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Plasma Science, IEEE Transactions on

Issue 3 • Date Jun 1990

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Displaying Results 1 - 25 of 51
  • A high-power, traveling wave tube amplifier

    Page(s): 546 - 552
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    High-power X-band traveling-wave tube amplifiers (TWTs) have been fabricated and tested. The tubes have gains ranging from 13 to 35 dB at 8.76 GHz and output powers ranging from 3 to 100 MW. The amplifiers are driven by the interaction of a slow space-charge wave, propagating on an electron beam, with an electromagnetic wave supported by the structure. The electron beam, which is produced from a magnetic-field-immersed field-emission cathode, has an energy of 850 keV, a current in the 1-kA range, and a pulse duration of 100 ns. The amplifiers are designed to operate as narrow-band devices in the TM01 mode. A report is presented on the amplifier characteristics, and their performance is compared with calculated performance using conventional TWT theory. The scaling of the gain and bandwidth with the beam current are approximately as expected from theory, but the absolute magnitude of the gain is somewhat greater than expected View full abstract»

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  • Relativistic Klystron amplifiers driven by modulated intense relativistic electron beams

    Page(s): 553 - 569
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    An overview is provided for the novel relativistic klystron amplifiers which are under active study at the Naval Research Laboratory. These amplifiers are driven by an annular intense relativistic electron beam (500-kV, 10-kA range), which is modulated by an external RF source (1.3-GHz, 100-kW range). Experiments, theory, simulation, and simple models are presented to illustrate the unusual properties of such devices which result from the intense space charge of the beam. Chief among them are electrostatic insulation against vacuum breakdown at high power levels, efficient current modulation, short bunching length, and amplitude and phase stability of the output signal. Many of these unexpected features were revealed in two separate experiments: one with a lower current beam (5 kA, 2-cm beam radius), and the other one with a higher current beam (16 kA, 6.6-cm beam radius). Three gigawatts of RF power at 1.3 GHz were generated with the large diameter beam at an efficiency of 35% with 37-dB gain. These experiments are reviewed, along with a combination of particle simulation results and analytic models which facilitate the interpretation. Special attention is paid to the unfamiliar features of these amplifiers, and the critical problems which must be solved before such amplifiers can fulfil their potential in a wide range of applications are addressed View full abstract»

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  • Growth and frequency pushing effects in relativistic magnetron phase-locking

    Page(s): 570 - 576
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    A magnetron-specific phase-locking model has been developed using the standard equivalent-circuit approach, which takes into account the unconventional magnetron growth characteristics as well as the frequency pushing effect. These effects owe their origin to the highly nonlinear electron-wave interaction; thus, they are believed to be more pronounced in relativistic magnetrons. The model predicts a wider locking bandwidth and a shorter locking time than those in conventional locking theory. The phase-locked amplitude resonance occurs, as the results indicate, at an injection frequency different from that of the free-running oscillator View full abstract»

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  • Temporal study of long-pulse relativistic magnetron operation

    Page(s): 594 - 602
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    A streak camera was used to make observations of apparent plasma motion in the interaction space of an A6 relativistic magnetron with an explosive emission cathode. The anode plasma luminosity velocity was measured to be 0.8 cm/μs, comparable with the gap closure velocity, which was indirectly measured to be 1.0-1.6 cm/μs, depending upon the magnetic field. The gap closure velocity appeared to be independent of the microwave oscillation. Long-pulse operation (hundreds of nanoseconds) as a function of the magnetron-modulator interaction is also presented. At 250 kV the A6 emitted microwave pulses out of a single resonator for up to 250 ns in duration, with peak power up to 90 MW and a characteristic single-spike pulse length of 150 ns View full abstract»

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  • Wave profile modification in the free electron laser with and without a waveguide

