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Plasma Science, IEEE Transactions on

Issue 2 • Date April 1988

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Displaying Results 1 - 25 of 35
  • Comments on the linear theory of the gyrotron

    Page(s): 116 - 121
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    The authors present alternative methods for the calculation of the starting current in a gyrotron, and an analysis of the excitation of parasitic modes in gyrotrons operating in a given mode. Numerical examples are presented which check the validity of some of the assumptions made, and the extension of the formalism to the investigation of the stability of an operating mode when parasitic modes can be excited (mode competition) is also discussed. Results from the problem formulation, which is based on the cold-cathode model of Borie and Jodicke (1987), are compared with both linear and fully nonlinear models. These results illustrate the importance of using a correct model for the field profile and the influence of numerical uncertainties on the results obtained using a linear theory for mode competition. It is noted that the results obtained using the nonlinear theory of mode competition are usually numerically more stable and easier to interpret.<> View full abstract»

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  • Electron density and temperature in the pulsed-orbitron-maser glow discharge

    Page(s): 270 - 274
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    In order to better understand the pulsed-glow discharge that provides the feed electrons for a pulsed-orbitron maser, the authors report using penetrating microwave beams and radially inserted Langmuir probes to make bulk and radial measurements of the number density in the plasma. The Langmuir probe was also used to measure the electron temperature across the plasma radius, and to investigate the effects of an externally applied magnetic field on the RF output of the orbitron. It was found that the emitted frequency is apparently not related to the electron plasma frequency by any simple linear relationship, and that an externally imposed magnetic field of as little as 70 G is required to kill the instability View full abstract»

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  • The orbit theory of electrostatic electron cyclotron resonance masers

    Page(s): 275 - 280
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    The orbit theory of the electrostatic electron cyclotron resonance maser (EECRM), which is similar to the orbitron maser, is presented based on models with both circular and noncircular steady-state electron orbits. In contrast to the nonlinear theory of the magnetostatic electron cyclotron resonance maser (MECRM), the concept of converting efficiency is introduced. The efficiencies for TE, TM, and TEM modes are calculated in various cases. Numerical analyses show that in the interaction process the radio-frequency (RF) fields attract energy mainly from the potential energy of electron beams. The nonlinear interaction mechanisms and the effects of some parameters are also studied View full abstract»

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  • Planar orotron experiments in the millimeter-wavelength band

    Page(s): 199 - 205
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    The planar orotron is introduced and shown to be a viable source of moderate power and of millimeter and submillimeter wavelength radiation. The resonator is a slow-wave structure consisting of a rectangular metal grating which is opposed by a planar conducting boundary. The device operates in the surface harmonic mode: electrons interact with axially traveling waves which evanesce above the grating surface, and the amplified radiation leaves the resonator in parallel with the beam axis. Operation in both the forward and backward mode is possible. The resonator cavity is designed to enhance longitudinal reflections, and thereby enhance the output power and efficiency. The output frequency and tuning range are determined by the grating parameters. Experiments performed in the backward mode have produced radiation from 30 to 110 GHz at power levels ranging from 100 W to 2 kW. The efficiencies vary from 1 to 7%. The measured frequencies are closely predicted by a theory which is also presented View full abstract»

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  • High-efficiency operation of free-electron-laser amplifiers

    Page(s): 167 - 171
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    A high-efficiency regime was studied for free-electron-laser (FEL) amplifier configurations that employ a helically symmetric wiggler and a uniform-axial-guide magnetic field. Efficiencies on the order of 50% are found to be possible in the presence of axial fields sufficiently strong that the Larmor period is shorter than the wiggler period. This operating regime is accessible for electron-beam energies greater than approximately 400 keV View full abstract»

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  • High-power and superpower Cerenkov masers

