Plasma Relativistic Microwave Research Articles

Effect of the Plasma Density on the Generation Efficiency and Changes in the Spectrum of a Plasma Relativistic Microwave Generator

Effect of the Plasma Density on the Generation Efficiency and Changes in the Spectrum of a Plasma Relativistic Microwave Generator

Physical analysis and numerical simulations of ultra wideband plasma relativistic microwave noise amplifier

The physical mechanism and output properties of the plasma relativistic microwave noise amplifier (PRNA) are studied numerically by using the all electromagnetic particle-in-cell (PIC) code. Firstly, the dispersion relation between the operating mode and the slow space charge wave of relativistic electron beam without coupling is simulated and analyzed. Simulation results show that both the plasma density <i>n</i><sub>p</sub> and radial thickness Δ<i>r</i><sub>p</sub> affect the dispersion characteristics markedly and their increasing can lead the frequency at the beam-wave resonant point to be enhanced. The beam voltage and current also affect the resonant frequency, but the effect is relatively slight. Secondly, variation of the linear growth rate and the bandwidth are then evaluated by using the linear theory. Calculations show that the PRNA has the virtue of wideband output. Its bandwidth can reach a GHz level. By adjusting the plasma parameters <i>n</i><sub>p</sub> and Δ<i>r</i><sub>p</sub>, the relativistic electron beam voltage and current, the operating frequency can be tuned over a wide frequency range. Therefore the PRNA also has virtue of fine frequency tunability. Based on the above calculation results, the whole PIC simulations of the PRNA are then carried out to verify the virtues of wideband microwave output and frequency tunability. The basic features of the field distributions of the operating in the evolution process and out coupling process are given. The bunching process and the energy release process of relativistic electron beam are also plotted. Simulations show that with a plasma density of 1.4×10<sup>19</sup> /m<sup>3</sup>, beam voltage and current of 500 kV and 2 kA and applied magnetic field of 2.0 T, 200 MW output microwave with efficiency about 20% can be obtained. The frequency ranges from about 7.0 to 9.0 GHz, the band width reaches 2 GHz. And the output mode is the TEM mode of the coaxial waveguide. Both <i>n</i><sub>p</sub> and Δ<i>r</i><sub>p</sub> affect the dispersion relations markedly and the output frequency increases clearly with <i>n</i><sub>p</sub> and Δ<i>r</i><sub>p</sub> increasing. The influence of beam voltage and current on the output frequency are both relatively small and the gap distance between the plasma and electron beam has little effect on the output frequency. The research results will provide useful reference for further designing the PRNA.

On radiation dynamics in plasma relativistic microwave amplifier at the edge of electron beam pulse

We consider dynamics of radiation of a plasma microwave amplifier on a surface wave at the leading edge of the relativistic electron beam pulse. It has been shown that depending on the electron density of the plasma, different modes of operation of the amplifier are possible. At low plasma density, there is an early activation of the amplifier with large gain at the leading edge of the electron beam pulse and a significant decrease in the amplification of the signal when the electron energy reaches a plateau. With a higher plasma density, a later activation takes place with an almost constant output amplitude. The effect of ponderomotive force on plasmas was discussed.

Effect of the Erosion of Collector Surface on the Operation of a Pulse-Periodic Plasma Relativistic Microwave Generator

The pulse-periodic operation regime of a plasma relativistic microwave generator is considered. The reasons for the increase of average frequency of the output microwave radiation in a pulse train created in the same conditions of initial plasma generation are analyzed. The effect of solid body erosion that occurs due to the interaction of the relativistic electron beam with the collector surface on the output microwave radiation is studied. The analysis of experimental data is accompanied by numerical simulations by the electromagnetic PIC code KARAT. It is shown that increasing the amount of mater extracted from the surface of the collector during erosion leads not only to an increase of the plasma density, but it can also lead to qualitative changes of the parameters of the output radiation, which are caused by the change of microwave generation regime. In particular, the generation of an ion background that is inhomogeneous in the radial direction caused a change of conditions of plasma–beam interaction during the second half of the pulse and a change of the spectral density of radiation at the main frequency. Numerical simulations were used to demonstrate this for two different admixtures, iron and graphite.

Spectra of Plasma Relativistic Microwave Amplifier of Monochromatic Signal

The ratio of the radiation energy of the plasma relativistic microwave amplifier at a frequency of the input signal to the noise energy is measured. In the absence of the input signal, the noise radiation is detected. It is shown that, under certain conditions, the amplifier noise energy sharply decreases, as compared to the noise energy measured in the absence of the input signal.