    Page(s): 424 - 436
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    The modification of the transverse wave profile by the free electron laser (FEL) interaction in the collective Raman regime is considered. The gain is calculated and the transverse wave profile is found by solving for the eigenvalues and the actual eigenmodes of the system. When a waveguide is employed, a strong FEL interaction is shown to couple various vacuum waveguide modes, as was demonstrated by A. Bhattacharjee et al. (1988). When no waveguide or cavity is present, a coupling parameter which measures the strength of the interaction is identified. Expressions are derived for the gain for large and small values of the coupling parameter, corresponding to a strong optical guiding and large diffraction, respectively. Comparing the present results for the Raman regime with previous results for the strong pump regime, it is shown that the gain scales differently in each case, depending on both the regime of operation and on the beam geometry. The linear analysis is valid when the signal is small, and is useful mainly when the gain is high prior to saturation View full abstract»

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  • Operation of a large-orbit high-harmonic gyro-traveling-wave tube amplifier

    Page(s): 313 - 320
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    The high-harmonics gyro-traveling-wave tube (gyro-TWT) is a high-power (⩾1-kW) millimeter wave amplifier based on the synchronous interaction of a beam of large-orbit axis-encircling electrons with a high-order cylindrical waveguide mode. Since the interaction occurs at a high harmonic of the cyclotron frequency, the intense magnetic fields required for the conventional fundamental-mode gyro-TWT are not required. A proof-of-principle experiment designed to demonstrate the interaction of a 150-mA, 350-keV electron beam with the TE81 mode of a cylindrical waveguide is described. Principal results include a small signal gain of 10 dB, an interaction bandwidth of 4.3%, and a saturated power transfer from electron beam to wave of 0.5 kW. Additional measurements include the dependence of gain on electron beam current and the measurement of the beam's γ, β 4F, β||, and Δβ||. Sufficient agreement between the experimental results, the simulation codes, and an analytic description of the interaction is demonstrated to permit the design of high-performance millimeter wave amplifiers View full abstract»

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  • AC space-charge effects in gyroklystron amplifiers

    Page(s): 273 - 285
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    The effects of AC space charge on multicavity gyroklystron amplifiers are studied. It is found that when AC space charge is included in the analysis of weakly relativistic multicavity gyroklystron amplifiers, the optimized nonlinear efficiency becomes a function of beam current. For a cold beam (no velocity spread), the efficiency is maximum at zero current and decreases monotonically as the current increases. The zero current limit of the optimized efficiency when AC space-charge effects are included is not the same as the optimized efficiency with no space charge; it is significantly higher. This behavior is regularized when velocity spread is taken into account; in that case, the nonlinear efficiency increases with beam current until it reaches a maximum, then falls off slowly. The increase in efficiency is attributed to enhanced bunching associated with the saturation of the space-charge instability in the drift region; the reduction in efficiency at high current occurs because space charge induces an additional velocity spread in the beam View full abstract»

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  • Prebunched high-harmonic gyrotron

    Page(s): 343 - 349
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    A scheme for harmonic wave generation using a prebunched electron beam has been demonstrated. The prebunched electron beam has been used to further increase the efficiency of the authors' axis-encircling high-harmonic gyrotron. The proof-of-principle experiment was performed at the third harmonic with a TE312 mode at 27.7 GHz. The conversion power of 6.7 kW was significantly greater than that used in the nonprebunched experiment. Also, mode competition was effectively suppressed. As expected, the unsaturated output power is proportional to the square of the electron beam current and the start of oscillation current is essentially zero. A linear theory, derived by taking into account the spread of the guiding center and the spread of the axial velocity, gives good agreement with the experimental results View full abstract»

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  • A reproducible rep-rate high-power magnetron microwave tube

    Page(s): 577 - 579
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    Observations are reported of the reproducible operation of an intense relativistic electron-beam-driven high-pulsed-power magnetron microwave tube at rep-rates of 1 Hz. These are the first observations of a reproducible rep-rate high-power magnetron, and microwave tubes in general, under several specific conditions: ultra-high vacuum; with a compact tunable modulator; and using extraction into the TE10 mode which radiates a polarized pencil beam. The experiments were performed in the 2 π mode. Power levels approaching 300 MW were achieved in short pulses (less than 100 ns) View full abstract»

