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  • 1990-1994  (19)
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Material
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Year
  • 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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    Paper (German National Licenses)
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  • 2
    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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    Paper (German National Licenses)
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  • 3
    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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    Paper (German National Licenses)
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  • 4
    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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    Paper (German National Licenses)
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  • 5
    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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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Space charge instabilities in gyrotron beams (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 714-729 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Space charge instabilities in gyrotron electron beams are studied via both the solution of the linear dispersion equation and by nonlinear particle simulation. The linear analysis addresses the stabilizing effects of energy spread in the beam and the inhomogeneity of the external magnetic field. The nonlinear multifrequency simulation demonstrates that for a uniform magnetic field, the energy spread induced by many unstable waves will be larger than the corresponding spread induced by a single unstable wave. This emphasizes the need to design electron guns that are stable to this mode.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870817
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    Paper (German National Licenses)
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  • 7
    Electronic Resource
    Electronic Resource
    Experimental realization of millimeter-wave amplification by a sheet beam free electron laser (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 1439-1441 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report an observation of millimeter-wave (94 GHz) amplification in a sheet beam, short period, planar wiggler, free electron laser amplifier. The observed gain is about 5 dB for a 530 keV, 4 A beam through a 54 cm wiggler. Wave energy absorption was also observed when the beam energy is off-resonance. Experimental results are in good agreement with numerical simulation. This amplifier configuration has potential for producing equally high output power but at relatively low voltage compared with longer period free electron lasers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111909
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    Paper (German National Licenses)
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  • 8
    Electronic Resource
    Electronic Resource
    Self-focusing and Raman scattering of laser pulses in tenuous plasmas (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1440-1452 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The propagation and self-focusing of short, intense laser pulses in a tenuous plasma is studied both analytically and numerically. Specifically, pulses of length of the order of a few plasma wavelengths and of intensity, which is large enough for relativistic self-focusing to occur, are considered. Such pulses are of interest in various laser plasma acceleration schemes. It is found that these pulses are likely to be strongly affected by Raman instabilities. Two different regimes of instability, corresponding to large and small scattering angles, are found to be important. Small-angle scattering is perhaps the most severe since it couples strongly with relativistic self-focusing, leading the pulses to acquire significant axial and transverse structure in a time of the order of the self-focusing time. Thus it will be difficult to propagate smooth self-focused pulses through tenuous plasmas for distances longer than the Rayleigh length, except for pulse duration of the order of the plasma period.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860884
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    Paper (German National Licenses)
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  • 9
    Electronic Resource
    Electronic Resource
    Nonlinear theory of stable, efficient operation of a gyrotron at cyclotron harmonics (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 4473-4485 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One of the main obstacles in achieving stable, efficient operation at the cyclotron harmonics in a gyrotron is mode competition with parasitic modes at the fundamental frequency. In this article, the nonlinear dynamics of mode interactions in such a system are studied using a multifrequency, time-dependent model. The results of numerical simulations for a second harmonic gyrotron are presented by considering two starting scenarios: (a) fast voltage rise or an instant turn-on case, and (b) slow voltage rise case. For the first case, it is demonstrated that for a certain range of operating parameters, the presence of a parasitic mode at the fundamental can be helpful in the excitation of the second harmonic operating mode. In the second case, it is found that the unstable operating region increases with the value of the rise time constant of the electrode voltages. Stable, efficient gyrotron operation at the second harmonic is demonstrated using the numerical study.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860562
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    Paper (German National Licenses)
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  • 10
    Electronic Resource
    Electronic Resource
    Relativistic plasma microwave electronics: Studies of high-power plasma-filled backward-wave oscillators (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2286-2292 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The area of relativistic plasma microwave electronics has only recently generated renewed interest. New experimental data are presented demonstrating that the presence of a low-density background plasma in a relativistic backward-wave oscillator leads to several beneficial effects, including (a) enhanced interaction efficiency (40%), (b) operation at very low and possibly zero guiding magnetic field, (c) tunability by controlling the plasma density, (d) high degree of spectral coherency, and (e) operation well above the vacuum limiting current.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860198
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    Paper (German National Licenses)
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