    Page(s): 206 - 216
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    The linear and nonlinear theory of the efficient operation of high-power (gigawatt) and superpower (50 GW) Cerenkov masers is presented. Issues such as breakdown, plasma production, and coupling to the output device are discussed. The relative merits of dielectric Cerenkov masers and plasma Cerenkov masers are considered. The principal design tool is a new particle simulation model that was developed to investigate Cerenkov masers. The novel aspects of this model are briefly described along with a comparison of calculated and experimental results. The agreement between calculations and measurements is generally good. Designs for a high-power and superpower plasma Cerenkov masers are also described View full abstract»

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  • Particle simulation of a high-power gyrotron oscillator

    Page(s): 129 - 134
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    A self consistent and time-dependent particle code has been developed to simulate the beam-wave dynamics in a gyrotron oscillator. The code is first applied to investigate the effect of the self-consistent field profile on the scaling of the cavity-filling rate with beam current. The fixed-field theory predicts that the transient-wave growth rate depends linearly on the beam current. The simulation results agree with the theoretical prediction at low beam currents. As the beam current increases, the modified field profile changes the nature of the gain scaling from linear to nonlinear. At higher beam currents, the excited wave is observed to exhibit the behavior of the modulated oscillation, due to unequal couplings to the forward- and backward-going waves by the beam. Associated with such amplitude modulation is a periodic variation of the self-consistent field profile. At still higher beam currents, the system becomes chaotic, showing the effect of periodic doubling. Simulation results are presented for the efficiency, the self-consistent field profile, and the scaling of the transient growth rate with the beam current View full abstract»

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  • The gain and efficiency improvement of a free-electron laser by an optical klystron configuration

    Page(s): 172 - 176
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    The feasibility of a high-grain amplifier in an optical klystron configuration at wavelengths of 1-2 mm was investigated using a computer simulation. A one-dimensional single-particle model for the free-electron laser (FEL) interaction was used in the numerical studies. It was found that for a three-cavity optical klystron gain of at least 30 dB and relatively high efficiency is possible for relatively modest electron-beam parameters (V⩽500 kV, J≈100 A/cm2) View full abstract»

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  • Operation of a large-orbit high-harmonic multicavity gyroklystron amplifier

    Page(s): 155 - 161
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    An experimental four-cavity, fifth-harmonic gyroklystron is described that separates at 11.3 GHz in a 1.2-KG magnetic field. This device used longer drift regions than previous configurations. Principal results reported include 45 dB of small-signal gain and 0.6 kW of output power. Other results include the measurement of phase noise characteristics and the identification of spurious output signals. An improved beam diagnostic which determines the pitch of the electron orbits was used together with a fluorescent uranium-glass witness plate and beam-current diagnostics to characterize the electron beam. This information was used to perform analytic modelling and computer simulation of the amplifier. The experimental results are compared with the predictions of the analytic theory and simulation, and substantial agreement is demonstrated View full abstract»

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  • Microwave emission and beam propagation measurements in a high-power relativistic electron beam-plasma system

    Page(s): 217 - 224
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    Microwave emission was measured from a system consisting of an unmagnetized plasma and a propagating electron beam. A 93-cm2 velvet cathode, with an anode-cathode gap of 5.9 cm, injects the electron current into the plasma through an aluminized Mylar anode. Measurements were made of the diode voltage and current in the 6-μV water dielectric accelerator and net current through the beam-plasma system. The unmagnetized plasma is produced by a 90-μs, 90-Å current pulse, emitted from a thermionic LaB6 electron source, that preionizes argon fill in a 1-m-long, 15-cm-diameter Lucite tube. A microwave spectrometer detects the radio-frequency output in the 2-18, 18-26, and 26-47 GHz bands, filters, and then separates into narrower subbands. The emission takes place in two distinct phases. The 2-GHz output rises promptly with the current pulse and then decays. At 6-GHz and above, a low-level microwave prepulse appears simultaneously with the 2-6 GHz output. This output rises sharply 25 ns after the current pulse begins and includes frequencies out to and beyond 40 GHz. The radio-frequency output falls off before the current pulse ends. The microwave intensity decays monotonically with frequency View full abstract»