Broadband Plasma Relativistic Microwave Source with a Short Pulse Duration

This study continues a series of works on creating a broadband plasma microwave sources on the basis of a smooth waveguide. Various methods for creating microwave sources and various methods for feedback uncoupling are considered with the purpose of generating continuous-spectrum microwave radiation. It is shown by numerical simulations that the plasma relativistic oscillator can be transformed into a noise amplifier by changing the parameters of the system. The numerical simulations allow one to determine the operating parameters of the experimental devices that are planned to be created and investigated in the future. It is shown that, when the time of the wave passage along the microwave oscillator exceeds the duration of the relativistic electron beam (REB) pulse, the microwave spectrum is continuous. However, if the wave passage time is much shorter than the REB pulse duration, then a line microwave spectrum will be observed at frequencies at which the oscillator length is a multiple of the half-wave, because microwave generation occurs on the longitudinal types of plasma oscillator modes. The tuning of the average microwave frequency from 3 to 9 GHz is achieved at a power of about 40 MW.

Effect of the Distance of Plasma–Beam Interaction on the Oscillation Regimes in a Plasma Relativistic Microwave Oscillator

We study the effect of the distance of plasma–beam interaction on the parameters of the output radiation of a plasma relativistic microwave generator (PRMO). In the experimental PRMO based on the Sinus 550-80 accelerator, the characteristics of the output microwave radiation (spectral width, spectral density of the amplitude of the radiation) vary during a single pulse of the relativistic electron beam (REB). We analyze and numerically simulate the experimental data; the geometric and physical parameters of the device used in the simulation are close to those in the experiments. We show that increase in the distance of the plasma–beam interaction (the length of the system) causes the duration of the effective microwave pulse generation to approach that of the REB despite the presence of ion background and the plasma density profile modification, i.e., no disruption or change in the generation regime occurs during the REB pulse, while the microwave radiation spectrum broadens.

Change in the Generation Mode of the Plasma Relativistic Microwave Oscillator

Time evolution of the parameters of output radiation of a plasma relativistic microwave oscillator (PRMO) based on the Sinus 550-80 accelerator during a single relativistic electron beam (REB) pulse was studied experimentally. Analysis of the experimental data was accompanied by numerical simulations under conditions close to the experimental ones. It is shown that qualitative changes in the parameters of the PRMO output radiation are associated with a change in the generation mode. The first half of the REB pulse is characterized by the maximum output radiation power and a broadband spectrum consisting of a great number of harmonics. The nonlinear beam–plasma interaction limits the growth of the field amplitude, decreases the number of plasma electrons, and leads to the emergence of longitudinal plasma inhomogeneity. The appearance of an ion background in the drift tube volume and the formation of a radially nonuniform plasma waveguide alter the conditions of beam–plasma interaction in the second half of the pulse. The second half of the REB pulse is characterized by a decrease in the output microwave power and the narrowing of the generation frequency band. Spectral analysis demonstrates preservation of a stable narrow spectral component throughout the microwave pulse.

Radiation Spectra of the Plasma Relativistic Microwave Oscillator

The spectral composition of high-power microwave radiation of the plasma relativistic oscillator in the range of 1−4 GHz was studied experimentally as a function of the plasma density and the cavity length and configuration. The duration of the generated microwave pulse was in the range of 250−400 ns, and its total energy reached 15 J. The main features of the spectral composition for different cavity configurations are explained.

Ultrawideband Plasma Relativistic Microwave Source

An ultrawideband (UWB) plasma relativistic microwave source based on the amplification of self-noise of a relativistic electron beam in plasma is described. There is a fundamental difference of the described source from the existing high-power UWB radiation sources. The UWB plasma microwave source allows one to obtain higher energy per pulse than existing sources (up to 10 J). In addition, the mean frequency of the microwave signal generated by the UWB plasma microwave source can be tuned from pulse to pulse (e.g., from 2 to 3.5 GHz). It is important that the microwave pulse energy remains unchanged after tuning the frequency.

Plasma Masers: Status Quo and Development Prospects

Consideration is given to the research status and prospects for development of plasma relativistic microwave electronics as a basis for plasma masers. Possible applications are determined for plasma masers as high-power sources of microwave pulses. Design features of masers and their radiation characteristics are considered with a view to the type of device: self-oscillator, external signal amplifier, or noise amplifier.

Excitation of Surface Plasma Waves in Microwave Radiation Sources by Electron Beam with Allowance for Thermal Velocity Spread

Based on the hydrodynamic model, in the linear approximation, the problem of excitation by an electron beam of surface waves in electrodynamic systems of plasma relativistic microwave electronics has been considered with consideration of electron velocity spread. Complex instability increments have been determined for complex parameters of the beam-plasma system. Two instability regimes differing in the dynamics of beam plasma oscillations during instability development have been observed: Compton (singleparticle) and Raman (collective) instabilities. The role of thermal effects in an electron beam and its density in the formation of a particular mode of beam instability has been analyzed.

