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  • 1
    Electronic Resource
    Electronic Resource
    Properties of diffraction gratings used as output couplers in a quasi-optical gyrotron (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 2089-2102 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The design of a diffration grating used as an output coupler for a Fabry–Pérot resonator is presented. We then consider the problem of determining the distortion and the cross-polarization of the incident Gaussian beam. Different types of gratings and planes with straight or curvilinear grooves and elliptical, have been designed and built. The output pattern from a resonator using such a grating has been measured experimentally. We found that the elliptical grating generates the least distortion and cross-polarization. This result is attributed to the geometry of the grooves and is in excellent agreement with numerical calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353156
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  • 2
    Electronic Resource
    Electronic Resource
    Electron beam instabilities in gyrotron beam tunnels (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2421-2430 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electron beam instabilities occurring in a gyrotron electron beam can induce an energy spread which might significantly deteriorate the gyrotron efficiency. Three types of instabilities are considered to explain the important discrepancy found between the theoretical and experimental efficiency in the case of quasi-optical gyrotrons (QOG): the electron cyclotron maser instability (ECMI), the electrostatic Bernstein instability (BI) and the Langmuir instability (LI). When the magnetic field gradient in drift tubes of QOG is low, the ECMI can develop in the drift tube at very low electron beam currents. Experimental measurements show that with a proper choice of absorbing structures in the beam tunnel, this instability can be suppressed. At high beam currents, the BI can induce a significant energy spread at the entrance of the interaction region. The induced energy spread scales approximately linearly with the electron beam density and for QOG one observes that the beam density is significantly higher than the beam density of an equivalent cylindrical cavity gyrotron. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872918
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  • 3
    Electronic Resource
    Electronic Resource
    Global approach to the spectral problem of microinstabilities in tokamak plasmas using a gyrokinetic model (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3929-3949 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A solution to the full two-dimensional eigenvalue problem of electrostatic microinstabilities in a tokamak plasma is presented in the framework of gyrokinetic theory. The approach is the generalization of methods previously developed for a cylindrical system [S. Brunner and J. Vaclavik, Phys. Plasmas 5, 365 (1998)]. By solving the spectral problem in a special Fourier space adapted to the curved geometry, orbit width as well as Larmor radius can be kept to all orders. For a first numerical implementation, a large aspect ratio plasma with circular concentric magnetic surfaces is considered. A root finding algorithm for identifying the eigenfrequencies, based on a higher order Nyquist method, enables straightforward implementation on a parallel computer. Illustrative results for ion temperature gradient-related instabilities are presented. These include scaling studies of the radial width, and toroidicity and magnetic shear scans, as well as the effects of nonadiabatic trapped electron dynamics. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873113
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  • 4
    Electronic Resource
    Electronic Resource
    Measurement of the parallel velocity distribution function of the electron beam in a quasi-optical gyrotron by Electron Cyclotron Emission (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3501-3506 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The parallel velocity distribution function of the weakly relativistic electron beam of a quasi-optical gyrotron has been determined by measuring the Doppler-shifted Electron Cyclotron Emission (ECE) at an angle θ=15° with respect to the external magnetic dc-field. Due to the Doppler shift, the frequency of the spontaneous cyclotron emission at the fundamental (ν0=100 GHz) is upshifted to 140 GHz. A broadening of the spectrum up to 10 GHz [full width at half maximium (FWHM)] was measured. The measured mean frequency agrees well with the theoretical predictions, but the observed line-width, and hence the parallel velocity distribution function, is 2–3 times larger than expected. Considerations on ECE-measurements of the electron beam energy spread, performed at larger angles θ, are also discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871500
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  • 5
    Electronic Resource
    Electronic Resource
    Operation of a quasi-optical gyrotron with a Gaussian output coupler (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3492-3500 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The operation of a 92 GHz quasi-optical gyrotron having a resonator formed by a spherical mirror and a diffraction grating placed in −1 order Littrow mount is presented. A power of 150 kW with a Gaussian output pattern was measured. The Gaussian content in the output was 98% with less than 1% of depolarization. By optimizing the magnetic field at fixed frequency, a maximum efficiency of 15% was reached.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871499
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  • 6
    Electronic Resource
    Electronic Resource
