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Plasma Science, IEEE Transactions on

Issue 3  Part 1 • Date June 2004

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Displaying Results 1 - 25 of 62
  • Table of contents

    Page(s): c1 - 834
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  • IEEE Transactions on Plasma Science publication information

    Page(s): c2
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  • A tribute to Georges Mourier (1923-2003)

    Page(s): 838 - 840
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  • Startup scenarios in high-power gyrotrons

    Page(s): 841 - 852
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (416 KB) |  | HTML iconHTML  

    To realize continuous-wave (CW) operation of millimeter-wave gyrotrons at megawatt (MW)-power levels, these devices must operate in very high-order modes. To excite such an operating mode and to drive it into the regime of MW-level operation with high efficiency requires careful consideration of the startup scenario through which the operating parameters of the device are brought to their nominal values. In the present paper, several common startup scenarios and the most important physical effects associated with them are discussed. Then, the paper presents the results of startup simulations for a 140-GHz, MW-class gyrotron developed by Communications and Power Industries (CPI) for electron-cyclotron plasma heating and current drive experiments on the "Wendelstein 7-X" stellarator. The simulations were done with MAGY, a multifrequency, self-consistent code developed at the University of Maryland. Simulations tracking six competing modes show that, with a proper choice of operating parameters, stable excitation of the desired TE28,7-mode at 1 MW level can be realized, despite the presence of dangerous parasites in the resonator spectrum. These results are in approximate agreement with experimental tests, in which the gyrotron demonstrated reliable operation at power levels up to 900 kW. PACS numbers: 84.40.Ik, 52.75 Ms. View full abstract»

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  • 165-GHz coaxial cavity gyrotron

    Page(s): 853 - 860
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    The basic investigations on a coaxial cavity gyrotron operating at 165 GHz have been accomplished and the feasibility of manufacturing a 2-MW continuous-wave coaxial cavity gyrotron at 170 GHz has been demonstrated. In the last measurements, the data required for fabrication of an industrial tube have been completed. The influence of misalignment of the insert on gyrotron operation has been investigated. The limitation of the high-voltage performance of the electron gun due to buildup of a Penning discharge has been suppressed and the leakage current to the insert has been measured as a function of the retarding collector voltage. View full abstract»

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  • Ohmic losses in coaxial gyrotron cavities with corrugated insert

    Page(s): 861 - 866
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    Ohmic losses in coaxial gyrotron cavities with corrugated insert are calculated on the basis of two theories-the surface impedance model (SIM) and the singular integral equation (SIE). It is found that SIE predicts significantly lower losses. View full abstract»

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  • Demonstration of a high-power long-pulse 140-GHz gyrotron oscillator

    Page(s): 867 - 876
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    A high-power long-pulse 140-GHz gyrotron oscillator has been designed, constructed, and tested. Key features of the gyrotron include an internal converter to transform the TE28,7,1 operating mode to a Gaussian output beam, a single-stage-depressed collector, and a chemical-vapor-deposition diamond output window. Peak output powers up to 930 kW at 34% efficiency have been demonstrated at 5-ms pulse lengths. Due to power supply limitations, long-pulse operation was not possible for beam currents above 25 A. At 25-A beam current and 500-kW output power, pulse lengths up to 700 s in duration were achieved. The gyrotron has been shipped to the Wendelstein 7-X facility in Greifswald, Germany, where long-pulse demonstrations up to 180s will be carried out at the 930-kW power level. View full abstract»

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  • Studies of the 1.5-MW 110-GHz gyrotron experiment

    Page(s): 877 - 883
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    Results of a 1.5-MW 110-GHz short-pulse (3 μs) gyrotron experiment are reported. The gyrotron magnetron injection gun operated at full voltage (96 kV) and current (40 A), producing up to 1.4 MW at 110 GHz in the TE22,6 mode. The operation of the TE22,6 mode, as well as nearby modes, was measured as a function of magnetic field at the cavity and at the electron gun to produce a mode map. Significant mode competition was found, but the measured efficiency of 37% in the TE22,6 mode, without a depressed collector, is close to the design value of 39%. The beam alpha, the ratio of transverse to axial velocity in the electron beam, was measured with a probe. The alpha value was found to be 1.33 when the gyrotron was operating at conditions for achieving the highest output power level (1.4 MW.) This value of alpha is less than the design value of 1.4, possibly accounting for the slightly reduced experimental efficiency. The output power and efficiency, as a function of magnetic field, beam voltage, and beam current, are in good agreement with nonlinear theory and simulations with the MAGY code. These results are promising for the development of an industrial version of this gyrotron capable of long pulse or continuous-wave operation. View full abstract»

