K. R. Chu

National Taiwan University, T’ai-pei, Taipei, Taiwan

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Publications (185)218.93 Total impact

  • M. K. Shen, K. R. Chu
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    ABSTRACT: Electromagnetic wave behavior in a conducting medium is a thought-provoking subject for a graduate-level electrodynamics course. Here, we focus on electromagnetic waves incident upon a conductor and highlight how the same dispersion relation, spanning 20 orders of magnitude in frequency, transforms the conductor from a perfectly reflecting to a perfectly transparent medium according to the classical free-electron model. We show that the spectral responses of the conductor can be divided into three radically different regimes. This article presents a graphic illustration of wave reflection, transmission, and penetration properties for copper in these regimes, along with physical interpretations and a brief discussion on the limitations of the free-electron model.
    01/2014; 82(2).
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    ABSTRACT: Microwave applicators are widely employed for materials heating in scientific research and industrial applications, such as food processing, wood drying, ceramic sintering, chemical synthesis, waste treatment, and insect control. For the majority of microwave applicators, materials are heated in the standing waves of a resonant cavity, which can be highly efficient in energy consumption, but often lacks the field uniformity and controllability required for a scientific study. Here, we report a microwave applicator for rapid heating of small samples by highly uniform irradiation. It features an anechoic chamber, a 24-GHz microwave source, and a linear-to-circular polarization converter. With a rather low energy efficiency, such an applicator functions mainly as a research tool. This paper discusses the significance of its special features and describes the structure, in situ diagnostic tools, calculated and measured field patterns, and a preliminary heating test of the overall system.
    Review of Scientific Instruments 01/2014; 85(8):084703-084703-5. · 1.60 Impact Factor
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    ABSTRACT: Relativistic electronics research in recent years has produced powerful millimeter waves on the MW level, while also extending the frequency range into the terahertz (THz) region and beyond. These developments have opened up new horizons in applications. The current study is motivated by the associated need for high-power absorbers not readily available at such frequencies. Our focus is on effective absorber schemes which can handle high power while also possessing a structural simplicity for easy implementation. In and above the THz region, the electrical conductivity can no longer be treated as a real constant. We begin with a derivation of the field penetration depth applicable to all frequencies. Requirements to meet the intended criteria are then determined from the wave penetration and reflection properties. Design examples in the 1-1000 GHz range are illustrated, which consist of a thin lossy conducting layer on the surface of a pyramidal shaped metal base. It is shown in theory that such structures can function effectively in the millimeter and THz regions.
    Physics of Plasmas 10/2013; 20(10):3301-. · 2.38 Impact Factor
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    ABSTRACT: Electron cyclotron maser interactions at terahertz (THz) frequencies require a high-order-mode structure to reduce the wall loss to a tolerable level. To generate THz radiation, it is also essential to employ cyclotron harmonic resonances to reduce the required magnetic field strength to a value within the capability of the superconducting magnets. However, much weaker harmonic interactions in a high-order-mode structure lead to serious mode competition problems. The current paper addresses harmonic mode competition in the gyrotron backward wave oscillator (gyro-BWO). We begin with a comparative study of the mode formation and oscillation thresholds in the gyro-BWO and gyromonotron. Differences in linear features result in far fewer 'windows' for harmonic operation of the gyro-BWO. Nonlinear consequences of these differences are examined in particle simulations of the multimode competition processes in the gyro-BWO, which shed light on the competition criteria between modes of different as well as the same cyclotron harmonic numbers. The viability of a harmonic gyro-BWO is assessed on the basis of the results obtained.
