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  • 1
    Electronic Resource
    Electronic Resource
    Universal efficiency and gain computations for high-gain free-electron-laser amplifiers (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 7503-7509 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The free-electron-laser (FEL) equations are reduced to a set of one-dimensional, normalized equations that allow a universal (dimensional) analysis. In universal parameters, numerical integration of the FEL equations indicates a relatively constant saturated ponderomotive wave amplitude independent of both the normalized wiggler potential amplitude and the injected signal level. The constant ponderomotive wave amplitude and an empirical fit for the universal saturation length as a function of normalized wiggler potential amplitude and gain permits unnormalized design calculations for saturated power and saturated length over a wide parameter range. Tapering is considered by deriving analytical expressions for the intrinsic efficiency and taper length. Design values for a high-gain, high-efficiency, tapered amplifier at 280 and 560 GHz are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347567
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  • 2
    Electronic Resource
    Electronic Resource
    High efficiency cavity design of a 170 GHz gyrotron for fusion applications (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 209-216 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The design of high power, continuous wave (cw), 170 GHz gyrotron cavities is considered. The anticipated degradation of efficiency with beam velocity spread places a premium on the optimization of efficiency. For parameters of interest achievement of high efficiency requires utilization of a high quality cavity. Two options are considered: a barrel cavity and a long simple tapered cavity operating at low voltage. The cavities are examined for their sensitivity to velocity spread, their mode competition, and their maximum Ohmic power dissipation. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872133
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  • 3
    Electronic Resource
    Electronic Resource
    A study of parametric instability in a harmonic gyrotron: Designs of third harmonic gyrotrons at 94 GHz and 210 GHz (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2839-2846 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Mode competition can present a major hurdle in achieving stable, efficient operation of a gyrotron at the cyclotron harmonics. A type of mode interaction in which three modes at different cyclotron harmonics are parametrically coupled together is analyzed here. This coupling can lead to parametric excitation or suppression of a mode; cyclic mode hopping; or the coexistence of three modes. Simulation results are presented for the parametric instability involving modes at the fundamental, second harmonic, and third harmonic of the cyclotron frequency. It is shown that the parametric excitation can lead to stable, efficient operation of a high-power gyrotron at the third harmonic. Based on this phenomenon, two practical designs are presented here for the third harmonic operation at 94 and 210 GHz. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871182
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  • 4
    Electronic Resource
    Electronic Resource
    Development and applications of a plasma waveguide for intense laser pulses (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2149-2155 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The demonstration of optical guiding of high-intensity laser pulses in a plasma fiber waveguide [C. G. Durfee III and H. M. Milchberg, Phys. Rev. Lett. 71, 2409 (1993)] has opened the way to new advances in the development of nonlinear optics-based short-wavelength light sources, soft x-ray lasers, and compact laser-driven charged particle accelerators, and offers a new practical realm in which to study, control, and apply nonperturbative laser–matter interactions at ultrahigh intensity. An overview of selected experimental and theoretical results and their applications is presented. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871668
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  • 5
    Electronic Resource
    Electronic Resource
    Physical mechanism of enhanced stability from negative shear in tokamaks: Implications for edge transport and the L-H transition (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2221-2223 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The enhancement of stability to ballooning modes from negative shear in tokamaks is shown to be a simple consequence of the orientation of the convective cell with respect to the toroidally outward effective gravity, g↘. For modest positive shear, convective cells remain oriented along g↘ as they map along field lines. In contrast, for negative shear or very positive shear convective cells twist strongly away from g↘ and are less strongly driven. The twist of convection cells is controlled by the profile of the vertical magnetic field along the outer midplane, Bz. Twist is a minimum in regions where Bz is independent of the major radius. Transport should be highest in such locations. Resistive ballooning modes in the tokamak edge are strongly stabilized by modest values of negative shear. Tokamak discharges with finite values of βp develop regions of local negative shear on the outside midplane of the plasma torus. This local negative shear should self-stabilize resistive ballooning modes at finite values of the poloidal beta. This effect may impact the transition to high confinement operation (H-mode). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871928
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  • 6
    Electronic Resource
    Electronic Resource
