Plasma-filled Backward-wave Oscillator Research Articles

Dispersion relation of a magnetized plasma-filled backward wave oscillator

A linear theory and a more general dispersion relation of electromagnetic radiation from a magnetized plasma-filled backward wave oscillator with sinusoidally corrugated slow-wave structure driven by a solid intense relativistic electron beam have been given. The comparisons show good agreement with the previous works when B0 → ∞ and ωb=0 from this dispersion relation.

Frequency and power response of high-power plasma-filled backward-wave oscillators

Backward-wave oscillators (BWO) have long been used as voltage-tunable signal sources, and more recently as high-power microwave (HPM) sources. The frequency at which a BWO oscillates is commonly viewed as being determined by the circuit geometry and the electron beam parameters. However, several techniques used to improve the efficiency and reduce the size of HPM BWOs, such as the Pasotron, cause the frequency to vary in discrete steps with changes in the beam parameters, and a plasma in these device can cause the frequency to chirp (increase significantly) during the pulse. End reflections in low-loss circuit designs and excessive plasma generation in these new source geometries produce nonclassical frequency and power behavior.

Interaction between moderately high power microwaves and plasma in corrugated wall waveguides

Local protuberances in negatively biased probe current (NBPC) after the incidence of moderately high power microwave pulses in a plasma-filled corrugated waveguide are observed. The aim of the present experiment is a fundamental study in the research of high power plasma-filled backward wave oscillators. Protuberances in NBPC profile and the peaks of standing waves of the RF electric field are observed always at the identical axial positions where the radius of the corrugated wall is the minimum. The electron energy distribution after the incidence of the microwave approaches the Druyvesteyn distribution rather than conventional Maxwellian distribution that is observed before microwave incidence. The protuberances in NBPC are found to consist of a uniform increase in electron temperature and of a localized rise in plasma density that is calculated from ion saturation currents. Physical reasons for the observations are discussed in some detail.

The Rayleigh hypothesis applicability criterion for periodic plasma-filled waveguides

The necessary and sufficient conditions for applicability of spatial harmonic expansions (the Rayleigh hypothesis) for plasma wave branches in periodic plasma-loaded waveguides have been obtained for the first time, based on the exact solution for the relevant boundary-value problem. Conditions of oscillation appearance beyond the scope of such expansions and their expected effect on the operation of plasma-filled backward-wave oscillators are discussed.

Linear Analysis of Axisymmetric Modes in Plasma-Filled Waveguides with Finite Strength Axial Magnetic Fields

Numerical analyses on dispersion relations of the axisymmetric quasi- TM 0 s and quasi- TE 0 s modes in a plasma-filled cylindrical waveguide and a corrugated wall metal slow wave structure are carried out under the condition of finite axial magnetic fields. It is shown that a hybrid mode given by a linear combination of extraordinary and ordinary modes satisfies the boundary conditions of plasma at metal surface. There are two genera in the branches in the dispersion relation. One of them is conventional electromagnetic modes and the other is Trivelpiece-Gould modes. Present analysis will be helpful to understand the various unsolved problems in high power plasma-filled backward wave oscillators.

Ponderomotive effects in plasma-filled backward-wave oscillators

A numerical model is presented for analyzing plasma-filled backward-wave oscillators (BWOs) operating near cutoff. The model allows for the investigation of the effects of the ponderomotive potential of the high-frequency electromagnetic waves on the motion of plasma electrons. As a result of their motion the electron plasma density is modified, and this affects the high-frequency radiation by modifying the dispersion characteristics of the slow wave structure. Two approaches for modeling the plasma are considered, a fluid model and a particle-in-cell model. Nonlinear simulations are performed to investigate the possible excitation of plasma waves over a range of background plasma densities. Results from nonlinear simulations show that for low plasma densities, electrons clump in regions of low high-frequency electric field. At somewhat higher densities nonlinear instabilities of the Raman type are excited. The model does not indicate the cause of the observed efficiency enhancement in plasma filled backward wave oscillators.