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  • High-power dielectric Cherenkov maser oscillator

    Page(s): 507 - 512
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    Results are presented of experiments conducted using the dielectric Cherenkov maser (DCM) oscillator. The device consists of a cylindrical metallic waveguide with an inner radius of 3.64 cm which is partially filled with a dielectric liner. Traveling through the lined waveguide is an annular relativistic electron beam. Liners of dielectric constant ε=10, 5, and 2.3 were investigated for liner thicknesses of 4 and 6 mm. The 6-mm-thick, ε=10 liner generated an RF output of 200 MW for 20 ns at 3.8 Hz with electron-beam parameters of 700 kV, 12 kA, and pulse duration of 100 ns. The maximum measured power output for the other configurations was 80 MW at a frequency of 7 GHz (ε=5), with several megawatts of power output from the polyethylene liner (ε=2.3) at a frequency of ~9 GHz View full abstract»

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  • `Spiking' radiation in the Columbia free electron laser

    Page(s): 447 - 450
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    A report is presented on the observation of spikes of high-intensity radiation emitted from a 2-mm wavelength Raman free electron laser oscillator. The spikes are correlated with a well-developed sideband spectrum, including several sideband harmonics, together with the carrier. A pulse width ~150 ps is obtained from analysis of data obtained with a two-slit Young's experiment. An even shorter pulse width results from a numerical model. Using an elementary model of the spike, it is estimated that the peak spike pulse power is ~100 MW View full abstract»

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  • Traveling-wave-tube simulation: The IBC code

    Page(s): 482 - 489
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    An interactive beam-circuit code (IBC) is presented, to run interactively on fast PCs or workstations, for purposes of first-cut design of traveling-wave tubes (TWTs) at small and large amplitudes. The new physics parts are the use of particle-in-cell methods to obtain the space-charge forces, and the following of the electron beam over the full length of the tube. The model is fully nonlinear and one-dimensional, with the transverse space-charge fields approximated by one mode. The slow-wave circuit is modeled by a transmission line. All variables are displayed continuously, such as the velocity displacement of all the particles (phase space), beam charge and current densities, space-charge field, circuit field, voltage and current, circuit power, and the location of the added loss. Some initial runs are presented View full abstract»

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  • Design of high-average-power near-millimeter free electron laser oscillators using short period wigglers and sheet electron beams

    Page(s): 399 - 415
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    The design and feasibility of a 1-MW continuous-wave (CW) free electron laser (FEL) oscillator are reviewed. The proposed configuration includes a short-period (Iw~ 1 cm) planar wiggler, a sheet electron beam, a 0.5-1.0-MV thermionic electron gun, a hybrid waveguide/quasi-optical resonator, commercial DC power supplies, and a depressed collector. Cavity ohmic RF losses are estimated to be extremely low (⩽10-100 W/cm2) at 1/MW output power, while thermal heat transfer studies conservatively indicate that wall cooling up to 1500 W/cm2 should be possible. Experiments have convincingly verified theory and simulations which predict that negligible body currents will be achievable with low-emittance low-space-charge sheet beams. High-voltage sheet beam gun design studies indicate that the required beam quality can be achieved with CW compatible devices. The spent beam energy distribution is consistent with highly efficient spent beam energy recovery, and the proposed resonator cavity should provide mode discrimination and beam/RF separation capability View full abstract»

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  • Pulse shapes for absolute and convective cyclotron-resonance-maser instabilities

    Page(s): 286 - 300
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    A linear analysis is presented for pulse shapes produced by a spatial and temporal delta-function disturbance of cyclotron-resonance-maser modes for the case where the initial equilibrium state is free of radiation. A pinch-point analysis based on the theory of R.J. Briggs (1964) and A. Bers (1983) is employed. Numerical and analytical techniques are developed for the straightforward calculation of pinch-point coordinates in a reference frame moving with arbitrary velocity in the axial direction. Examples analyzed include the absolute instability in the waveguide operating mode, in higher harmonics of the operating mode, and in lower-frequency waveguide modes when the operating mode is a higher-order waveguide mode. Effects of waveguide wall resistance on pulse shapes and the effectiveness of such resistance in suppressing or reducing the growth rates of absolute instabilities are also analyzed View full abstract»

