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  • 1
    Electronic Resource
    Electronic Resource
    Coherent synchrotron radiation in a cylindrical waveguide with a helical wiggler (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1306-1317 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spectral and temporal radiation characteristics of an axially extended (finite-size), transversally accelerated, charge distribution propagating on fixed helical trajectories through a wiggler are derived both in free-space and in a cylindrical waveguide. If the charge distribution scale length is short compared to the radiation wavelength, the electron bunch essentially behaves as an accelerated point charge, and coherently radiates spontaneous synchrotron radiation. In a waveguide, two very different radiation processes are possible. At grazing, where the axial bunch velocity matches the electromagnetic wave group velocity, the single output radiation pulse is extremely short, and chirped over the full interaction bandwidth; the pulse duration is determined by group velocity dispersion. In the free-space limit, the widths of the Doppler upshifted and downshifted radiation pulses are determined by slippage. At grazing, the radiation power level is considerably higher than that expected from the conventional coherent synchrotron scaling.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870729
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  • 2
    Electronic Resource
    Electronic Resource
    Transform-limited coherent synchrotron radiation wavepackets in a chirped pulse free-electron laser (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2446-2456 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel source of transform-limited pulses of coherent electromagnetic radiation relying on the synchrotron radiation process in a fast wave guiding structure is investigated theoretically. An ultrashort electron bunch transversally accelerated by a periodic external field is considered. At grazing, where the bunch and group velocities are matched, the duration of the resulting ultrawideband chirped pulse is governed by group velocity dispersion instead of slippage. Because of the intimate connection between the rate of chirping and the bandwidth, the corresponding pulse duration is shown to be very close to the Fourier transform limit. In addition, the propagation of such chirped pulses through a guiding structure with negative group velocity dispersion is investigated both theoretically and computationally. The spectral and temporal characteristics of the chirped and compressed pulses are derived analytically. Detailed computer calculations complement this theoretical analysis. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871706
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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    Far-infrared heterodyne scattering to study density fluctuations on the DIII-D tokamak : Eighth topical conference on high—temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 3007-3010 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A far-infrared collective scattering system has been installed and is operational on the DIII-D tokamak. The system consists of a 245 GHz (λ= 1.22 mm) twin frequency laser and uses heterodyne detection techniques which enables us to determine the propagation direction of fluctuations in the laboratory frame of reference. At present, three receiver channels are available to study poloidal propagating fluctuations along the entire midplane of DIII-D in a wave number range of 2≤ k ≤16 cm−1. The beam geometry of the system dictates a wave number resolution of Δk ≈ 0.7 cm−1 and a radial spatial resolution of ± 20 cm at k=10 cm−1. Fluctuation spectra and propagation direction measured during Ohmic operation were found to depend on plasma parameters. Large plasma toroidal rotation was measured which correlated with modifications in the fluctuation spectra. During H-mode operation, a dramatic drop of low-frequency fluctuations (〈 250 kHz) is observed coincident with the drop in D-alpha light. This is followed by the appearance of a high- frequency component possibly connected with profile modification and rotation effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141972
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  • 8
    Electronic Resource
    Electronic Resource
    Development and operation of a backward wave oscillator based FIR scattering system for DIII-D : Eighth topical conference on high-temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 3010-3013 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The laser-based far-infrared scattering system (λ=1.22 mm) on DIII-D is to be upgraded by the addition of a backward wave oscillator or carcinotron. By supplying up to 300 mW at λ=1.06 mm, the carcinotron is expected to increase the scattered power and signal-to-noise ratio by more than ten times. In addition, the laser has suffered from significant frequency pulling during high density discharges which has necessitated using undesirable geometry to obtain accuracy in the low-frequency part of the fluctuation spectrum. However, the carcinotron-based system will use an IF tracking system to simultaneously remove the excessive phase noise, frequency drift typical of high-frequency tubes, and the effects of frequency pulling. This should allow more reliable measurements of low-frequency asymmetric spectra under all discharge conditions. The local oscillator will be provided by a frequency doubled and locked klystron, to enable heterodyne detection of the scattered signal. Results of extensive laboratory tests of the carcinotron-based scattering apparatus are to be presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141762
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  • 9
    Electronic Resource
    Electronic Resource
    UCLA FIR diagnostic systems on TEXT and developments for TEXT Upgrade (abstract) : Eighth topical conference on high−temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 3071-3071 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A multipurpose multiview far-infrared diagnostic system is being designed and constructed for the TEXT Upgrade. The system's capabilities, designed to enhance the applications of the present FIR system on TEXT, will include two-view interferometry, polarimetry, and collective scattering in order to measure the plasma density profile, current profile, and density fluctuation distribution. The interferometer/polarimeter system will possess high spatial (Δx=1.5 cm) and high-density (≤5×1010 cm−3) resolution. Tomographic reconstruction techniques will be employed to investigate both the equilibrium and perturbed profiles of plasma density. This represents an extension of the present work on TEXT where MHD effects such as sawteeth, Mirnov oscillations, and disruptions have been studied. Two-view multichannel collective scattering (1≤k⊥≤20 cm−1) will allow the kr and kθ spectra to be measured simultaneously. Heterodyne detection permits the wave propagation direction and multiple modes to be resolved unambiguously. Cross correlation of signals from each view may produce significant improvement in the scattering system spatial resolution. A reflectometry system will also be installed to study fluctuations and provide more localized measurement. This work was supported by U.S. DOE contract No. DE-FG03-86ER-53225 and DE-FG03-88ER-53267.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141733
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  • 10
    Electronic Resource
    Electronic Resource
    Poloidal asymmetry and gradient drive in core electron density and temperature fluctuations on the Texas Experimental Tokamak-Upgrade (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2013-2021 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electron temperature and density fluctuations are measured in the core of the Texas Experimental Tokamak-Upgrade (TEXT-U) [P. H. Edmonds, E. R. Solano, and A. J. Wootton, in Proceedings of the 15th Symposium on Fusion Technology, Utrecht (Elsevier Science, Amsterdam, 1989), Vol. 1, p. 342] plasma across the poloidal cross section. The high spatial resolution of the heavy-ion beam probe (HIBP) and correlation radiometry of electron cyclotron emission (CRECE) reveal that both the density and temperature fluctuations are strongly poloidally asymmetric. Temperature fluctuation measurements indicate a broadband drift wave feature localized near the plasma equatorial plane on both the high- and low-field sides, which is consistent with density fluctuation measurements by far infrared (FIR) scattering. In contrast, the HIBP observes this feature localized only to the low-field side. Excellent spatial resolution allows us to investigate whether changes in the gradient affect the fluctuation amplitudes. We find that indeed, the temperature fluctuations increase with the electron temperature gradient. Results also link density fluctuations to changes in the density gradient. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871998
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