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Plasma Science, IEEE Transactions on

Issue 3 • Date June 1996

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Displaying Results 1 - 25 of 77
  • A tribute to George Bekefi (1925-1995)

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    Freely Available from IEEE
  • The multiplet cavity: A buncher for broad-bandwidth klystron amplifiers

    Page(s): 928 - 934
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    The bandwidth of a klystron output cavity scales (approximately) as p0.8P0.2, where p is the beam perveance and P is the beam power. For high-perveance (p>10 μpervs) high-power (P>10 kW) electron beams, it is relatively straightforward to design a broad-band output cavity. However, the design of the input cavity of the broad-band klystron is in some ways more difficult. The purpose of the input cavity is to produce a velocity-modulated electron beam with a frequency-dependent modulation amplitude that will optimize the bandwidth of the entire klystron system, while providing a magnitude of velocity modulation large enough to minimize the length of the klystron. This paper shows how a multiplet (multiple cavity) buncher cavity can be designed to provide broad-band (>20%) operation while keeping short the drift section length of the klystron View full abstract»

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  • Use of UV to reduce particle trapping in process plasmas

    Page(s): 1133 - 1136
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    The use of ultraviolet (UV) radiation to reduce the negative charge on dust particles and modify force balance in particle traps in processing plasmas is investigated theoretically. Possible photophoresis associated with the UV flux is also discussed. The use of UV to facilitate moving contaminant particles from traps is considered View full abstract»

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  • Design of multimegawatt millimeter-wave converters for operation at high gyroharmonics

    Page(s): 825 - 837
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    The authors discuss a feasibility study for the production of multimegawatt RF power from a spatiotemporally modulated gyrating relativistic electron beam at a high harmonic of the frequency of modulation. Parameters are given for optimizing the production of power at 148.5 GHz, the thirteenth harmonic of an 11.424-GHz cyclotron autoresonance traveling wave interaction that accelerates the beam. With an injected 16-A 200-kV beam, to which 7.2 MW is added in the accelerator, it is predicted that a power output of 2.2 MW at 148.5 GHz can be realized, after taking into account ohmic wall losses. This requires an injected laminar beam with small initial axial velocity spread, and an output structure that effectively suppresses competing modes. In this device a tapered guide magnetic field no stronger than 8.2 kG is required. Means to produce the required high-quality electron beam, and to allow only the TE13.1 design mode to propagate in the output structure are discussed. A similar design for a fifteenth harmonic converter at 171.4 GHz is also described View full abstract»

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  • A model of injection-locked relativistic klystron oscillator

    Page(s): 935 - 937
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    By the use of a simple model, we explicitly incorporate the coupling between the driver cavity and the booster cavity in a relativistic klystron amplifier (RKA). We show that this RKA configuration may turn into an injection-locked oscillator only when the beam current is sufficiently high. Other features revealed by this model include: the downshifted frequency mode (“0” mode) is unstable whereas the upshifted frequency mode (“π” mode) is stable; the growth rate of the “0” mode is relatively mild so that the oscillation can start only in an injection-locked mode; the oscillation does not require the presence of reflected electrons; and the separation of the cavities must be sufficiently short. These, and other features, are found to be in qualitative agreement with the recent experiments on the injection-locked relativistic klystron oscillator (RKO) that were conducted at Phillips Laboratory View full abstract»

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  • Current modulation in relativistic klystron amplifiers

    Page(s): 924 - 927
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    A brief review is first provided of the assumptions and approximations involved in Uhm's recent theory (1993) of current modulation in klystron amplifiers. Next, this theory is employed to obtain a solution to a cubic equation that approximates the klystron drift distance to the point of maximum current modulation, and also to obtain the corresponding current modulation amplitude in terms of relevant klystron parameters View full abstract»

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  • Mode competition and control in free electron lasers with one- and two-dimensional Bragg resonators

    Page(s): 770 - 780
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    We present a time domain analysis of the longitudinal and azimuthal mode dynamics which occur in relativistic free electron lasers and cyclotron autoresonance masers with one-dimensional (1-D) and two-dimensional (2-D) Bragg resonators. It is shown that when the 1-D oscillator is moderately above threshold, a single longitudinal mode generation regime is established due to nonlinear mode competition. The process of longitudinal mode selection is more effective if the region of synchronous interaction between the electron beam and the forward propagating wave is extended inside the Bragg reflectors. Further above the oscillation threshold, multimode chaotic behavior occurs. In the second part of this paper, it is shown that both traditional 1-D and novel 2-D Bragg resonators can produce radiation at a single frequency with a one-mode azimuthal distribution which corresponds to spatial synchronization of the electromagnetic radiation. In 1-D systems, electronic mode selection occurs via nonlinear mode competition. In contrast, electrodynamic mode selection occurs in 2-D Bragg resonators, resulting in the production of a single azimuthally symmetric mode after the linear stage of evolution. The 2-D Bragg resonator is shown to retain its selectivity when its radius greatly exceeds the radiation wavelength View full abstract»