Excitation of electromagnetic waves in a plasma-filled backward wave oscillator in the Ku region

The effect of plasma on backward wave oscillator (BWO) emission in a 12–18 GHz frequency range is given. It is shown that the Ku band emission corresponds to the cylindrically symmetric transverse magnetic TM01 and not the higher mode. Due to the slowing down of waves with the increase in plasma density there occurs a BWO mode switching, i.e., BWO switches from a bounded mode to an unbounded one. Here the analysis for the switched BWO in the unbounded mode is given. The expressions for the temporal and spatial growth rates in the presence of plasma are obtained. The efficiency calculations are also given. Comparison of the results with that of a previous experiment [IEEE Trans. Plasma Sci. 18, 497 (1990)] has been carried out.

Pulse shortening causes in high power BWO and TWT microwave sources

Pulse shortening, an effect where the microwave output power from a high-power tube terminates or significantly degrades well before the end of the electron beam pulse, severely limits the energy per pulse and average power capability of many high power microwave (HPM) sources. The cause of pulse shortening varies from device to device, and different causes can simultaneously contribute to the observed power reduction behavior which tends to obscure the underlying mechanisms and possible solutions. In this paper, we show a variety of experimental situations that lead to pulse shortening in HPM sources. The mechanisms of the different pulse shortening triggers are examined in detail in high-vacuum traveling wave tubes (TWT) and plasma-filled backward-wave oscillators (BWO). We find that there are many different causes of pulse shortening such as arcing, mode competition, beam instability, etc. However, the most commonly observed situation that leads to pulse shortening is the combination of sufficiently high power electron beams and poor vacuum conditions that lead to plasma generation. The presence of plasma significantly modifies the beam coupling to the circuit, which can affect the microwave production efficiency on very short time scales. The situations lending to pulse shortening and possible solutions are presented.

Initial plasma-filled backward-wave oscillator experiments using a cathode-mounted plasma prefill source

The introduction of plasma into the slow-wave structure (SWS) of a backward-wave oscillator (BWO) has been shown to increase microwave power output and generation efficiency, as well as provide several other benefits. Researchers at Niigata University in Japan have performed a linear analysis that has shown that the optimal plasma-filled BWO configuration is one in which the plasma is confined close to the axis of the SWS, while the electron beam driving the device is kept close to the walls. Previous schemes for preinjecting plasma have utilized sources mounted downstream of the SWS and have relied upon the converging field lines of the electron beam's guiding magnetic field to compress the plasma close to the axis as it entered the SWS. This paper presents data from experiments using a plasma source mounted internal to the cathode structure of a high-power BWO, thus providing injection of the plasma directly on axis in the strong, uniform field region of the BWO. Data describing plasma prefill density at different axial positions within the SWS prior to operation is presented. The plasma prefill has been found to both enhance and reduce the microwave generation efficiency, depending on the plasma density. Details of the novel plasma injection system are also provided.

Experiment on a large-diameter plasma-filled backward-wave oscillator

Abstract A large-diameter plasma-filled backward-wave oscillator (BWO) is investigated experimentally. The parameters of slow wave structure are chosen so that the oscillation frequency is about 20 GHz at 60 keV beam energy. Plasma is produced by the beam and has favorable effects for beam propagation and Cerenkov oscillations. The output power of the BWO with plasma is observed to be three to six times that of vacuum BWO. The power level is several kilowatts and the efficiency is about 0.01%. For Cerenkov oscillations of a large-diameter BWO, the beam energy mainly determines the starting conditions for oscillation. The output power is strongly enhanced when the guiding magnetic field approaches the fundamental electron cyclotron resonance. This mechanism is closely related to the anomalous Doppler cyclotron resonance. The maximum power of 480 kW with an efficiency of 5% is achieved even for a relatively low beam energy of 60 keV.

