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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 1530-1534 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Generation of a uniform density plasma has been a key consideration in the design of microwave/plasma processing systems. A circularly polarized wave can generate a plasma with good azimuthal uniformity as well as provide strong resonant interaction with the plasma electrons. In this article, we report the development of a three-port polarization converter which efficiently converts the TE10 wave of a standard rectangular waveguide into a circularly polarized TE11 wave of a cylindrical waveguide. Employing the same principle for mode conversion, the converter is also made to function as a protective device of the high power microwave source by providing a separate port for the return and damping of the reflected wave. Such a converter has been analyzed, constructed, and tested. At the operating frequency of 2.45 GHz, test results indicate a 25 dB return loss, 97% end-to-end polarization conversion efficiency, and better than 20 dB rejection of the reflected wave. The structural simplicity of the device allows high power operation as well as easy construction for system research and applications. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 2404-2406 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel scheme producing multiple, coherently phased, high charge (nC), short duration (ps) electron bunches in a radio-frequency (rf) photoinjector is presented. In this configuration, the mirror photocathode is an integral part of an optical resonator. The ultrashort laser pulse is reflected off the photocathode and recirculated through the optical cavity at a subharmonic of the rf drive frequency after extracting a photoelectron bunch. This new technique can dramatically increase the effective quantum efficiency of metals and produce high current, high brightness, prebunched electron beams ideally suited for high power coherent microwave generation. A proof-of-principle experiment operating at 0.250 GHz and using a frequency-quadrupled, mode-locked Nd:YAG laser has shown the production of a train of coherently phased photoelectron bunches, with a measured effective quantum efficiency enhancement of 4.2. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 60 (1992), S. 292-294 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Computer simulation results reveal that the superradiant modes in free-electron laser amplifiers with high gain and long interaction length tend to degrade the amplifier performance. If these modes are kept at low levels, the amplifier efficiency can be significantly improved. This observation can be employed to interpret the recent MIT reversed guide field free-electron laser experiment.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2403-2408 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Mode interactions in the gyrotron traveling wave tube amplifier (gyro-TWT) are examined with particle simulations and compared with experimental observations. Studies focus on the competition between the absolute and convective instabilities. Detailed diagnostics on mode structures, field growth profiles, and evolution of electron distribution in momentum and energy spaces allow an in depth look at the basic physical processes, such as the mechanism of mode competition and suppression. Mutual verifications between the theory and experiment have strengthened understanding of the multimode behavior and added to the predictability of high-power gyro-TWT's.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    International journal of infrared and millimeter waves 2 (1981), S. 415-425 
    ISSN: 1572-9559
    Keywords: wide-band fast wave gyro-TWA ; tapered magnetic field ; bandwidth ; contoured waveguide ; streaming electrons
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A simple scheme is proposed for the construction of a wide-band fast wave gyrotron travelling wave amplifier. The key features of this scheme include a contoured waveguide, a tapered magnetic field, and injection of input signals in opposite direction of the streaming electrons. Bandwidths at tens of percents, and gain at tens of decibals, are projected.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    International journal of infrared and millimeter waves 2 (1981), S. 373-393 
    ISSN: 1572-9559
    Keywords: gyrotron travelling wave amplifier ; oscillations ; threshold currents ; oscillation suppression
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The operation of the gyrotron travelling wave amplifier is based on the convective cyclotron maser instability. It is found that this convective instability may become absolute (nonconvective) at a sufficiently high current level, resulting in oscillation instead of amplification. This threshold current for the transition depends sensitively on the applied magnetic field. The axial wavelength and the characteristic frequency of oscillation at the onset of absolute instability are given. It is found that momentum spread has virtually no effect on the threshold current. A small amount of resistive wall loss, however, raises the threshold current significantly. Oscillations due to partial reflection at the ends of the system are also examined. Preliminary experimental results on both types of oscillations are reported and are found to be in good agreement with the theory.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    International journal of infrared and millimeter waves 2 (1981), S. 395-413 
    ISSN: 1572-9559
    Keywords: Gyrotron amplifier ; Bandwidth ; Velocity spread ; Wall resistivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The small signal gain and bandwidth of the gyrotron travelling-wave amplifier (gyro-TWA) operating at the fundamental cyclotron harmonic are examined. The analytical and numerical studies focus on the effects of velocity spread in the electron beam and of a distributed wall resistivity in the waveguide. It is found that wall resistivity reduces the forward gain of the amplifier only by an amount approximately equal to 1/3 of the corresponding cold tube loss. Significant increase in bandwidth may result, under certain conditions, from the presence of wall resistivity. A moderate amount (5–10%) of velocity spread does not reduce the peak gain significantly in general, but may reduce the bandwidth by an amount depending on the applied magnetic field. Based on considerations of such factors as power, stability, gain, bandwidth, and efficiency, operating parameters for the gyro-TWA are suggested.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    International journal of infrared and millimeter waves 3 (1982), S. 705-723 
    ISSN: 1572-9559
    Keywords: gyrotron ; open resonator
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A multiple reflection approach is employed to examine the properties of the lowQ open resonators commonly used in high power gyrotrons. The resonant frequency, diffractionQ, and RF field profile are derived in closed forms in the lowQ limit. Formation of the eigenmode in the resonator is shown as the result of constructive wave interference. The lower limit ofQ in the context of a conventional definition ofQ is derived.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    International journal of infrared and millimeter waves 3 (1982), S. 645-665 
    ISSN: 1572-9559
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Gyrotron oscillators are of great interest as sources of high power mm wave radiation for electron cyclotron resonance heating and current drive in magnetic fusion research devices. Gyrotrons capable of efficiently generating cw power ≳ 1 megawatt will be required in future magnetic fusion studies. However, as gyrotron power approaches the megawatt level, a very large, overmoded cavity must be employed in order to keep ohmic power losses in the cavity at an acceptable level, and the problem of mode overpopulation becomes severe. Also, it becomes increasingly important to optimize gyrotron efficiency for a number of important reasons including minimizing the problem of collecting the electron beam energy. In the present paper, a detailed experimental and theoretical study of mode competition and mode locking in an overmoded gyrotron is presented. Efficiency enhancement (to 60%) and high peak power (430 kW) were achieved in the TE2,4,1 mode using magnetostatic profiling in the cavity. With selective mode suppression, peak power of 475 kW was generated in the TE0,4,1 mode.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    International journal of infrared and millimeter waves 5 (1984), S. 37-56 
    ISSN: 1572-9559
    Keywords: Gyrotron ; oscillator ; slotted structure ; harmonics millimeter waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A small signal theory is presented of the harmonic gyrotron oscillator with a slotted resonator. The magnetron-type wall structure has the effect of creating a strong harmonic field near the electron orbits and, as a result, greatly reducing the beam voltage and start-oscillation power required for harmonic operations. Characteristics of operation and methods of optimization are studied and numerically illustrated. It is shown that at a given harmonic the startoscillation beam power is lower at a lower beam voltage.
    Type of Medium: Electronic Resource
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