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  • An axisymmetric model for an RF plasma in contact with a surface having a nonuniform secondary electron emission coefficient

    Page(s): 1072 - 1078
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    An axisymmetric analytical model of an RF glow discharge was developed. This model allowed exploration of various effects: radial losses at the side wall, mismatch of secondary electron emission coefficients on the surface of a powered electrode, the aspect ratio and dimension of the plasma chamber, and operational parameters such as gas pressure and bias voltage. The plasma density decreased in most cases by a factor of two to three from the axis to the periphery as a consequence of the radial losses. A maximum in the radial plasma density distribution appeared when the substrate had a different secondary electron emission coefficient than the platen. A maximum in plasma density at the axis also appeared when the electrode separation was varied at constant electrode radius because the plasma diffusion loss between electrodes dominates when the electrode separation is small and plasma radial diffusion loss dominates when the electrode separation is large View full abstract»

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  • Large-signal characteristics of a wide-band dielectric-loaded gyro-TWT amplifier

    Page(s): 718 - 726
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    The bandwidth of a gyrotron traveling wave amplifier (gyro-TWT) has been significantly increased by partially filling the interaction waveguide with dielectric to reduce the circuit's dispersion. The proof-of-principle experiment was designed for X-band, and employs the fundamental mode of rectangular waveguide loaded with dielectric slabs along the narrow sidewalls. The amplifier yields a peak output power of 55 kW with 11% efficiency, 27 dB saturated gain, and an unprecedented untapered gyro-TWT constant-drive bandwidth of 11% and saturated bandwidth exceeding 14%. The single-stage amplifier is completely zero-drive stable. The 95-kV 5-A electron beam was produced by a single-anode magnetron injection gun with pz=0.6, as determined by the EGUN code, and Δυzz=4%, determined as the best fit to the gyro-TWT large-signal simulation data. Simulation studies predict that by lowering the velocity spread to Δυ zz=2%, the amplifier performance will be further enhanced to a constant-drive bandwidth of 20% with 15% efficiency View full abstract»

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  • Simulation of the velocity spread in magnetron injection guns

    Page(s): 982 - 991
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    The velocity spread associated with phase mixing due to dc space charge in a magnetron injection gun (MIG) is investigated. A simple model is introduced to describe the mixing process. Simulations are performed by using the results of the EEGUN trajectory calculation for initial conditions at the entrance of the drift region. Results for a 170 GHz gun are obtained and compared with EGUN simulations. This new model provides a more accurate and efficient approach for analyzing the velocity spread due to mixing in MIG's View full abstract»

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  • Oscillations and chaos in plasma-filled diodes

    Page(s): 1005 - 1014
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    A plasma-filled diode has four different regimes of operation: stable, oscillatory, chaotic, and unstable. So far only a special case of charge neutrality at the entrance electrode (α=ni0/n e0=1) has been considered. In the present investigations the analysis has been extended to the more general case of α≠1, i.e., when there is a net positive or negative charge present at the entrance electrode. The device is frequently used as a model of modern microwave tubes such as vircators and split-cavity oscillators View full abstract»

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  • Effects of tapering on gyrotron backward-wave oscillators

    Page(s): 636 - 647
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    Computer modeling has been utilized to guide gyrotron backward-wave oscillator (gyro-BWO) experiments at the University of Michigan over a wide range of tapered interaction regions and tapered magnetic fields. E-GUN code is used to examine beam and diode characteristics, while MAGIC is used to analyze the dynamics of the problem, such as particle kinematics and microwave power production. Several innovative techniques are used to create matching boundary conditions for a backward propagating wave. MAGIC simulations predict optimum performance of the gyro-BWO operating in a TE01 mode within a combination of uniform interaction region and a tapered axial magnetic field which increases 7.5% in the direction of beam propagation. Experiments have been performed to investigate the effects of tapering magnetic fields and tapered interaction region radii on the high-power microwave emission from the gyro-BWO over the frequency range from 4.0 to 6.0 GHz. These experiments were performed on the Michigan Electron Long-Pulse Accelerator (MELBA) with parameters: V=-0.7 to -0.9 MV, Idiode=1-10 kA, Itube=1-4 kA, Te-beam =0.4-1.0 μs. Tapered interaction regions of 37%, 23%, 9.4%, and 6.4% were built and tested to determine their effect on microwave power, pulselength, and inferred energy compared to the uniform interaction region. Magnetic tapering trim coils with a range of -10.6%<ΔB/B0<+15.0% were constructed which allow the orientation of the field taper to be changed without breaking the vacuum. The peak microwave power from individual shots was from 30 to 55 MW. Experiments on magnetic field tapering indicate that positive tapered fields improve microwave power and energy output View full abstract»