Experimental study of a plasma-filled backward wave oscillator

Experimental studies of a plasma-filled X-band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from <or=0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X-band Tm/sub 01/ mode prevailed when the background plasma density was below 6*10/sup 11/ cm/sup -3/. At a critical background plasma density of approximately=8*10/sup 11/ cm/sup -3/ power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse.<<ETX>>

Effect of plasma on efficiency enhancement in a high power relativistic backward wave oscillator

A linear theory of the excitation of electromagnetic waves in a plasma-filled backward-wave oscillator driven by an intense relativistic electron beam is presented. It is found that the spatial growth rate of backward-wave instability exhibits a resonant increase for a particular value of fill-plasma density. Results are compared to the results of an experiment by K. Minami et al. (1988) on a high-power backward-wave oscillator. >

Relativistic plasma microwave electronics: Studies of high-power plasma-filled backward-wave oscillators

The area of relativistic plasma microwave electronics has only recently generated renewed interest. New experimental data are presented demonstrating that the presence of a low-density background plasma in a relativistic backward-wave oscillator leads to several beneficial effects, including (a) enhanced interaction efficiency (40%), (b) operation at very low and possibly zero guiding magnetic field, (c) tunability by controlling the plasma density, (d) high degree of spectral coherency, and (e) operation well above the vacuum limiting current.

Observation of Trivelpiece-Gould modes in a plasma-filled backward wave oscillator.

Trivelpiece-Gould modes (TG modes) have been observed with frequencies up to the background plasma frequency in a plasma-filled X-band backward wave oscillator. The TG mode emission was broadband with observed frequencies ranging from 2.6 GHz, the lower limit of our frequency diagnostic, to ${\mathit{f}}_{\mathit{p}}$, the plasma frequency. Mode competition between the TG modes and the X-band ${\mathrm{TM}}_{01}$ mode prevailed when the background plasma density was below 6\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (${\mathit{f}}_{\mathit{p}}$\ensuremath{\sim}7 GHz). At a critical background plasma density of ${\mathit{n}}_{\mathrm{cr}}$\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (${\mathit{f}}_{\mathit{p}}$\ensuremath{\sim}8 GHz) a sudden power enhancement appeared simultaneously in both X band and the TG modes. Power enhancements in S and J bands were up to 8 and 3 dB, respectively, with the X band enhanced up to a factor of 6.

Linear analysis of a finite length plasma-filled backward wave oscillator

Absolute instability in a plasma-filled backward wave oscillator with sinusoidally corrugated slow wave structure driven by an intense relativistic electron beam has been analyzed numerically. The maximum spatial growth rate of the plasma-filled waveguide is found to be larger than that of an optimally designed vacuum structure. The excitation of a finite length structure is investigated. Although the spatial growth rates obtained in the linear theory increase with plasma density in the finite length plasma-filled structure for the fundamental TM01 mode of oscillation, negative global spatial growth rates prevent the coherent oscillation for intermediate ranges of plasma density, beam current, and even for structure length.

Cherenkov-cyclotron resonance interaction in a high-power plasma-filled backward-wave oscillator.

A possible explanation of high-power microwave radiation from a plasma-filled backward-wave oscillator in the presence of an axial magnetic field has been provided. The increase in power output with magnetic field can be explained by taking into account the simultaneous existence of Cherenkov and cyclotron resonances.

Efficiency enhancement of a plasma-filled backward-wave oscillator by self-induced distributed feedback.

It is shown that the high efficiency of a plasma-filled backward-wave oscillator can be accounted for by a self-induced distributed feedback mechanism, which is set up by a density grating formed in the plasma by the counterpropagating electromagnetic waves. The efficiency increases with the plasma density, up to a critical density, where the oscillations are quenched.

Absolute instability for enhanced radiation from a high-power plasma-filled backward-wave oscillator.

The linear theory of electromagnetic radiation from a backward-wave oscillator with a plasma-filled, sinusoidally corrugated waveguide driven by a relativistic electron beam has been derived and analyzed numerically. The presence of plasma can cause a substantial increase in the spatial growth rate of the absolute instability. For high plamsa densities, however, the absolute instability is suppressed and only the convective instability remains. The predicted radiation enhancement can be attributed to a decrease in the group velocity of the backward wave in the presence of a plasma.

Emission of plasma cyclotron waves in plasma-filled backward-wave oscillators.

Grâce a la simulation numerique des oscillateurs a ondes regressives remplis de plasma on trouve qu'a l'interieur d'une certaine gamme de valeurs pour le champ magnetique, le taux de croissance de l'interaction cyclotron faisceau-plasma est significativement plus eleve que l'oscillation d'onde regressive conventionnelle

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

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. >