    Physics of Plasmas 10/2012; 19(10). · 2.38 Impact Factor
  • S. H. Kao, C. C. Chiu, K. R. Chu
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    ABSTRACT: We present a theoretical study on some of the key physics issues associated with over-moded, sub-terahertz, and terahertz (THz) gyrotron oscillators. Simulations of a large number of fundamental and second cyclotron harmonic modes yield a broad database for physics observations as well as a test of relevant scaling laws. Linear properties over a broad magnetic field range exhibit a number of interesting trends. Nonlinearly, despite the possibility of multimode excitation, each mode is found to exist as a dominant single mode in a narrow magnetic field range, and a significant fraction of these modes are due to second cyclotron harmonic interactions. The wall resistivity, while a relatively minor concern for sub-terahertz gyrotrons, is shown to play a radically different role in the THz regime. It affords a linear advantage to the harmonic modes while also significantly degrading their output efficiencies. These results are interpreted in terms of the nature of harmonic mode competition and the scaling laws for the cavity quality factors.
    Physics of Plasmas 02/2012; 19(2). · 2.38 Impact Factor
  • S H Kao, C C Chiu, K F Pao, K R Chu
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    ABSTRACT: Cyclotron harmonic interactions are a key physics issue of critical importance to the generation of terahertz radiation via the electron cyclotron maser instability for practical magnetic field strengths. We present an inherent mechanism, as well as a deciding factor, which governs the competition between low- and high-harmonic interactions. Multimode simulations reveal the physical process in which a significant advantage develops for the lower-harmonic interaction, which eventually dominates in the fully nonlinear stage. The results also suggest a start-up scenario for persistent higher-harmonic operation.
    Physical Review Letters 09/2011; 107(13):135101. · 7.73 Impact Factor
  • K.R. Chu, W.M. Manheimer
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    ABSTRACT: The dispersion relation for the dissipative trapped-electron mode including the effects of magnetic shear, finite ion Larmor radius, banana-plateau velocity space separation, magnetic curvature drift resonance, ion-ion collisions, etc. is used to examine the plasma stability of recent tokamak experiments. Regimes of importance of various effects in these experiments are also discussed. Instabilities mostly occur at kθ ρi ~ 1, where kθ is the azimuthal wave number and ρi is the ion Larmor radius. The often used anomalous heat conduction coefficient, , predicts losses comparable to those observed in TFR, ST, and ATC.
    Nuclear Fusion 01/2011; 18(1):29. · 2.73 Impact Factor
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    ABSTRACT: The non-linear theory of one-dimensional mode-coupling stabilization for the dissipative trapped-electron instability is presented. This non-linear process couples unstable modes at low wave number to damped waves at high wave number. The regions of parameter space in which this process is applicable are carefully specified and the saturated wave spectra and anomalous transport coefficients are calculated numerically for a range of parameters of interest for large tokamak operation. The transport coefficients calculated are appreciably smaller than the usual γ/k2 estimates in moderate- and high-temperature regimes.
    Nuclear Fusion 01/2011; 16(2):203. · 2.73 Impact Factor
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    ABSTRACT: Physics and performance issues of a TE01-mode gyrotron traveling-wave amplifier are studied in theory. For a high order mode, absolute instabilities on neighboring modes at the fundamental and higher cyclotron harmonic frequencies impose severe constraints to the device capability. Methods for their stabilization are outlined, on the basis of which the performance characteristics are examined in a multidimensional parameter space under the marginal stability criterion. The results demonstrate the viability of a high-order-mode traveling-wave amplifier and provide a roadmap for design tradeoffs among power, bandwidth, and efficiency. General trends are observed and illustrated with specific examples.
    Physics of Plasmas 11/2010; 17(11):113104-113104-6. · 2.38 Impact Factor
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    ABSTRACT: We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotoron oscillator. It is shown through the particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. A physical interpretation is given.
    Infrared Millimeter and Terahertz Waves (IRMMW-THz), 2010 35th International Conference on; 10/2010
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    ABSTRACT: We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotoron oscillator. It is shown through the particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. A physical interpretation is given.