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  • Prebunched high-harmonic FEL for short-pulse millimeter wave emission

    Page(s): 808 - 815
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    A novel wide-band millimeter-wave source is described where a “point-charge” electron beam rotating within a bifilar wiggler will generate a short-pulse TEmn wavepacket at the mth-harmonic of the fundamental free electron laser (FEL) frequency. In this first-order interaction, all electrons lose energy to the wave if the electron bunch size is much less than the wavelength. For a grazing intersection of the beam mode with the waveguide mode, there is no slippage of the electron bunch in the wave and the narrow width of the broad-band output pulse is limited only by the waveguide's dispersion. A linear theory of this new interaction is presented along with the design for a prebunched tenth harmonic FEL proof-of-principle experiment that is predicted to generate a 140-ps pulsewidth 30-GHz wavepacket at kilowatt power levels View full abstract»

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  • Mode selective properties of coaxial gyrotron resonators

    Page(s): 596 - 605
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    In this paper, the mode-selective properties of coaxial gyrotron resonators are discussed for different cavity configurations. Expressions are derived for the ohmic loading of a coaxial conductor with longitudinal corrugations where the RF field inside the corrugations is fully taken into account, and for the diffractive quality factor in coaxial cavities with tapered inner conductor relative to that of an empty cylindrical cavity. The competing action of a kind of “surface mode” that appears in corrugated structures is investigated by considering its diffractive and ohmic quality factors. Additional mode competition problems that can arise in coaxial cavities with corrugated inner conductor due to second cyclotron harmonic interaction are investigated and solutions to the problem are proposed View full abstract»

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  • High-efficiency relativistic backward wave oscillator: theory and design

    Page(s): 843 - 851
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    Backward wave oscillators (BWOs) driven by high-current relativistic electron beams are capable of producing high-power coherent radiation in the centimeter and millimeter wavelength regions. However, the efficiency of these devices is usually limited to 15-20% when a homogeneous slow-wave structure is used. Utilizing a two-section slow-wave structure, where the spatial period of the second section is larger than that of the first section, a BWO efficiency of greater than 50% was calculated. A conceptual design of a high-efficiency S-band BWO driven by a 500-kV 5-kA electron beam has been developed and analyzed View full abstract»

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  • Study of high-power Ka-band second-harmonic gyroklystron amplifier

    Page(s): 666 - 670
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    The self-consistent nonlinear theory of two-cavity high-harmonic gyroklystron amplifier has been developed. The efficiency and gain of a second-harmonic gyroklystron were calculated numerically. The results obtained were used to choose the optimal parameters of the experimental second-harmonic tube. The experimental study was carried out to test high-harmonic amplifier concept. Two-cavity 35 GHz second harmonic gyroklystron with the TE021 cavity mode has been designed and tested in pulse operation. Output power of about 260 kW with efficiency 18% and 17 dB gain have been produced at 72 kV beam voltage and 20 A beam current. Bandwidth of about 0.1% has been observed. The restriction of the output power, efficiency, and gain was caused by spurious oscillations excited in the second cavity in the TE011 mode at the fundamental cyclotron frequency View full abstract»

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  • Mode competition in fourth-harmonic magnicon amplifiers

