[Show abstract][Hide abstract] ABSTRACT: To generate terahertz radiation via the electron cyclotron maser instability, harmonic interactions are essential in order to reduce the required magnetic field to a practical value. Also, high-order mode operation is required to avoid excessive Ohmic losses. The weaker harmonic interaction and mode competition associated with an over-moded structure present challenging problems to overcome. The axis-encircling electron beam is a well-known recipe for both problems. It strengthens the harmonic interaction, as well as minimizing the competing modes. Here, we examine these advantages through a broad data base obtained for a low-power, step-tunable, gyrotron oscillator. Linear results indicate far more higher-harmonic modes can be excited with an axis-encircling electron beam than with an off-axis electron beam. However, multi-mode, time-dependent simulations reveal an intrinsic tendency for a higher-harmonic mode to switch over to a lower-harmonic mode at a high beam current or upon a rapid current rise. Methods are presented to identify the narrow windows in the parameter space for stable harmonic interactions.
Physics of Plasmas 04/2015; 22(4):043109. DOI:10.1063/1.4916907 · 2.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The surface resistance of metals, and hence the Ohmic dissipation per unit area, scales with the square root of the frequency of an incident electromagnetic wave. As is well recognized, this can lead to excessive wall losses at terahertz (THz) frequencies. On the other hand, high-frequency oscillatory motion of conduction electrons tends to mitigate the collisional damping. As a result, the classical theory predicts that metals behave more like a transparent medium at frequencies above the ultraviolet. Such a behavior difference is inherent in the AC conductivity, a frequency-dependent complex quantity commonly used to treat electromagnetics of metals at optical frequencies. The THz region falls in the gap between microwave and optical frequencies. However, metals are still commonly modeled by the DC conductivity in currently active vacuum electronics research aimed at the development of high-power THz sources (notably the gyrotron), although a small reduction of the DC conductivity due to surface roughness is sometimes included. In this study, we present a self-consistent modeling of the gyrotron interaction structures (a metallic waveguide or cavity) with the AC conductivity. The resulting waveguide
attenuation constants and cavity quality factors are compared with those of the DC-conductivity model. The reduction in Ohmic losses under the AC-conductivity model is shown to be increasingly significant as the frequency reaches deeper into the THz region. Such effects are of considerable importance to THz gyrotrons for which the minimization of Ohmic losses constitutes a major design consideration.
Physics of Plasmas 01/2015; 22(1):013108. DOI:10.1063/1.4905627 · 2.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Open cavities are often employed as interaction structures in a new generation of coherent millimeter, sub-millimeter, and terahertz (THz) radiation sources called the gyrotron. One of the open ends of the cavity is intended for rapid extraction of the radiation generated by a powerful electron beam. Up to the sub-THz regime, the diffraction loss from this open end dominates over the Ohmic losses on the walls, which results in a much lower diffraction quality factor (Q(d)) than the Ohmic quality factor (Q(ohm)). Early analytical studies have led to various expressions for Q(d) and shed much light on its properties. In this study, we begin with a review of these studies, and then proceed with the derivation of an analytical expression for Q(d) accurate to high order. Its validity is verified with numerical solutions for a step-tunable cavity commonly employed for the development of sub-THz and THz gyrotrons. On the basis of the results, a simplified equation is obtained which explicitly expresses the scaling laws of Q(d) with respect to mode indices and cavity dimensions. (C) 2014 AIP Publishing LLC.
Physics of Plasmas 10/2014; 21(10). DOI:10.1063/1.4900415 · 2.14 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
American Journal of Physics 01/2014; 82(2). DOI:10.1119/1.4837435 · 0.96 Impact Factor
[Show abstract][Hide abstract] 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-. DOI:10.1063/1.4825147 · 2.14 Impact Factor
[Show abstract][Hide abstract] 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). DOI:10.1063/1.4757215 · 2.14 Impact Factor
[Show abstract][Hide abstract] 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). DOI:10.1063/1.3684663 · 2.14 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1063/1.3505945 · 2.14 Impact Factor
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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. DOI:10.1063/1.3227649 · 2.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Precise measurement of the power generated by a virtual cathode oscillator has been a difficult task because of the ultra short duration of the microwave pulse and the complexity of its frequency spectrum. Here, we report a resolution of this problem through the time-frequency analysis of the output signal. The results reveal that the output spectrum is not only broadband in content, but also display a composition which varies significantly in a stepwise manner over the duration of the pulse. The resulting time-resolved spectrum sheds light on the physical processes involved. It also allows a proper account for the frequency sensitivity of the measuring apparatus and thereby an accurate determination of the output power from the experimental data.
[Show abstract][Hide abstract] 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.