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  • Quasi-linear analysis of Smith-Purcell free-electron amplifier

    Page(s): 225 - 233
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    Calculations are presented of the scattering of an electromagnetic wave from a periodic structure above which flows an electron beam. The reflected fields are computed and found to comprise two separate contributions: (1) the reflection in the homogeneous case (without the beam); and (2) the contribution of the beam. Both are shown to depend on the structure by means of its reflection properties, as expressed by a reflection matrix. The beam contribution is shown to be exponentially dependent on the longitudinal position. This contribution also includes the exponential decay, which depends on the distance between the beam and the structure, and is a characteristic of Smith-Purcell devices. Expressions for the local and global gains are obtained. The local gain is found to be proportional to the first velocity derivative of the electron distribution function. Considerations of nonlinear effects introduce spatial dependence to the expression for the local gain. This dependence is determined by a nonlinear diffusion equation. The gain in the nonlinear regime is found to depend on the first and second velocity derivative of the distribution function View full abstract»

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  • Gigatron

    Page(s): 258 - 263
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    The gigatron is a new RF amplifier tube designed for linac (linear accelerator) collider applications. It employs the lasertron concept in which a bunched electron beam is extracted from a modulated cathode and accelerated through a DC diode structure. The resulting beam is fully modulated at high energy without the requirements of a modulator or drift region that characterize all conventional amplifier tubes. RF energy is then extracted from the beam in an output coupler. Because there is excellent bunching of the beam current, very high efficiency can, in principle, be achieved. Three design features permit extension of the lasertron concept to very high frequencies. First, a ribbon-beam geometry mitigates space-charge depression and facilitates efficient output coupling. Second, a traveling-wave output coupler is used to obtain optimum coupling to the ribbon beam. Third, a gated field-emitter array is employed for the modulated cathode. A prototype device that is currently being developed is discussed View full abstract»

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  • High-power pseudospark and BLT switches

    Page(s): 317 - 323
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    A review is presented of recent developments in a new group of high-power hollow-electrode switches, including the pseudospark and the backlighted thyratron (BLT). Experiments demonstrate that for several key high-power switching performance factors, the pseudospark and BLT switches are superior to either high-pressure spark gap switches or thyratrons or, in some cases, both. High performance has been demonstrated in peak current (>100 kA), current rate of rise (>10 12 A/s), switching precision, trigger efficiency, current reversal (100%), and recovery time. Several electrical and optical trigger methods have been demonstrated and are described View full abstract»

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  • Operational features and microwave characteristics of the Vircator II experiment

    Page(s): 177 - 184
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    The Vircator II oscillating virtual-cathode microwave source operates with diode voltages between 600 and 800 kV and diode current between 50 and 120 kA. Maximal microwave output power between 200 and 500 MW is achieved when the diode aspect ratio, cathode surface, charge voltage, and extraction coupling are arranged to simultaneously (1) maximize diode voltage, (2) satisfy magnetic insulation criteria, (3) avoid nonuniform or unstable electron emission, and (4) optimize microwave transmission from the virtual cathode to the launching antenna. Broadband radiation between 0.4 and 5.5 GHz is generated. The central frequency follows the beam plasma frequency. It is tuned by varying the current density with anode-cathode gap adjustments View full abstract»

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  • Effect of external fluctuation on nonequilibrium MHD power generation

    Page(s): 324 - 325
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    The effects of externally induced small fluctuations in the input thermal energy and/or the seed fraction are examined using numerical simulations based on two-temperature MHD (magnetohydrodynamic) equation system. A quasi-one-dimensional approach is adopted for a disk generator. A generator with a high enthalpy extraction rate is considered, and its design parameters are set so that the full ionization of seed contained in the plasma is realized. The fluctuations in seed fraction and gas pressure influence the performance of a generator in several different ways View full abstract»