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    ABSTRACT: Performance of a fundamental cyclotron harmonic, TE01-mode gyrotron traveling-wave amplifiers (gyro-TWTs) is studied theoretically under the marginal stability criterion. The interaction circuit is shown in Fig. 1. As is illustrated in Fig. 2 for the parameters in Table I, absolute instabilities on both lower and higher order modes and at the fundamental and second cyclotron harmonic frequency present severe constraints to the stable operating current. General methods for stabilizing these unwanted oscillations are outlined, on the basis of which the parametric dependence of gain, power, bandwidth and efficiency on the beam velocity ratio, circuit lengths, wall resistivity, and velocity spread is obtained under the marginal stability criterion. The results allow a tradeoff among gain, power, bandwidth, and efficiency to reach a desired design.
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    ABSTRACT: Efficiency enhancement in a gyrotron backward wave oscillator (gyro-BWO) with a nonlinearly tapered waveguides is investigated with a self-consistent code. It is shown that the interaction efficiency can be enhanced significantly to a value as high as 61%. Mechanism of the efficiency enhancement is interpreted physically via phase space diagrams.
  • S.H. Kao, C.C. Chiu, K.R. Chu
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    ABSTRACT: We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotron backward-wave oscillator (gyro-BWO). It is shown through particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. A physical interpretation is given.
    Infrared, Millimeter, and Terahertz Waves, 2009. IRMMW-THz 2009. 34th International Conference on; 10/2009
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    ABSTRACT: Electron beam quality is essential to the performance of millimeter-wave gyroamplifiers, particularly the gyrotron traveling-wave tube amplifier, which is extremely sensitive to the electron velocity spread and emission uniformity. As one moves up in power and frequency, the quality of the electron beam becomes even more critical. One aspect of the electron beam formation technology which has received relatively little attention has been the performance analysis of the electron beam itself. In this study, a 100 kV, 8 A magnetron injection gun with a calculated perpendicular-to-parallel velocity ratio of 1.4 and axial velocity spread of 3.5% has been designed, tested, and analyzed. It is shown that the equipment precision and a fully relativistic data analysis model afford sufficient resolution to allow a verification of the theoretical predictions as well as a quantitative inference to the surface roughness of the cathode used.
    Physics of Plasmas 09/2009; 16(9):093111-093111-5. · 2.38 Impact Factor
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    ABSTRACT: By delivering unprecedented power and gain, the gyrotron traveling-wave amplifier (gyro-TWT) offers great promise for advanced millimeter wave radars. However, the underlying physics of this complex nonlinear system is yet to be fully elucidated. Here, we report a new phenomenon in the form of nonlinearly driven oscillations. A zero-drive stable gyro-TWT is shown to be susceptible to a considerably reduced dynamic range at the band edge, followed by a sudden transition into driven oscillations and then a hysteresis effect. An analysis of this unexpected behavior and its physical interpretation are presented.
    Physics of Plasmas 12/2008; 15(12):123109-123109-4. · 2.38 Impact Factor
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    ABSTRACT: Improving the electron beam power and quality is essential to increasing the power and performance of the gyro-TWT. We have designed, and tested, a new magnetron injection gun (MIG) with nominal alpha=1.5 and calculated axial velocity spread (via EGUN) of 3.9% (neglecting cathode surface roughness, emission non-uniformity, and other defects). The gun is designed to nominally operate at 100 kV with ~8 Amp beam current at a cathode current density of 30 A/cm<sup>2</sup> and uses Scandate impregnated cathodes. Beam analyzer tests show that the gun performs with excellent beam quality and that the design is flexible with respect to maintaining the beam quality with variable operating parameters such as voltage, current, and compression. This is a low compression (only 12) thin beam design. The surface quality of advanced high current density cathodes can be inferred from the beam quality measurements. Advanced gyro-TWT design using the improved MIG is discussed.