Influence of Plasma Unsteadiness on the Spectrum and Shape of Microwave Pulses in a Plasma Relativistic Microwave Amplifier

Dependence of the shape of a microwave pulse in a plasma relativistic microwave amplifier (PRMA) on the initial plasma electron density in the system is detected experimentally. Depending on the plasma density, fast disruption of amplification, stable operation of the amplifier during the relativistic electron beam (REB) pulse, and its delayed actuation can take place. A reduction in the output signal frequency relative to the input frequency is observed experimentally. The change in the shape of the microwave signal and the reduction in its frequency are explained by a decrease in the plasma density in the system. The dynamics of the plasma density during the REB pulse is determined qualitatively from the experimental data by using the linear theory of a PRMA with a thin-wall hollow electron beam. The processes in a PRMA are analyzed by means of the KARAT particle-in-cell code. It is shown that REB injection is accompanied by an increase in the mean energy of plasma electrons and a significant decrease in their density.

Nonlinear dynamics of beam–plasma instability in a finite magnetic field

The nonlinear dynamics of beam–plasma instability in a finite magnetic field is investigated numerically. In particular, it is shown that decay instability can develop. Special attention is paid to the influence of the beam−plasma coupling factor on the spectral characteristics of a plasma relativistic microwave accelerator (PRMA) at different values of the magnetic field. It is shown that two qualitatively different physical regimes take place at two values of the external magnetic field: B 0 = 4.5 kG (Ω ~ ωB p ) and 20 kG (Ω B ≫ ωp). For B 0 = 4.5 kG, close to the actual experimental value, there exists an optimal value of the gap length between the relativistic electron beam and the plasma (and, accordingly, an optimal value of the coupling factor) at which the PRMA output power increases appreciably, while the noise level decreases.

Microwave pulse shortening in plasma relativistic high-current microwave electronics

We describe mechanisms of microwave pulse shortening in radiation sources with the power of about 108 W based on interaction between relativistic electron beams of nanosecond duration and preformed plasma. The shortening is mainly due to the electron return flow through the plasma, which leads to a multiple decrease in the linear gain of the microwave by the relativistic electron beam and in the reflection coefficient of the plasma wave that provides the generator feedback. The ways to eliminate the effect of microwave pulse shortening are proposed.

Visualization of the microwave beam generated by a plasma relativistic microwave amplifier

A method based on the detection of emission of a dielectric screen with metal microinclusions in open air is applied to visualize the transverse structure of a high-power microwave beam. In contrast to other visualization techniques, the results obtained in this work provide qualitative information not only on the electric field strength, but also on the structure of electric field lines in the microwave beam cross section. The interpretation of the results obtained with this method is confirmed by numerical simulations of the structure of electric field lines in the microwave beam cross section by means of the CARAT code.

Plasma relativistic microwave noise amplifier with the inverse configuration

The results of numerically simulating a high-power plasma maser—the wideband microwave noise amplifier—are reported. The configuration and parameters of the device are chosen with a view to its possible future use for generating pulses with a duration of 3 ns and pulse frequency bandwidth of 0.3 to 2GHz. In the pulse-periodic mode, it is possible to generate radiation in a continuous range of 2 to 12GHz with an average pulse power of 20MWand pulse energy efficiency of 6%.

Noise characteristics of a plasma relativistic microwave amplifier

Reasons for the occurrence of microwave noise at the output of a plasma relativistic amplifier have been analyzed. It is found that, in the absence of an input signal, the emission spectrum of the plasma relativistic microwave amplifier is similar to that of an electron beam in vacuum. It is concluded that microwave noise at the output of the amplifier appears as a result of amplification of the intrinsic noise of the electron beam. The emission characteristics of a relativistic electron beam formed in a magnetically insulated diode with an explosive emission cathode in vacuum have been studied experimentally for the first time. An important point is that, in this case, there is no virtual cathode in the drift space.

On the development of plasma relativistic microwave oscillator without strong magnetic field

A method for generating microwave pulses with a power of ∼1 GW and a frequency controlled in a band from 2 to 15 GHz during the interaction of relativistic electrons with plasma without strong magnetic field is proposed in the numerical model. The combination of a broadband plasma relativistic microwave oscillator and a monochromatic magnetically insulated transmission line oscillator (MILO) allows the development of a device combining advantages of both prototypes.

Output power variations in a plasma relativistic microwave amplifier during a 500-ns relativistic electron beam current pulse

A relativistic plasma microwave amplifier with a gain of about 30 dB and an output power of about 60–100 MW in the frequency range from 2.4 to 3.2 GHz is studied experimentally. The total duration of the output microwave pulse is equal to the duration of the current pulse of the driving relativistic electron beam (500 ns); however, the maximum output power is observed only within 200 ns. It is shown that variations in the output microwave power during the current pulse of the annular relativistic electron beam are caused by variations in the beam radius and thickness. Analysis of the experimental data and results of numerical simulations show that the thickness of the electron beam is determined by the density of the cathode emission current.