Tunable plasma relativistic microwave amplifier

A stable regime of the amplification of a slow plasma wave in a plasma waveguide during the injection of a high-current relativistic electron beam is obtained. For an input-signal frequency of 9.1 GHz, there exists a range of plasma densities in which the spectrum of the output microwave radiation lies in a 0.5-GHz-wide band. For a 40-kW input power at a frequency of 9.1 GHz, the maximum output power is 8 MW. It is shown experimentally for the first time that the beam-plasma amplifier can operate at frequencies of 9.1 GHz and 12.9 GHz. The range of plasma densities in which the regime of amplification is observed agrees with the results of calculations based on linear theory.     

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.     

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.     

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.     

Normal Doppler effect in experiments on the interaction of relativistic electron beams with plasma: Plasma relativistic microwave amplifier

The Cherenkov interaction of a high-current relativistic electron beam with a spatially bounded plasma was studied experimentally. In the generation of electromagnetic radiation, an important role is played by the counterpropagating plasma wave produced due to the reflection from the end of the plasma column. It is shown that, at the resonant value of the magnetic field, the normal Doppler effect occurs and the amplitude of the counterpropagating wave decreases. This effect was used to design and create a plasma relativistic microwave amplifier in which 10% of the beam energy is converted into radiation. The radiation frequency is 9.1 GHz, and the radiation spectrum width (±0.17%) is determined by the microwave-pulse duration. The maximum radiation power is 100 MW, the gain factor being 32 dB.     

Study of the development of relativistic plasma bunches in a long mirror trap by optical and X-ray imaging and numerical simulations

The work presents experimental results demonstrating the feasibility of autoresonance acceleration of electrons in a long mirror trap with a reverse magnetic field. It is shown that gyromagnetic autoresonance results in the formation of a plasma bunch with average electron energy of several hundred keV, which is confined for a long time in the trap. The results of computer simulations of the regime of reverse gyromagnetic autoresonance agree well with the experimental data.     

A 50-MW broadband plasma microwave amplifier

Plasma Physics Reports - A pure amplification regime (without accompanying generation) at two frequencies of 9.1 and 13 GHz is achieved in a plasma relativistic microwave amplifier. It is shown...     

Spectroscopic measurements and numerical simulations of the electrode plasma of an electrode microwave discharge in hydrogen

The structure of an electrode microwave discharge in hydrogen at pressures of 1–8 torr and incident powers of 20–100 W is studied using optical spectroscopy. A two-dimensional computer code is developed for self-consistently simulating a self-sustained steady-sate electrode microwave discharge ignited at the end of the inner conductor of a coaxial line. The model is based on simultaneously solving time-dependent Maxwell’s equations, the balance equations for charged particles, and a homogeneous Boltzmann equation. The numerical results referring to the electrode region of the discharge are in fair agreement with the experimental data. This confirms the early suggestion (inferred from experimental data) of the combined “self-sustained-non-self-sustained” character of the electrode discharge. It is shown that the self-sustained discharge domain is located in the electrode region of the discharge.     

Control over the radiation spectrum of a microwave plasma relativistic oscillator

General features of the operation of microwave oscillators based on the Cherenkov resonance interaction of a high-current relativistic electron beam with a preformed plasma are considered. Emphasis is placed on the presence of longitudinal modes of the plasma-beam resonator that make it possible to tune the radiation frequency. Methods by which the radiation frequency can be varied severalfold continuously or in discrete controlled steps and the width of the spectrum of simultaneously generated frequencies can be changed substantially are described. The results of numerical simulations are compared with available experimental data.     

The use of a high-current electron beam in plasma relativistic microwave oscillators

Relativistic microwave electronics faces the problem of using high currents of relativistic electron beams; i.e., it is possible to use beams the current of which is lower than that of actually existing high-current accelerators. We show the possibility of increasing the power of radiation generated in a plasma relativistic microwave oscillator (PRMO) due to an increase in the absolute value of current. For the beam currents close to the value of limiting vacuum current, the efficiency of microwave generation decreases; therefore, we study PRMO schemes with a high value of limiting vacuum current, i.e., schemes with a small gap between a hollow relativistic electron beam and the waveguide wall. The results of the experiment and numerical simulation are discussed.     

Present status of the theory of relativistic plasma microwave electronics

Theoretical research on high-power microwave sources based on stimulated emission from relativistic election beams in plasma waveguides and resonators is reviewed. Both microwave amplifiers and oscillators are investigated. Two mechanisms for stimulated emission—resonant Cherenkov emission from a relativistic electron beam in a plasma and nonresonant Pierce emission arising from the onset of a high-frequency Pierce instability—are studied theoretically. The theory developed here is motivated by recent experiments carried out at the Institute of General Physics of the Russian Academy of Sciences and is aimed at creating high-power pulsed plasma microwave sources [both narrowband (Δω/ω<0.1) and broadband (or noisy, Δω/ω≈1)] based on high-current relativistic electron beams. Although the paper is devoted to theoretical problems, all analytic estimates and numerical calculations are made with real experiments in mind and theoretical results are compared with reliable experimental data. Special attention is paid to the opportunity to progress to short (millimeter) and long (decimeter) wavelength ranges. Some factors that influence the formation of the wave spectra excited by relativistic electron beams in plasma sources are discussed.     

