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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
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    Self-consistent theory of a harmonic gyroklystron with a minimum Q cavity (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 3858-3863 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper, the energy extraction stage of the gyroklystron [in Advances in Electronics and Electron Physics, edited by C. Marton (Academic, New York, 1979), Vol. 1, pp. 1–54], with a minimum Q cavity is investigated by using a self-consistent radio-frequency (rf) field model. In the low-field, low-current limit, expressions for the self-consistent field and the resulting energy extraction efficiency are derived analytically for an arbitrary cyclotron harmonic number. To our knowledge, these are the first analytic results for the self-consistent field structure and efficiency of a gyrotron device. The large signal regime analysis is carried out by numerically integrating the coupled self-consistent equations. Several examples in this regime are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865769
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  • 9
    Electronic Resource
    Electronic Resource
    Experimental measurements of competition between fundamental and second harmonic emission in a quasi-optical gyrotron (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2544-2546 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A quasi-optical gyrotron (QOG) designed for operation at the fundamental ( fce(approximately-equal-to)100 GHz) exhibits simultaneous emission at fce and 2fce (second harmonic). For a beam current of 4 A, 20% of the total rf power is emitted at the second harmonic. The experimental measurements show that the excitation of the second harmonic is only possible when the fundamental is present. The frequency of the second harmonic is locked by the frequency of the fundamental. Experimental evidence shows that when the second harmonic is not excited, total efficiency is enhanced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859377
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  • 10
    Electronic Resource
    Electronic Resource
    Optimization of gyroklystron efficiency (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 1274-1281 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this article, the optimization of gyroklystron efficiency is investigated by employing a two–step procedure. As a first step, the prebuncher is analyzed using a small signal approximation, since the cavity(ies) here serve mainly to modulate the velocities of the electrons slightly, which will be bunched in the field-free drift section(s). It is found that the electrons entering the energy extraction cavity can be characterized entirely by only two dimensionless parameters: a bunching parameter q and a relative phase ψ. The numerical simulation of the extraction cavity, based on the nonlinear pendulum equations describing the interaction between the electrons and the rf field, supplemented by the initial conditions specified by q and ψ, constitutes the second step. The final result of this two-step analysis is the efficiency, η⊥,opt optimized with respect to q, ψ, and the magnetic detuning parameter Δ. This efficiency depends only on the normalized cavity length μ and the normalized rf field F of the energy extraction section. The efficiency as well as the conditions required to attain this optimum (qopt,Δopt, and qopt) are presented as contour plots on the (F,μ) plane and can be used efficiently to design gyroklystrons of any frequency and output power.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865876
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