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  • Phase-stability analysis of the magnetron-driven vircator experiment

    Page(s): 580 - 585
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    A slow envelope approximate solution to the Van der Pol equation is evaluated as a model for a driven high-power oscillator experiment. It is shown that Adler's inequality gives a necessary but not sufficient condition for achieving phase-locking between the driving relativistic magnetron and the driven high-power cavity vircator oscillations. The amplitude of the entrained (phase-locked) oscillations is found as a function of the injected magnetron power, the initial frequency detuning, and other system parameters. The stability of these oscillations is examined. Not all entrained states are stable. Parameteric boundaries between stable and unstable states are given. Combination oscillations containing both magnetron and vircator frequency components are predicted and observed to occur when the initial detuning between the two sources is too large to allow entrainment View full abstract»

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  • Plasma effects in a free electron laser

    Page(s): 466 - 471
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    The introduction of a plasma and a strong guide magnetic field in a free electron laser (FEL) slows down the phase velocity of radiation, significantly reducing the requirements on beam energy for generating frequencies below the electron-cyclotron frequency (ω1≲ωc). Around plasma resonance (ω1p), the FEL mode couples to two-stream instability (TSI), attaining a large growth rate, comparable to that of the wiggler-free TSI. At plasma densities comparable to beam density, the beam-induced local depression in the electron density of the plasma acts as a waveguide for guiding any high-frequency radiation when the beam current is ≳17 kA View full abstract»

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  • Kinematics and structure of a plasma-focus current sheath during its radial implosion stage

    Page(s): 646 - 650
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    Measurements of the light intensity emitted by the current sheath in a plasma focus device are presented. For operating deuterium pressures in the range of 1-4 mbar, radial velocity and acceleration data are given, and, making some assumptions about the characteristics of the radiation emerging from the plasma, its transversal density profile is derived. An interpretation of the structure thus obtained is also given. It is consistent with the strong shock wave theory when the acceleration of the current sheath is taken into account View full abstract»

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  • Multimode simulations without particles in the quasi-optical gyrotron

    Page(s): 369 - 386
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    A set of coupled nonlinear differential equations, involving the slow amplitude and phase variation for each mode, is used to simulate the multimode dynamics in the quasi-optical gyrotron. The interaction among various modes is mediated by coupling coefficients of known analytic dependence on the normalized current I, the interaction length μ, and the frequency detunings Δi corresponding to the competing frequencies ωi. The equations include all the possible resonant combinations of up to four different frequencies, ωijkl≃0, among a set of N participating modes, keeping terms up to fifth order in the wave amplitudes. The formalism is quite general and can be used to study mode competition, the existence of a final steady state and its stability, and its accessibility from given initial conditions. It is shown that when μ/β≫1, μ can be eliminated as an independent parameter. The control space is then reduced to a new normalized current I and the desynchronism parameters νiiμ for the interacting frequencies. Numerical simulations for cold beams of various cross sections demonstrate that νi is the most important parameter for the system behavior. Overmoding is not determined by the frequency separation δω among the cavity modes per se, but by the separation among the corresponding desynchronism parameters View full abstract»

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  • Voltage drop over a vacuum arc and the cathode-spot brightness

    Page(s): 682 - 684
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    A spectral investigation of low-current DC vacuum arc voltage is presented. High-speed streak photographs of cathode-spot light emission with high time resolution were obtained and are discussed. Characteristic frequencies of cathode-spot instabilities are derived from average frequency spectra of the arc voltage View full abstract»

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  • Special complex open-cavity and low-magnetic-field high-power gyrotron