    Vacuum Electronics Conference, 2008. IVEC 2008. IEEE International; 05/2008
  • Source
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    ABSTRACT: Stability issues have been a major concern for the realization of broadband tunability of the gyrotron backward-wave oscillator gyro-BWO. Multimode, time-dependent simulations are employed to examine the stability properties of the gyro-BWO. It is shown that the gyro-BWO is susceptible to both nonstationary oscillations and axial mode competition in the course of frequency tuning. Regions of nonstationary oscillations and axial mode competition are displayed in the form of stability maps over wide-ranging parameter spaces. These maps serve as a guide for the identification and optimization of stable windows for broadband tuning. Results indicate that a shorter interaction length provides greater stability without efficiency degradation. These theoretical predictions have been verified in a Ka-band gyro-BWO experiment using both short and long interaction lengths. In the case of a short interaction length, continuous and smooth tunability, in magnetic field and in beam voltage, was demonstrated with the high interaction efficiency reported so far. A maximum 3-dB tuning range of 1.3 GHz with a peak power of 149 kW at 29.8% efficiency was achieved. In a comparative experiment with a longer interaction length, the experimental data are characterized by piecewise-stable tuning curves separated by regions of nonstationary oscillations, as predicted by theory. © 2007 American Institute of Physics.
    Physics of Plasmas 09/2007; · 2.38 Impact Factor
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    ABSTRACT: In recent years, superconducting RF cavities have been increasingly adopted for use in modern accelerator and light-source facilities. The microwave power is coupled into the accelerating cavity typically through a waveguide or a coaxial power coupler. The multipactor discharge in the RF power coupler constitutes one of the most serious problems to the reliable operation of superconducting cavities. Due to the lack of multipacting-free RF structures, RF processing of the RF power coupler becomes a required and routine procedure to ensure multipacting-free operation of a superconducting RF system. However, for a superconducting cavity with negligible wall losses, variations of the beam loading lead to significant magnitude and phase variations of the wave reflected from the cavity. This greatly complicates the multipacting behavior in the RF power coupler. Hence RF processing under fixed conditions, as is conventionally done, is inadequate for multipactor processing of RF power couplers used in superconducting cavities. Here we report the concept, design, and demonstration of a novel tunable reflecting load for multipactor processing of the RF power coupler. The load realistically models the RF cavity through a broad tuning range of the reflection coefficient, both in magnitude and in phase. It thus allows, for the first time, off-line multipactor processing of the RF power coupler under machine operational conditions before it is attached to the superconducting cavity. A high power prototype was assembled in-house using standard WR1800 rectangular waveguide components. Results of cold and high-power test up to 50 kW, CW were found to be in good agreement with theoretical predictions.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
  • Source
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    ABSTRACT: Selective suppression of high order axial modes of the gyrotron backward-wave oscillator gyro-BWO is investigated in theory and in experiment. The gyro-BWO interaction is much more efficient in a down-tapered interaction structure, while it is also more susceptible to the problem of axial mode competition in such a structure. Because higher order axial modes at a higher oscillation frequency penetrate deeper into the interaction structure, application of distributed wall loss at the downstream end of the interaction structure is shown to be effective for selective suppression of these modes with minor effects on the efficiency of the desired fundamental axial mode. A stable gyro-BWO operating in a single mode throughout the entire beam pulse is demonstrated on the basis of this principle. Theoretical and experimental results are found to be in good agreement. © 2007 American Institute of Physics.
    Physics of Plasmas 07/2007; · 2.38 Impact Factor

Publication Stats

1k Citations
218.93 Total Impact Points


  • 2010–2014
    • National Taiwan University
      • Department of Physics
      T’ai-pei, Taipei, Taiwan
  • 2011
    • United States Naval Research Laboratory
      Washington, Washington, D.C., United States
  • 1988–2009
    • National Tsing Hua University
      • Department of Physics
      Hsinchu, Taiwan, Taiwan
  • 1994–2008
    • University of California, Davis
      • Department of Electrical and Computer Engineering
      Davis, CA, United States
  • 2007
    • National Synchrotron Radiation Research Center (NSRRC)
      • Light Source Division
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2006
    • National Hsinchu University of Education
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2002
    • University of Maryland, College Park
      • Department of Electrical & Computer Engineering
      College Park, MD, United States
  • 2000
    • Tsinghua University
      • Department of Physics
      Beijing, Beijing Shi, China
  • 1989–1992
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, CA, United States