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  • Efficiency and sideband observations of a Raman FEL oscillator with a tapered undulator

    Page(s): 162 - 166
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    Power and spectral measurements are reported from the Columbia Raman free-electron laser (FEL) oscillator experiment. High-power radiation pulses (~12 MW, 100 ns) are generated at a wavelength of ~2.5 mm, using a 750-kV electron beam injected into a helical undulator. The undulator is made up of a 40-cm long constant-period (1.45 cm) section followed by an equal length of tapered undulator. The period is decreased by 7.6% in such a way that the on-axis field remains constant. It is reported that the taper allows an increase in total power efficiency from ~4 to ~12%. Most noteworthy is that the tapered undulator reduces the sideband radiation compared with a constant-period undulator FEL which is studied in the same configuration. The power was measured calorimetrically and compared with the results of a 1-D Raman code. The reduction of sideband power observed in the experiment was consistent with computational results obtained with a 2-D sideband code View full abstract»

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  • Theoretical study of the folded waveguide

    Page(s): 305 - 311
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    A three-dimensional (3-D) algorithm for solving Maxwell's equations is applied to the analysis of folded waveguides used for fusion plasma heating at the ion cyclotron resonance frequency (ICRF). In this method a finite-difference method is used with a successive overrelaxation (SOR) convergence scheme and a method of treating boundaries that allows the cavity to have an arbitrary shape. A rigorous analysis of the magnetic-field structure in the folded waveguide is presented. The results are compared with experimental measurements in vacuum. To study breakdown problems, a much simpler two-dimensional (2-D) model is adopted. It is found from the 2-D analysis that the geometry, shape, and thickness of the vanes play an important role in avoiding voltage breakdown problems View full abstract»

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  • Operation of cusptron at fundamental and harmonic cyclotron frequencies

    Page(s): 149 - 154
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    Microwave radiation at the fundamental and harmonic electron cyclotron frequencies generated by a cusptron oscillator is reported. A low-energy, axis-rotating beam of 28-30 kV, 0.8-3.5 A, 4 μs, and 60 pps interacts with a single RF mode, both in a circular cavity and in a six-vane circuit by the negative mass instability. In fundamental and second-harmonic frequency generation with a circular circuit, the independently excited modes are TE11s and TE21s, with radiation power of more than 1.8 kW and an electronic efficiency of approximately 7.5%. Employing a six-vane circuit, microwave radiation of 6.0 GHz (sixth harmonic) and 3.9 GHz (fourth harmonic) is also independently generated with more than 10.4 and 4.0 kW radiation power, respectively. Corresponding electronic efficiencies are approximately 10.0 and 9.5% View full abstract»

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  • Numerical simulations of the reditron

    Page(s): 185 - 191
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    The reflected-electrons discrimination microwave generator (reditron) is a high-power, narrow-band, and single-mode microwave generator that makes exclusive use of the oscillatory character of the virtual-cathode of a relativistic electron beam. The complex, nonlinear character of the virtual-cathode device necessitates particle-in-cell plasma simulation techniques. Investigations indicate two sources of the radiation: (1) the trapped electrons reflexing between the real and virtual cathodes, and (2) the oscillation of the virtual cathode. In the conventional design, the two mechanisms coexist and interfere with each other destructively, causing degradation of the efficiency of microwave generation. The authors have investigated a configuration with a slotted, thick anode and an external magnetic field, which effectively eliminates the reflexing electrons. Two-dimensional particle-in-cell simulations showed that such a configuration exploits the oscillation of the virtual cathode exclusively, and it generates single-mode, narrow bandwidth, and high-power microwave radiation with a potential efficiency over 10%. It was found that further optimization could be achieved by the use of a density (current) modulated electron beam at appropriate frequencies View full abstract»

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  • Direct experimental observation of π-mode oscillation in a relativistic magnetron

