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  • 1
    Electronic Resource
    Electronic Resource
    Turbulence and beam size effects on profile reflectometry (abstract) : Proceedings of the 12th topical conference on high temperature plasma diagnostics (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 1076-1076 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A two-dimensional code based on the Wentzel–Kramers–Brillouin approximation is used to simulate the ability of reflectometry techniques to reproduce the average density profile in an ignition device to test engineering concept (ITER) size plasma with density turbulence. Analysis is mainly performed for frequency modulation [(FM) swept frequency] systems, with some comparison with the amplitude modulation techniques. The effects of turbulence with nonperpendicular reflection and plasma rotation are analyzed. Perturbation effects seem to appear for the FM technique, although the real impact for reflectometry in ITER will strongly depend on the parameters of turbulence and rotation. The influence of the turbulence level, fluctuation wavelengths, and antenna beam size on the density profile determination has been studied in a static plasma with perpendicular launching. The vector average of the reflected e field improves the resultant profiles in turbulent plasmas, while an inappropriate choice of the antenna beam size may cause additional errors. The code has also been used to simulate correlation measurements. The results show the correlation of the reflectometry signals for different turbulence parameters. Errors in the correlation length increase when two-dimensional effects become important, although the homodyne signal works better than the phase. Along with the use of these studies for turbulence analysis and code validation, the correlation measurements can be used as a robust method for the determination of the signal time delay and therefore of the density profile in highly turbulent plasmas with parasitic reflections. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149371
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  • 2
    Electronic Resource
    Electronic Resource
    Wavelet bicoherence: A new turbulence analysis tool (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3017-3032 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A recently introduced tool for the analysis of turbulence, wavelet bicoherence [van Milligen, Hidalgo, and Sánchez, Phys. Rev. Lett. 16, 395 (1995)], is investigated. It is capable of detecting phase coupling—nonlinear interactions of the lowest (quadratic) order—with time resolution. To demonstrate its potential, it is applied to numerical models of chaos and turbulence and to real measurements. It detected the coupling interaction between two coupled van der Pol oscillators. When applied to a model of drift wave turbulence relevant to plasma physics, it detected a highly localized coherent structure. Analyzing reflectometry measurements made in fusion plasmas, it detected temporal intermittency and a strong increase in nonlinear phase coupling coinciding with the L/H (low-to-high confinement mode) transition. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871199
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  • 3
    Electronic Resource
    Electronic Resource
    Turbulence and beam size effects on reflectometry measurements (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2657-2665 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional code based on the WKB (Wenzel–Kramers–Brillouin) approximation is used to simulate reflectometry measurements in plasmas with turbulence. In this work we aim to understand, first the role of turbulence in the determination of density profiles with reflectometry and, second, the ability of reflectometry techniques to give reliable information on the characteristics of the turbulence itself. The effects on the profile determination of a rotating turbulence structure with nonperpendicular reflection are analyzed. The influence of the turbulence level, fluctuation wavelengths and antenna beam size on the density profile determination has been studied in static plasma with perpendicular launching. The code has been used to simulate correlation measurements. The results show the correlation of the reflectometry signals for different turbulence parameters. Errors in the correlation length increase when two-dimensional effects become important, though the homodyne signal works better than the phase. The correlation simulations also show the way for new methods to determine the group delay and therefore the density profile. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1368142
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  • 4
    Electronic Resource
    Electronic Resource
    Electrostatic and magnetic fluctuations in the proximity of the velocity shear layer in the TJ-I tokamak (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 4007-4011 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The structure of the electrostatic and magnetic turbulence changes in the proximity of the naturally occurring velocity shear layer in the TJ-I tokamak [Nucl. Fusion 30, 717 (1990)]. Both a modification in the fluctuation levels and autocorrelation times of the Langmuir probe ion saturation current and a decorrelation in the broadband magnetic fluctuations, have been observed in the proximity of the shear layer location. However, no direct correlation between the sheared poloidal flows and turbulence characteristics has been established.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860305
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  • 5
    Electronic Resource
    Electronic Resource
    Broadband heterodyne reflectometer at the W7-AS stellarator (1994)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 2284-2290 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A broadband heterodyne reflectometer, operating in the frequency range 75–110 GHz in extraordinary mode polarization, has been installed and operated at the Wendelstein 7-AS stellarator for the study of density fluctuations and density profile determination. At 2.5 T it covers the density range between 5×1018 and 5×1019 m−3. The system differs from the usual setup as it combines the advantages of heterodyne detection with broadband capability avoiding the limitations of phase-locked sources and permits the unambiguous determination of the phase delay independent of amplitude fluctuations in the reflected beam. After a first downconversion to an intermediate frequency of about 6 GHz, two additional mixing steps lead to an intermediate frequency of 60 MHz, which carries the phase delay from the plasma as a phase modulation. The phase information is yielded numerically from sin/cos detection at 60 MHz. Due to the balanced detection scheme, the drift as well as the broadband noise of the signal and local oscillators are canceled. The system has a dynamic range of about 60 dB and is within this range almost insensitive to amplitude fluctuations of the reflected beam. Examples are given demonstrating the potential of the system installed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144678
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  • 6
    Electronic Resource
    Electronic Resource