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  • Frequency-tunable CW gyro-BWO with a helically rippled operating waveguide

    Page(s): 884 - 889
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    Operation of a continuous wave gyrotron backward-wave oscillator (gyro-BWO) with a helically rippled operating waveguide has been experimentally studied. The gyro-BWO exploits a dc oil-cooled magnet with magnetic field up to 0.5 T and utilizes a weakly relativistic (20 keV) electron beam produced by a magnetron injection gun. Stable generation at the second cyclotron harmonic with a maximum power of 7 kW and an efficiency of 15% at a frequency of 24.7 GHz was achieved. Smooth oscillation frequency tuning by varying the magnetic field was measured to be as wide as 5% at the half-power level. The first gyro-BWO operation with a single-stage energy recovery system was realized. The use of a depressed collector provided an efficiency increase up to 23% and an opportunity for reduction of the main power supply voltage down to 10 kV. View full abstract»

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  • Evolution of an electron beam with azimuthal density nonuniformity in a cylindrical beam tunnel

    Page(s): 890 - 898
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    We present the results on the deformation of the cross section of an electron beam in a cylindrical beam tunnel with ∂/∂z=0 and homogeneous magnetic field, due to azimuthally nonuniform current density distribution. For this purpose, a new three-dimensional, self-consistent, trajectory code has been used. In addition, a simple theoretical model has been seen to be adequate for the qualitative and quantitative interpretation of the numerical results. It has been found, that the azimuthal nonuniformity causes effects clearly associated with&oarr;E×&oarr;B drifts, but in absolute terms these effects are too small to affect the performance of a gyrotron. View full abstract»

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  • Reflections in gyrotrons with axial output

    Page(s): 899 - 902
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    Influence of reflections on operation of gyrotrons with axial output is studied both theoretically and experimentally. By way of example the Fukui large orbit gyrotron with a permanent magnet operating in third harmonic at frequency 89 GHz is considered. In the case of strong reflection (|R|=0.6), extreme sensitivity of output power on the reflection phase is found. A qualitative agreement between theory and experiment is observed. View full abstract»

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  • A high harmonic gyrotron with an axis-encircling electron beam and a permanent magnet

    Page(s): 903 - 909
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (552 KB) |  | HTML iconHTML  

    A gyrotron with an axis-encircling electron beam is capable of high-frequency operation, because the high-beam efficiency is kept even at high harmonics of the electron cyclotron frequency. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already operated successfully at the third, fourth, and fifth harmonics. The frequencies are 89.3, 112.7, and 138 GHz, respectively, and the corresponding cavity modes are TE311, TE411, and TE511. The permanent magnet system is quite novel and consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in the cavity and electron gun regions. The magnetic field in the cavity region can be varied from 0.97 to 1.18 T. At the magnetic field intensities, the output powers at the third and the fourth harmonics are 1.7 and 0.5 kW, respectively. The gyrotron is pulsed, the pulse length is 1 ms and the repetition frequency is 1 Hz. The beam energy is 40 kV and the beam current is 1.2-1.3 A. Beam efficiencies and emission patterns have also been measured. In this paper, the experimental results of the gyrotron are described and compared with computer simulations. View full abstract»

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  • Development of high-frequency, highly stable gyrotrons as Millimeter to submillimeter wave radiation sources

    Page(s): 910 - 916
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    High-frequency, frequency-tunable, medium-power gyrotrons (Gyrotron FU series) are being developed in Fukui University as millimeter to submillimeter wave sources. The gyrotron series has achieved frequency tuneability in a wide range from 38 to 889 GHz and medium output power from 0.1 kW to several kilowatts in pulse mode (the pulse width is 1 ms and repetition rate 1 Hz) and several tens of watts in CW mode. Stabilizations of both amplitude and frequency have been achieved, for the application to many fields. High purity mode operations by installation of carefully designed cavity were also achieved for highly efficient conversion to Gaussian beam. In this paper, the advances in recent few years are summarized. View full abstract»

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  • Design considerations for powerful continuous-wave second-Cyclotron-harmonic coaxial-cavity gyrotrons

    Page(s): 917 - 928
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (640 KB) |  | HTML iconHTML  