    Page(s): 957 - 963
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    In the magnicon RF amplifier, the drive and gain cavities are cylindrical deflection cavities which operate in a rotating TM110 -mode and spin up an electron beam to high transverse momentum. As a result of the rotating-mode interaction, the electron beam entry point into the output cavity rotates about the axis at the drive frequency. The gyrotron-like output cavity can be operated at m times the drive frequency by using a mode with an azimuthal index of m, as this mode rotates at m-1 times its RF frequency, thus maintaining synchronism with the electron beam. Previous frequency-multiplying magnicons have used m=2; in this paper it is shown that magnicons with m=4 may be practical, provided one also operates the output cavity at the m/2 harmonic of the cyclotron frequency. Operation at higher harmonics lowers the frequency of the deflection cavities allowing lower RF fields, reducing cavity breakdown problems; lower magnetic fields, reducing magnet cast and complexity; and a larger electron beam, relaxing beam quality constraints. On the other hand, higher order azimuthal-index magnicon modes interacting at higher order cyclotron interactions are subject to competition with nonsynchronous (gyrotron) modes and are more sensitive to electron beam scanning angle spread. A time-dependent multimode gyrotron code has been modified to examine competition in the output cavity between the phase-synchronous operating mode and other nonsynchronous modes which interact via the conventional gyrotron interaction. Calculations have been carried out for fourth-harmonic magnicons with TM410 and TE411 mode output cavities and include the effects of mode competition with nonsynchronous modes as well as a spread in entry-point angles of the scanning electron beam View full abstract»

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  • Numerical simulation of nonstationary processes in intense helical electron beams of gyrotrons

    Page(s): 992 - 998
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    Experiments with helical electron beams (HEB) of powerful gyrotrons displayed the existence of intense nonstationary processes in HEB. A version of numerical simulation of HEB's is proposed, which is based on the method of large particles. Some results of the analysis are given with account of the electrons reflected from the magnetic mirror and locked in the adiabatic trap. One of the effects detected is intense bombardment of the cathode by the trapped electrons. Besides, formation of quasi-stationary states whose parameters (velocity spread, pitch-factor) differ significantly from those given by the static theory was observed. In the absence of the reflected particles, the solution acquires a stable state. The HEB parameters in this case are almost the same as the static parameters View full abstract»

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  • Stability of a 95-GHz slotted third-harmonic gyro-TWT amplifier

    Page(s): 735 - 743
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    A low-magnetic-field moderate-voltage gyrotron amplifier has been designed for stable high-performance operation at 95 GHz. A slotted interaction circuit is utilized to achieve strong amplification near the third cyclotron harmonic frequency. The start-oscillation conditions were determined by an analytical theory and confirmed by a multimode particle-in cell simulation code. The dominant threat to the amplifier's stability is from a third-harmonic peniotron backward-wave interaction. A slow-timescale particle-tracing simulation code predicts the three-section slotted third-harmonic gyro-TWT, which utilizes an 11.6-kG magnet and a 50-kV 3-A υz=1.4 axis-encircling electron beam with an axial velocity spread of 6% will yield an output power of 30 kW with an efficiency of 20%, a saturated gain of 40 dB, and a constant-drive bandwidth of 2% View full abstract»

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  • Prototype microwave source for a relativistic klystron two-beam accelerator

    Page(s): 938 - 946
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    A test facility is established at Lawrence Berkeley National Laboratory (LBNL) to study RF power sources for linear colliders based on the relativistic klystron two-beam accelerator (RK-TBA) concept. A 24-m long prototype source, the RTA, will be constructed in this facility to study physics, engineering, and cost issues related to RK-TBAs. The RTA will generate 200-ns 180-MW RF (11.4-GHz) pulses from each of eight output ports. The major components of the RTA include a 2.8-MeV 1.2-kA induction injector, transverse beam modulator, adiabatic compressor, and RF extraction section. The beam energy is increased to 4 MeV and the RF bunch length is shortened from 240° to 110° in the adiabatic compressor. The 8-m long extraction section includes 40 induction accelerator cells to maintain beam energy at an average 4 MeV, eight equally spaced RF output structures, and a ppm quadrupole focusing system. In this paper, we describe the RTA and present results of component testing and computer simulations View full abstract»

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  • A hybrid hydrodynamic-like model for pseudospark discharge