    Swept frequency reflectometer for correlation studies in the TJ-I tokamak (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 487-491 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An improved correlation reflectometer technique has been developed and implemented on TJ-I to measure the radial correlation lengths of the density turbulence. The method uses homodyne detection and is based on the slow sweeping of one oscillator while the other remains at fixed frequency. The decay of the coherence shows maxima and minima as the distance between the two probing points increases. The maxima correspond to the relevant coherence values. The correlation length for the overall turbulence as well as the correlation length for the different frequency intervals can be directly determined in a single shot. By the operation in the Q-band (33–50 GHz) extraordinary mode, the system reaches most of the plasma radius. Correlation lengths obtained at the edge are compared with those measured at deeper positions in the plasma. The proposed method can provide the values of the differential phase delay along the sweep, avoiding the effect of lost fringes even in the presence of turbulence. This opens the possibility of obtaining the information on the density profile if several channels are used.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144219
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  • 7
    Electronic Resource
    Electronic Resource
    Amplitude modulation reflectometry for large fusion devices : 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. 4654-4656 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: One of the main problems which arises in density profile measurements by reflectometry is the need for continuous tracking of the phase delay; fast density fluctuations and strong modulation on the amplitude of the reflected signal can cause "fringe jumps'' in the phase signal. Amplitude modulation reflectometry performs a time delay measurement by the determination of the phase delay of the modulating envelope of a millimeter wave reflected by the plasma. The phase delays involved are small, the measurement is not affected by the fluctuations and can be directly performed without complicated fringe counters; the method provides a promising possibility for real time determination of the plasma position and density profile. In the present paper the principles of the method are presented as well as the analysis on the effects of phase and amplitude fluctuations, dispersive effects, and accuracy requirements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143651
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  • 8
    Electronic Resource
    Electronic Resource
    Radial distribution of turbulence correlation lengths obtained by reflectometry in the TJ-I 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. 4657-4657 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A method for determination of radial correlation lengths of the electrostatic turbulence has been developed and applied to the TJ-I tokamak [T. Estrada, J. Sanchez, B. Brañas, and A. P. Navarro, Rev. Sci. Instrum. 61, 3034 (1990)]. The method uses homodyne detection and is based on the slow sweeping of one oscillator while the other remains at fixed frequency. The decay of the coherence can be observed as the distance between the two probing points increases. By the operation in the Q band (33–50 GHz) extraordinary mode, the system reaches most of the plasma radius. Correlation lengths obtained at the edge are compared with those measured at the plasma center. A critical issue for this comparison is the detailed knowledge of the local density gradient. The proposed method can provide the density profile information, avoiding the effect of lost fringes, even in the presence of turbulence. This fact can be used to determine the local density gradient which is needed to evaluate the separation between both reflecting points. Initial experimental results and a discussion on its degree of accuracy is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143652
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  • 9
    Electronic Resource
    Electronic Resource
    Broadband homodyne reflectometer devoted to density fluctuation measurements : Eighth topical conference on high−temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 3034-3036 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A microwave reflectometer has been used in the TJ-I tomakak (R=0.30 m, a=0.10 m, IpIp〈 50 kA, Bt 〈 1.5 T) to monitor density fluctuations in the plasma bulk. Experiments were made in the range 33–50 GHz, x mode. The system uses a single antenna for launching and receiving the microwave beam. Since broadband operation usually requires homodyne detection, different techniques were used to overcome the limitations due to this detection scheme. Radial profiles of the density turbulence were taken and the behavior of the spectra analyzed. A slow frequency sweeping has been performed to provide a method to normalize the signals reflected from different radial positions. In addition this technique can be used to obtain information on the absolute value of the phase oscillation amplitude. Spectra of the edge density fluctuations obtained by reflectometry and by Langmuir probes are compared. By the use of two reflectometers in the same antenna, radial correlation experiments can be performed. Some discussions on the ability of homodyne reflectometry to determine coherence lengths are also presented. A slow sweeping of the frequency is proposed to overcome the lack on the information given by the homodyne system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141718
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  • 10
    Electronic Resource
    Electronic Resource
    Heterodyne reflectometer for phase and time delay measurements in W7-AS : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 412-412 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A broadband heterodyne reflectometer operating in the frequency range 75–110 GHz in extraordinary mode polarization, is used at the W7-AS stellarator for density profile and density fluctuation studies. The system allows for broadband operation and permits unambiguous determination of the phase delay independent of the amplitude fluctuations in the reflected beam. The system uses two different gauss beam optics for both signal launch and detection with a slim beam waist of about 2 cm at the relevant plasma location. An amplitude modulation system has been incorporated operating at 133 MHz modulation frequency sharing with the phase sensitive system the main oscillators, antennas, and the receiver front end. AM detection is accomplished in the IF part by synchronous detection after recovery of the carrier by narrowband filtering. Time delay measurements can be done simultaneously with the carrier phase determination. The main purpose of the diagnostic in AM mode is fast edge density profile monitoring, conventional carrier phase detection is used for density fluctuation measurements and planned correlation studies. The agreement between the WKB approximation and the 1 D full wave numerical solutions for the W7-AS parameters and geometry guarantees the good localization of the measurements and permits the estimation of density fluctuation level from the measured phase fluctuations. Density fluctuation studies have been carried out in NBI and ECRH (L- and H-mode) plasmas. Fluctuation levels and spectral characteristics have been analyzed. In addition 2D effects possibly due to rotating turbulent structures have been observed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146363
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