    Particle-in-cell (PIC) simulations of beam instabilities in gyrotrons (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 3043-3048 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Extensive simulations are performed to investigate effects of electron cyclotron instabilities on the gyrotron beam quality, using two-dimensional axisymmetric particle-in-cell (PIC) codes. Both electrostatic and electromagnetic models, as well as realistic geometries of the gyrotron, are considered. It is found that a large beam density can lead to an electrostatic-instability-induced energy spread which substantially degrades the gyrotron efficiency. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872439
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  • 7
    Electronic Resource
    Electronic Resource
    Global linear gyrokinetic simulations in quasi-symmetric configurations (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3321-3333 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: First global linear study of electrostatic drift waves in two realistic quasisymmetric configurations, namely the Quasi-Axially symmetric Stellarator with three fields periods (QAS3) [P. Garabian and L. P. Ku, Phys. Plasma 6, 645 (1999)] and the Helically Symmetric eXperiment (HSX) [F. S. B. Anderson et al., Trans. Fusion Technol. 27, 273 (1995)], are presented. Effects of the shape of the plasma on the growth rate and frequency of the ion temperature gradient (ITG) driven mode are investigated by varying the quasi-symmetric configurations to an equivalent symmetric system. The calculations have been performed using a three-dimensional (3D) global gyrokinetic code in the magnetic configurations provided by the magnetohydrodynamic (MHD) equilibrium code VMEC [S. P. Hirshman and D. K. Lee, Comput. Phys. Commun. 39, 161 (1986)]. The plasma is modeled by gyrokinetic ions and adiabatic electrons. In QAS3, results are very close to those obtained for a tokamak. The drift waves are only slightly affected by the shape of the plasma or the local magnetic shear. On the other hand, results for the HSX configuration show a clear 3D effect, namely a strong toroidal variation of the drift wave mode structure. This variation is a clear structure of the 3D plasma shape. However, first results show that the growth rate of the ITG driven mode is largely unaffected by this effect. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1374585
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  • 8
    Electronic Resource
    Electronic Resource
    Velocity ratio measurement using the frequency of the gyro backward wave (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1315-1318 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The operating diagram of a low quality factor, 8 GHz TE°01 gyrotron exhibits oscillations between 6.8 and 7.3 GHz. These oscillations are identified as the backward wave component of the TE°21 traveling mode. As the resonance condition of this mode depends on the average parallel velocity 〈v(parallel)〉 of the beam electrons (ωBW(approximately-equal-to)Ωc/γ−k(parallel)〈v(parallel)〉), the measurement of ωBW for given Ωc and γ is used as a diagnostic for the beam electrons velocity ratio α=〈v⊥〉/〈v(parallel)〉. The values of α, deduced from ωBW through the linear dispersion relation for the electron cyclotron instability in an infinite waveguide, are unrealistic. A nonlinear simulation code gives α values that are in very good agreement with the ones predicted by a particle trajectory code (+10% to +20%). It is found numerically that the particles' velocity dispersion in v⊥ and v(parallel) increases ωBW. This effect explains part of the discrepancy between the values of α inferred from ωBW without velocity dispersion and the expected values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859825
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  • 9
    Electronic Resource
    Electronic Resource
    dc space-charge-induced frequency up-shift in a quasioptical gyrotron (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 519-521 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent experiments on a 100 GHz quasioptical gyrotron have shown that for a large resonator setup [Phys. Fluids B 2, 1654 (1990)] the observed frequency up-shift between the starting current and a current of 10 A corresponds to a shift of 4–5 longitudinal modes. In this Letter it is shown that the interpretation of this frequency up-shift should involve the current-dependent electron beam voltage depression in the beam tunnel and the interaction region for both the single-mode and multimode time evolution codes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859901
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  • 10
    Electronic Resource
    Electronic Resource
    Experimental measurements on a 100 GHz frequency tunable quasioptical gyrotron (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1654-1661 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments on a 100 GHz quasioptical (QO) gyrotron operating at the fundamental (ω=Ωce) are described. Powers larger than 90 kW at an efficiency of about 12% were achieved. Depending on the electron beam parameters, the frequency spectrum of the output can be either single moded or multimoded. One of the main advantages of the QO gyrotron over the conventional gyrotron is its continuous frequency tunability. Various techniques to tune the output frequency have been tested, such as changing the mirror separation, the beam voltage, or the main magnetic field. Within the limitations of the present setup, 5% tunability was achieved. The QO gyrotron designed for operation at the fundamental frequency exhibits simultaneous emission at 100 GHz (fundamental) and 200 GHz (second harmonic). For a beam current of 4 A, 20% of the total rf power is emitted at the second harmonic.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859439
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