the last closed flux surface (LCFS), and ∼330 m/s near the wall. The IPOs slow down as they shrink in radial size from 4 cm at the LCFS to 0.5 cm near the wall. The IPOs appear in the SOL of both L and H mode discharges and are responsible for nearly 50% of the SOL radial E×B transport at all radii; however, they are highly reduced in absolute amplitude in H-mode conditions. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1406940
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  • 6
    Electronic Resource
    Electronic Resource
    Kelvin–Helmholtz instabilities in tokamak edge plasmas (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 3955-3965 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The parallel Kelvin–Helmholtz instability is investigated as a possible explanation for poloidal asymmetries of density fluctuations which reverse with the plasma current direction. It is shown that these modes are localized around the position where the radial gradient of parallel velocity is maximum. Two mechanisms lead to unstable Kelvin–Helmholtz modes; the acceleration of ions in a presheath and the anomalous Stringer spin-up due to asymmetries of the particle flux. Up–down asymmetries are explained by combining these two effects. Depending on the limiter configuration, the Stringer effect amplifies or weakens the flow due to presheath acceleration. This type of asymmetry reverses with the plasma current direction. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873659
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  • 7
    Electronic Resource
    Electronic Resource
    The Hurst exponent and long-time correlation (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1181-1183 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The rescaled range statistics (R/S) method is applied to the ion saturation current fluctuations measured by the Langmuir probe at the edge of Tore Supra [Equipe Tore Supra, in Proceedings of the 13th International Conference on Plasma Physics and Controlled Nuclear Fusion, Washington, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 549] to evaluate the Hurst exponent. Data block randomization is carried out to the data sets in order to investigate the relationship between the Hurst exponent and long-time correlation. It is observed that H is well above 0.5 in the long-time self-similar range. However, it is found that the information which leads to H〉0.5 is totally contained in the short-time correlation and no link to long times is found. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873927
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  • 8
    Electronic Resource
    Electronic Resource
    Nonlocal k-space interaction and the rare existence of coherent structures in the tokamak edge plasma (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 1255-1261 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The relation between turbulent density fluctuations occurring at two wave numbers is investigated at the plasma edge of the Tore Supra tokamak [G. Antar et al., Plasma Phys. Controlled Fusion 41, 947 (1998)]. To achieve this goal, light scattering is used with two laser beams performing a Fourier transform about two given wave vectors. Consequently a band-pass filter is applied to the spatial turbulent density fluctuations about each wave vector. When the separation is small between the analyzing wave numbers, it is found that the cross correlation between the two channels is dominated by the autocorrelation caused by the diagnostic resolution in the wave number space. For large separations, the instrumental resolution contribution is negligible but a cross correlation amplitude of about 1% to 2% is still detected. It reflects nonlocal coupling in the k space caused by eddy organization rarely taking place. In agreement with this behavior, a rather small deviation of the probability distribution from a Gaussian is reported. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1455636
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  • 9
    Electronic Resource
    Electronic Resource
    Omegatron mass spectrometer for analysis of ion concentrations in hydrogenic plasmas : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 623-626 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe an omegatron-type mass spectrometer used for measurement of the molecular and atomic ion concentrations as well as the ion species temperatures in hydrogenic plasmas. The experiments were performed in the PISCES-A linear plasma experiment with electron densities n(approximate)1011–1012 cm−3, ion temperatures Ti(approximate)2–8 eV, electron temperatures Te(approximate)3–30 eV, and magnetic fields B=0.8–1.9 kG. The resulting mass spectra show clear evidence of atomic hydrogen H+, as well as of the molecular ions H2+ and H3+, with H+ and H3+ usually being dominant. It is found that large electron repeller voltages ((approximate)−200 V) are important for preventing electrons from entering the diagnostic region and ionizing the residual gas. The relative peak heights for the different ions are found to vary as a function of ion repeller voltage, probably due to differences in ion temperature. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1310594
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  • 10
    Electronic Resource
    Electronic Resource
    A fast phosphor imaging diagnostic for two-dimensional plasma fluctuation measurements : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 953-956 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A plasma imaging diagnostic is being developed using a fast time response ZnO:Zn phosphor disk to image plasma density fluctuations in a two-dimensional (2D) region. The plasma sensor consists of an 8.9 cm diameter phosphor coated aluminum disk that is inserted into the plasma and is excited by incident electrons resulting in a cathodoluminescent emission image. The phosphor light distribution is then interpreted as plasma density fluctuations using sheath theory and the phosphor response function. The local luminance S of fluctuating phosphor light is dependent on the incident electron energy Ee and current density through the equation S(r,t)=e∫R(Ee)(v⋅n(circumflex))fe(r,v,t)d3v, where R(Ee) is the energy dependent response function of the phosphor. The phosphor persistence time of 1–10 μs (emulsion dependent) combined with fast intensified charge coupled device camera shutter speeds enables the imaging of plasma fluctuations on microsecond range time scales. Broadband turbulent phosphor light fluctuations (f≤500 kHz) have been measured with photodiode detectors and compared with Langmuir probe data. The 2D phosphor images show density profiles consistent with radial Langmuir probe measurements. Temporal and spatial resolution of the phosphor diagnostic enables 2D imaging of plasma turbulence and coherent modes. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1329658
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