    The mode-selection ability of coaxial gyrotron cavities with a longitudinally corrugated inner conductor is systematically examined, as regards powerful, continuous-wave (CW) operation at the second harmonic of the electron-cyclotron frequency. Suitable cavity geometries for the excitation of a high-order mode of eigenvalue around 100 at the second harmonic have been identified, employing coaxial inserts both of high and of low conductivity. Focusing on submillimeter-wave radiation, we investigate frequency and CW limitations on efficiency, output power, and cavity machining and we present pertinent design considerations. The feasibility of powerful and efficient, CW, submillimeter-wave operation of coaxial cavities at the second harmonic is illustrated by two realistic designs of a second-harmonic gyrotron delivering a power over 100 kW CW at 340 GHz with electronic efficiency around 29%. The performance of the designs is confirmed by independent numerical simulations, which in addition show weak sensitivity to a spread in the electron velocity ratio. View full abstract»

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  • Regime of nonresonant trapping in a CARM oscillator

    Page(s): 929 - 933
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    A method for realization of the regime of nonresonant electron trapping in an oscillator scheme of the cyclotron autoresonance maser (CARM) is proposed and theoretically studied. A possibility to achieve simultaneously a high electron efficiency and very weak sensitivity to the electron-beam quality is demonstrated for a moderately-relativistic millimeter-wavelength CARM. View full abstract»

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  • Coaxial gyrotrons: past, present, and future (review)

    Page(s): 934 - 946
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    Most of the present-day millimeter-wave gyrotrons developed for plasma experiments in controlled fusion reactors utilize cylindrical cavities operating in high-order modes. The choice of modes should obey certain restrictions dictated by the achievable mode selection and the maximum admissible level of the density of microwave ohmic losses in the cavity walls. Even with these restrictions, developers have successfully manufactured quasi-continuous-wave gyrotrons operating in the short millimeter wavelength bands that are capable of delivering microwave power on the order of 1 MW. To upgrade gyrotron power to the level of several megawatts, more complicated coaxial microwave circuits should be used. This statement is also valid for relativistic gyroklystrons, which are currently under development for driving future linear accelerators. This paper presents an overview of the history of the development of coaxial gyrodevices, a discussion of the physics-based issues which are the most important for their operation, a description of the state of the art in the development of coaxial gyrodevices for the above-mentioned applications, and a brief forecast for their future. View full abstract»

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  • A TE11 Ka-band gyro-TWT amplifier with high-average power compatible distributed loss

    Page(s): 947 - 956
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    Current amplifier research at the Naval Research Laboratory Vacuum Electronics Branch emphasizes techniques to extend the bandwidth and average power capability of gyro devices for millimeter wave radar applications. This paper will discuss the implementation of a wideband high-gain gyro-traveling wave tube amplifier design, with a measured peak output power of 78 kW, gain ∼60 dB, and a 3-dB bandwidth of 4.2 GHz (12%) at 52 kW in Ka-band. The 3-dB saturated bandwidth at 70 kW is 6 GHz (17%), which is also the instantaneous bandwidth with appropriately tailored input power (e.g., gain equalizer). The amplifier operates in the TE11 mode and for stabilization employs a high-average power compatible diffractive loading technique. View full abstract»

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  • Comparison of multistage gyroamplifiers operating in the frequency-multiplication regime with gyroamplifiers operating at a given Cyclotron harmonic

    Page(s): 957 - 969
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    The operation of gyrodevices at cyclotron harmonics is very attractive because of the possibility to reduce the magnetic field requirement by s times, where s is the cyclotron harmonic number. In recent years, two methods of harmonic operation in multistage gyroamplifiers have been actively studied: operation at a given harmonic in all stages and operation in the frequency-multiplying regime when the input stage operates at a lower harmonic than the output one. The present paper is aimed at making a comparative analysis of these two schemes of operation. To do this, a simple analytical method is developed, which allows one to qualitatively describe saturation effects in both schemes and compare such performance characteristics as the efficiency, gain, and bandwidth in both schemes. The results are of interest for evaluating the pros and cons of both schemes. View full abstract»

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  • Prebunching of electrons in harmonic-multiplying cluster-cavity gyro-amplifiers

    Page(s): 970 - 980
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    The use of a cluster of cavities in frequency multiplying gyro-amplifiers is described. An analytical theory has been developed to maximize the second harmonic current and optimize the drift section length for the case of a single low-Q input cavity, operating at the fundamental cyclotron harmonic, and bunching clustered cavities operating at the second-harmonic. MAGY simulations have been conducted to benchmark the theory and further study the detailed characteristics of cluster-cavity gyro-amplifiers. The theory and MAGY code simulations agree. In the small signal regime, the bandwidth of a cluster-cavity device (with a pair of cavities in the cluster) is twice that of a single cavity device, while both have the same peak bunching. With a gyro-TWT output section, a peak power of 247 kW, efficiency of 24.2% and bandwidth of 1.08% has been simulated using a cluster of cavities as a buncher. In addition, the power-bandwidth product is 105 kW×MHz, which is double that of the single cavity buncher case. We also investigate the effect of coupling between the cavities of a cluster, and the performance of a three-cavity cluster. View full abstract»