    Page(s): 1106 - 1119
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    A hybrid hydrodynamic-like formalism is presented for the study of the inception of breakdown and development of plasma bubble in the hollow cathode phase of a pseudospark switch discharge. It allows one to obtain the spatio-temporal evolution of the electronic and ionic densities, velocities, and energies in fast transient situations in low pressure discharges. This hybrid formalism allows the study of situations where the mean free path is not low compared to the spatial variations of the macroscopic parameters, and/or situations for which the transport energy (respectively, the transport velocity) is of the same order of magnitude as the thermal energy (respectively, the thermal velocity), as in the case of pseudospark discharge. The model is self-consistent and its formalism stands on a particular representation of the anisotropic nonlocal distribution function for electrons as having a beam component. The nonlocal electronic velocity is computed as a moment of the distribution function. The transport equations for mass and energy for electrons and ions are solved by the flux-corrected transport (FCT) method where the source terms are calculated from the nonlocal distribution function. The electric field is calculated by the Poisson's equation. The discharge physics is modeled in both main gap and hollow cathode simultaneously to permit a correct coupling of the charged particles, photons, and field between the two zones. Results show that the transport velocity and ionization growth for the case of highly anisotropic nonlocal distribution function is largely higher than that based on a pure hydrodynamic nearly Maxwellian distribution function. The energetic and high density positive ions impacting the inner walls and the back of the cathode hole participate in a composite effect yielding a cathode surface phenomena of super dense secondary emission of electrons which enhances the plasma bubble inception inside the cathode hole. We propose a criterion for switch operation in the pseudospark mechanism based on the formation of the plasma bubble, inside the cathode hole, known as the hollow cathode phase, as a transition phase between the breakdown phase and the conduction phase View full abstract»

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  • Theory of a rotating-mode coaxial infrared grating laser

    Page(s): 758 - 769
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    Analysis is presented for a novel interaction that makes possible a compact infrared grating laser configuration. The laser utilizes an annular axis encircling beam that is confined by an axial magnetic field and passes along a grating blazed axially on the center conductor of a coaxial resonator. A smooth cylindrical outer conductor completes the Fabry-Perot resonator to provide the necessary feedback for sustained oscillation. Linear analysis leads to an eigenvalue equation for cavity resonance frequencies, start-oscillation currents, and a gain-bandwidth relationship. The latter permits estimation of an upper bound on the ideal-beam power extraction efficiency. The nonlinear studies make use of analytical and numerical methods to gain a detailed understanding of electron motion in the presence of both axially focusing and radio frequency fields, and to determine the nonlinear efficiency. There is significant degradation of the interaction efficiency when the beam has finite annular thickness. Efficiency enhancement by means of a down-tapered axial field is demonstrated. A point design for lasing at ~160 μm in the far-infrared, utilizing a 100-kV beam with a pitch angle of 55° is presented. The start-oscillation current is a sensitive function of the resonator quality factor (or finesse); for a well-designed resonator, it is on the order of tens of Amperes View full abstract»

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  • Linearized field theory of a dielectric-loaded helix traveling wave tube amplifier

    Page(s): 895 - 904
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    A linearized relativistic field theory of a helix traveling wave tube (TWT) is presented for a configuration where either a thin annular beam or a solid beam propagates through a sheath helix enclosed within a loss-free wall in which the gap between the helix and the outer wall is filled with a dielectric. A linear analysis of the interaction is solved subject to the boundary conditions imposed by the beam, helix, and wall. In the case of the annular beam, the electrons are assumed to be strongly magnetized. In contrast, the effect of variations in the axial magnetic field are included in the electron dynamics for the solid beam analysis. Determinantal dispersion equations are obtained for the azimuthally symmetric modes which implicitly includes beam space-charge effects without recourse to a heuristic model of the space-charge field. Numerical solutions of the dispersion equations are discussed and compared with experiments View full abstract»

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  • A small signal theory of gyro-devices using the concept of space-charge waves

    Page(s): 707 - 717
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    The linear theory of the interaction between a thin annular beam of gyrating electrons and a traveling electromagnetic wave is developed. This theory is very close in its methodology to the classical small signal theory of O- and M-type devices. Just this fact makes it possible to consider a number of issues in the theory of gyro-devices, namely, space-charge wave propagation in drifting electron beams, noise and parametric phenomena, effects of input signal suppression (Kompfner dip) in gyro-traveling-wave-tubes, and gyro-backward-wave-amplifiers View full abstract»

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  • 35-GHz 25-kW CW low-voltage third-harmonic gyrotron

    Page(s): 613 - 619
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    A 50-kV third-harmonic gyrotron is shown to be capable of high efficiency. Operation at the third harmonic allows the required magnetic field for 35 GHz generation to be supplied by a 4.5-kG permanent magnet. Two gyrotrons employing sliced circuits for mode control have been evaluated with a large-signal nonself-consistent particle-tracing simulation code and found to be capable of producing 25 kW continuously. The preliminary design of a third-harmonic TE41 gyrotron utilizing a magnetron injection electron gun is predicted to yield a device efficiency of 17%, which can potentially be increased to 46% with an ideal single-stage depressed collector, while an axis-encircling electron beam from a Cusp electron gun is predicted to drive a third-harmonic TE31 gyrotron with a device efficiency of 23%, which can theoretically be increased to 45% through the use of an ideal depressed collector 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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