    Page(s): 326 - 333
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    A theory is proposed for the special complex cavity; it is in the form of a single resonant circuit having a TE0n&lrarr2;TE0,n+p mode converter and it features excellent mode selectivity, high power capability, and an asymmetric triangle profile of the RF field that is favourable to efficient operation for a 35-GHz second-harmonic gyromonotron employing this complex cavity with TE03 mode output are numerically illustrated and experimentally demonstrated. Power as high as 200 kW and efficiency as high as 30% have been obtained. These experimental results are record values for a gyrotron operating at the second-harmonic millimeter wavelength. Implications of the single-resonant complex cavity for the fundamental harmonic and third-harmonic high-average-power gyrotron design are discussed View full abstract»

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  • High-power microwave generation by excitation of a plasma-filled rippled boundary resonator

    Page(s): 497 - 506
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    An experimental demonstration of a strong enhancement of the interaction efficiency in a high-power relativistic backward-wave oscillator when plasma is injected is presented. Controlled plasma injection enhances the interaction efficiency for the vacuum case by a factor of up to eight to a value of about 40%. A linear theory of electromagnetic wave generation in plasma-loaded corrugated wall resonators is reviewed. A number of physical mechanisms are considered to account for the enhanced interaction, including two variations of a three-wave interaction involving the electron-beam slow space-charge wave, the slow electromagnetic waves in the structure, and the quasi-electrostatic waves in the plasma View full abstract»

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  • Linear theory of electromagnetic wave generation in a plasma-loaded corrugated-wall resonator

    Page(s): 537 - 545
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    A linear theory of the excitation of electromagnetic waves in a plasma-filled corrugated-wall waveguide with an arbitrarily large sinusoidal corrugation has been derived and analyzed numerically. The theory predicts that, when driven by an electron beam, the presence of a plasma in the slow wave structure will cause an increase in the oscillation frequency, and that the temporal growth rates of a high-frequency mode approach those of the fundamental mode for high plasma densities. The latter result may account for the high-frequency modes observed in the authors' plasma-filled backward-wave oscillator View full abstract»

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  • Dependence of relativistic backward wave oscillator properties on effective beam gamma

    Page(s): 490 - 496
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    The operation of a backward wave oscillator (BWO) is shown to be critically dependent on the energy of the slow space-charge wave of the electron beam. Experimental work parameterizing the dependence of microwave frequency on effective beam energy, γbeam, reveals that through an understanding of electron-beam dynamics, a BWO could be systematically tuned through a desired frequency range while maintaining a high power of a few hundred megawatts and narrow frequency bandwidth, which was 400 MHz. Through variation of γbeam, 1.2 to 1.5 for the experiment, the lack of scaling of peak microwave power with the kinetic energy of the electron beam for γbeam >1.32 was observed. This effect was previously found in numerical simulation. In order to explain this effect, the relationship of the beam current to the space-charge-limiting current for increasing γbeam is examined. Dramatic evidence of pulse shortening, a phenomenon known to relativistic oscillators, was also seen View full abstract»

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  • Free electron laser pumped by a powerful traveling electromagnetic wave

    Page(s): 437 - 446
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    A three-wave free-electron laser (FEL) was operated with a powerful 8.4-GHz electromagnetic pump wave replacing the usual magnetostatic wiggler. The presence of a uniform axial magnetic field B0 produced cyclotron-harmonic idler waves. Peaks in the emission spectrum corresponding to cyclotron harmonics were observed covering a frequency range from 16.5 to 130 GHz. The frequency spectrum of this novel FEL mechanism was tuned continuously by the variation of B0 View full abstract»

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  • Two-stream instability in free electron lasers

    Page(s): 416 - 423
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    The analysis of the interaction between electromagnetic waves and electron beams in semi-infinite wigglers is presented. The beam is assumed to include two cold streams of electrons; hence, its space-charge waves may be unstable (two-stream instability). In the wiggler, this instability is shown to yield much larger growth rates (up to seven times greater) than in the conventional one-stream free electron laser (FEL). Accordingly, the gain per pass is enhanced by orders of magnitude. The enhancement of the two-stream instability is shown to be most effective for short-period wigglers 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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