    Page(s): 234 - 236
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    The authors report the direct measurement of the operating mode of a relativistic magnetron. The S-band 2.65-GHz magnetron used has a washer cathode and is driven by a 1-MV 2.8-Ω 65-ns pulser. Power and pulse shape are measured simultaneously by calibrated couplers, cables, and crystal diodes. The frequency is measured as the heterodyne or difference frequency between a local oscillator and the extracted signal. This difference frequency is several-hundred megahertz. By directly measuring the phase relationship, any questions concerning perturbations of the resonance by the large current of relativistic electrons are avoided View full abstract»

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  • Magnetron injection gun scaling

    Page(s): 290 - 295
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    A set of tradeoff equations was simplified to obtain scaling laws for magnetron injection guns (MIGs). The constraints are chosen to examine the maximum-peak-power capabilities of MIGs. The scaling laws are compared with exact solutions of the design equations and are supported by MIG simulations in which each MIG is designed to double the beam power of an existing design by adjusting one of the four fundamental parameters View full abstract»

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  • Cubic dispersion relation for a relativistic backward-wave oscillator

    Page(s): 264 - 269
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    The cubic approximation to the dispersion relation for a relativistic backward-wave oscillator is obtained, and the utility and limits of the approximation are presented. The approximation is obtained by Taylor series expansion of the wave admittance in the dispersion relation for the transverse-magnetic and free-streaming modes of a relativistic, thin, hollow, cylindrical electron beam moving along the axis of a disc-loaded waveguide in a strong axial magnetic field. The resulting cubic dispersion relation yields instability growth rates and frequencies which fall off beyond their maximum more sharply with increasing wavenumber than for the complete dispersion relation. The approximation is found to be quite good near the operating points of contemporary high-power relativistic backward-wave oscillators, namely, for relatively long wavelength and small ratio of Budker's parameter to the relativistic gamma factor of the beam View full abstract»

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  • Self-consistent simulation of cyclotron autoresonance maser amplifiers

    Page(s): 122 - 128
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    A self-consistent, one-dimensional model of the cyclotron autoresonance maser (CARM) amplifier is developed, and numerical simulations based on this model are described. Detailed study results of the CARM gain and efficiency for a wide range of initial energy and velocity spreads are presented. The interaction efficiency is found to be substantially increased when the axial magnetic field is tapered. Efficiencies of greater than 41% are obtained for a 140-GHz CARM amplifier with a tapered axial magnetic field and a 700-kV 4.5-A electron beam with parallel velocity spreads of less than 1%. A discussion of the nonlinear bandwidth and interaction sensitivity to axial-field inhomogeneities is presented View full abstract»

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  • High-peak power Ka-band gyrotron oscillator experiments with slotted and unslotted cavities

    Page(s): 142 - 148
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    A Ka-band gyrotron oscillator powered by a compact pulseline accelerator has been operated using oscillator cavities with and without axial slots. The oscillator was operated at high voltage (~900 keV) and high current (~500 A) in the approximate frequency range of 20-50 GHz. The use of axial slots has been shown to suppress low-starting-current whispering-gallery modes, in particular, modes of the TEm2 type, allowing stable operation in a linearly polarized TE13 mode. A peak power of 35 MW has been observed at 6% efficiency View full abstract»

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  • Nonlinear space-charge waves on an intense relativistic electron beam

    Page(s): 249 - 257
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    The propagation characteristics of the large-amplitude space-charge waves on a modulated intense relativistic electron beam are calculated. The beam is assumed to propagate inside a straight drift tube, and is guided by a strong axial magnetic field. Only the analytical and simulation results of this relatively simple system are given. It is shown that the slow space-charge waves may cease to propagate if the modulating voltage is sufficiently high, whereas the fast waves are relatively unaffected by the nonlinearity. The limiting electron velocity inferred from the analytical theory is in excellent agreement with particle simulation. The implications are 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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