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  • Coaxial-waveguide gyrotron amplifier operating with high power and ultrahigh gain in Millimeter and submillimeter waves

    Page(s): 981 - 986
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (240 KB) |  | HTML iconHTML  

    A coaxial-waveguide gyrotron-traveling wave amplifier is proposed, which may be suitable for the operation in the spectrum of the short millimeter and submillimeter waves with good mode selectivity, high power, and ultrahigh gain. An example is presented in terms of the linear gyrokinetics and nonlinear simulations. Theoretical results show that tens of kilowatts with ultrahigh gain up to 67 dB might be achieved for the operation of the coaxial-waveguide mode TE28,16 in the fundamental cyclotron harmonic at the frequency of 140 GHz. The power enhancement by a tapered magnetic field is discussed. View full abstract»

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  • Linear theory of shot noise in harmonic gyroklystrons and gyro-TWT amplifiers

    Page(s): 987 - 993
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    Expressions for the equivalent input power level from bare shot noise in two types of gyro-amplifiers are calculated. The analysis is limited to the small gain, ignores correlations between electrons, and is further restricted to cold beams. The fields are calculated in a self-consistent manner directly from the electron beam as a first-order perturbation. In the first case, the equivalent input noise temperature for a harmonic gyro-traveling wave tube amplifier is derived. In the second case, the noise temperature of a harmonic gyroklystron input cavity is derived. A sample calculation is given for each using the TE021 and TE02 modes with a 60-kV 1-10 A annular electron beam operating near the second harmonic of the cyclotron frequency. View full abstract»

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  • Commissioning of the 34-GHz, 45-MW pulsed magnicon

    Page(s): 994 - 1001
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    A high-efficiency, high-power magnicon at 34.272 GHz has been designed and built as a microwave source to develop radio frequency (RF) technology for future multi-TeV electron-positron linear colliders. To develop this technology, this new RF source is being perfected for necessary tests of accelerating structures, RF pulse compressors, RF components, and to determine limits of breakdown and metal fatigue. The design of this high-power amplifier tube, as well as the first experimental results are presented. View full abstract»

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  • Folded waveguide traveling-wave tube sources for terahertz radiation

    Page(s): 1002 - 1014
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    Microfabricated folded waveguide traveling-wave tubes (TWTs) are potential compact sources of wide-band, high-power terahertz radiation. We present feasibility studies of an oscillator concept using an amplifier with delayed feedback. Simulations of a 560-GHz oscillator and experimental evaluation of the concept at 50 GHz are presented. Additionally, results from various fabrication methods that are under investigation, such as X-ray lithography, electroforming, and molding (LIGA), UV LIGA, and deep reactive ion etching are presented. Observations and measurements are reported on the generation of stable single-frequency oscillation states. On varying the feedback level, the oscillation changes from a stable single-frequency state at the threshold to multifrequency spectra in the overdriven state. Simulation and experimental results on amplifier characterization and dynamics of the regenerative TWT oscillator include spectral evolution and phase stability of the generated frequencies. The results of the experiment are in good agreement with the simulations. View full abstract»

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  • Linearized field theory of a Smith-Purcell traveling wave tube

    Page(s): 1015 - 1027
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    A linearized field theory of a grating-coupled Smith-Purcell traveling-wave tube (TWT) is presented. The configuration describes the propagation of a magnetized, sheet electron beam through a parallel-plate waveguide in which two opposing faces of the waveguide consists of either 1) a pair of identical diffraction gratings (referred to as a double-sided configuration) or 2) a diffraction grating along one face and a flat plate along the opposing face (referred to as a single-sided configuration). This is a two-dimensional formulation since we assume the configuration is uniform in the direction parallel to the slots in the grating. For each of these configurations, we derive the linearized dispersion equations for the vacuum structures and the wave-particle interaction in both the infinitely strong magnetic field limit and for an arbitrary (solenoidal) magnetic field. A discussion of the extraction efficiency based on phase-trapping arguments is also presented. An example of a W-band Smith-Purcell TWT is discussed. View full abstract»

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IEEE Transactions on Plasma Sciences focuses on plasma science and engineering, including: magnetofluid dynamics and thermionics; plasma dynamics; gaseous electronics and arc technology.

 

 

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