    Periodic magnetic focusing of sheet electron beamsf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1714-1720 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Sheet electron beams focused by periodically cusped magnetic (PCM) fields are stable against low-frequency velocity-shear instabilities (such as the diocotron mode). This is in contrast to the more familiar unstable behavior in uniform solenoidal magnetic fields. A period-averaged analytic model shows that a PCM-focused beam is stabilized by ponderomotive forces for short PCM periods. Numerical particle simulations for a semi-infinite sheet beam verify this prediction and also indicate diocotron stability for long PCM periods is less constraining than providing for space-charge confinement and trajectory stability in the PCM focusing system. In this article the issue of beam matching and side focusing for sheet beams of finite width is also discussed. A review of past and present theoretical and experimental investigations of sheet-beam transport is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870675
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  • 7
    Electronic Resource
    Electronic Resource
    Theory of relativistic backward wave oscillators operating near cutoff (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 730-740 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A numerical model for analyzing backward-wave oscillators (BWOs) operating near the upper edge of the transmission band is presented. The model is used to calculate starting currents for two finite length devices, an X-band BWO (f=8.4 GHz) and a J-band BWO (f=5.5 GHz). The operating frequency and efficiency predicted by the nonlinear numerical simulations are compared with experimental data for each device.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870818
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  • 8
    Electronic Resource
    Electronic Resource
    Theory of helix traveling wave tubes with dielectric and vane loading (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3145-3161 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A time-dependent nonlinear analysis of a helix traveling wave tube (TWT) is presented for a configuration where an electron beam propagates through a sheath helix surrounded by a conducting wall. The effects of dielectric and vane loading are included in the formulation as is efficiency enhancement by tapering the helix pitch. Dielectric loading is described under the assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. The vane-loading model describes the insertion of an arbitrary number of vanes running the length of the helix, and the polarization of the field between the vanes is assumed to be an azimuthally symmetric transverse-electric mode. The field is represented as a superposition of azimuthally symmetric waves in a vacuum sheath helix. An overall explicit sinusoidal variation of the form exp(ikz−iωt) is assumed (where ω denotes the angular frequency corresponding to the wave number k in the vacuum sheath helix), and the polarization and radial variation of each wave is determined by the boundary conditions in a vacuum sheath helix. The propagation of each wave in vacuo as well as the interaction of each wave with the electron beam is included by allowing the amplitudes of the waves to vary in z and t. A dynamical equation for the field amplitudes is derived analogously to Poynting's equation, and solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Electron beams with a both a continuous and emission-gated pulse format are analyzed, and the model is compared with linear theory of the interaction as well as with the performance of a TWTs operated at the Naval Research Laboratory and at Northrop–Grumman Corporation. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871590
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  • 9
    Electronic Resource
    Electronic Resource
    First operation of a wiggler-focused, sheet beam free electron laser amplifierf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1708-1713 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A wiggler-focused, sheet beam free electron laser (FEL) amplifier utilizing a short-period wiggler magnet has been proposed as a millimeter-wave source for current profile modification and/or electron cyclotron resonance heating of tokamak plasmas. As proposed, such an amplifier would operate at a frequency of approximately 100–200 GHz with an output power of 1–10 MW CW. Electron beam energy would be in the range 500–1000 keV. To test important aspects of this concept, an initial sheet beam FEL amplifier experiment has been performed using a 1 mm×2 cm sheet beam produced by a pulse line accelerator with a pulse duration of 100 ns. The 500–570 keV, 4–18 A sheet beam is propagated through a 56 period uniform wiggler (λw=9.6 mm) with a peak wiggler amplitude of 2–5 kG. Linear amplification of a 5–10 W, 94 GHz signal injected in the TE01 rectangular mode is observed. All features of the amplified signal, including pulse shape and duration, are in accordance with the predictions of numerical simulation. Amplified signal gain has been measured as a function of injected beam energy, current, and wiggler field amplitude and is also in good agreement with simulation results. Continuation of this experiment will involve studying nonlinear amplifier operation and adding a section of tapered wiggler.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870674
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  • 10
    Electronic Resource
    Electronic Resource
    Poloidal spin-up of tokamak plasmas from poloidal asymmetry of particle and momentum sources (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 337-344 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Transport equations for particle density, toroidal angular momentum, and poloidal rotation are derived in the presence of strong particle and momentum sources in general axisymmetric toroidal geometry. It is shown that the state with no poloidal rotation is unstable on flux surfaces where the local particle source rate is sufficiently poloidally asymmetric and, roughly, more rapid than the damping rate of poloidal rotation from magnetic pumping. In agreement with earlier results, it is also shown that a toroidally injected momentum source can drive poloidal rotation if the momentum deposition is poloidally asymmetric. Thus poloidal rotation could be induced in tokamaks by asymmetric particle fueling.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870836
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