Plasma relativistic microwave amplifier with smooth frequency tuning from 2.4 to 3.2 GHz

Earlier, it was shown that the plasma relativistic microwave amplifier can operate at two frequencies, 2 and 3.2 GHz. In the present work, it is shown that, by varying the plasma density from one microwave pulse to another, it is possible to amplify the input signals to a power of 50?C80 MW at any frequency in the range 2.4?C3.2 GHz.     

Repetitively rated plasma relativistic microwave oscillator with a controllable frequency in every pulse

A repetitively rated microwave oscillator whose frequency can be varied electronically from pulse to pulse in a predetermined manner is created for the first time. The microwave oscillator has a power on the order of 10⁸ W and is based on the Cherenkov interaction of a high-current relativistic electron beam with a plasma preformed before each pulse. Electronic control over the plasma properties allows one to arbitrarily vary the microwave frequency from pulse to pulse at a pulse repetition rate of up to 50 Hz.     

Experimental study of the plasma effect on the generation of microwave radiation in systems with a virtual cathode

Results are presented from experimental studies of the plasma effect on the generation of microwave radiation in systems with a virtual cathode. Using a triode with a virtual cathode as an example, it is shown that the cathode and anode plasmas reduce the generation efficiency; in particular, the power of the generated microwave radiation decreases and the radiation frequency and the microwave pulse duration change appreciably. It is demonstrated that, at high microwave powers, the power radiated into free space can be reduced by the plasma generated at the surface of the output window. This plasma appears due to discharges developing on the window surface under the combined action of bremsstrahlung, UV radiation, electrons and ions arriving from the beam formation zone, and the microwave electric field.     

Experimental study and numerical simulation of ion cyclotron heating of a hydrogen plasma in the T-11M tokamak

Results are presented from investigations of the possibility of heating a hydrogen plasma at the fundamental harmonic of the ion cyclotron frequency in the T-11M tokamak. The fluxes of charge-exchange atoms that escape from the plasma in the radial direction and across the toroidal magnetic field (transverse neutrals) were recorded by a Lakmus neutral particle analyzer. Measurements by the analyzer show that, during an RF pulse, the ion temperature increases by approximately 50–100 eV. Such plasma parameters as the ion temperature, rotation velocity, and isotopic composition were measured by a high-resolution spectrometer. According to the data from high-resolution spectroscopy, the ion temperature increases by approximately 150 eV. Results from numerical simulations of the ion cyclotron resonance heating of a hydrogen plasma in the T-11M tokamak are also given.     

Electromagnetic wave in a relativistic magnetized plasma

Results are presented from a theoretical investigation of the dispersion properties of a relativistic plasma in which an electromagnetic wave propagates along an external magnetic field. The dielectric tensor in integral form is simplified by separating its imaginary and real parts. A dispersion relation for an electromagnetic wave is obtained that makes it possible to analyze the dispersion and collisionless damping of electromagnetic perturbations over a broad parameter range for both nonrelativistic and ultrarelativistic plasmas.     

Emission spectra of a cherenkov plasma relativistic maser

The spectra of a plasma relativistic maser are measured. It is shown that the microwave frequency can be varied from 4 to 28 GHz by varying the plasma density from 4×10¹² to 7×10¹³ cm^?3 at a power of 30–50 MW. The relative width of the emission spectrum is within 50–80% for low plasma densities and 15–30% for high densities. Experimental results are compared with calculations.     

Coherent interaction of a relativistic electron beam with a plasma

Results are presented from experimental studies of the interaction of a modulated relativistic electron beam with a plasma. The electron energy spectra at the exit from the interaction chamber are measured for electron beams with energies of about 50 and 20 MeV. The coherent interaction of an electron beam with a microwave-driven plasma is studied. It is shown that, in strong electric fields that can be generated in the coherent interaction, the beam current is very sensitive to the phase of the microwave field.     

Numerical simulations of a high-current plasma lens

The dynamic processes by which an electrostatic plasma lens with a wide-aperture ion beam and electrons produced from the secondary ion-electron emission relaxes to a steady state is investigated for the first time by the particle-in-cell method. The parameters of a two-dimensional mathematical model were chosen to correspond to those of actual plasma lenses used in experimental studies on the focusing of high-current heavy-ion beams at the Institute of Physics of the National Academy of Sciences of Ukraine (Kiev, Ukraine) and the Lawrence Berkeley National Laboratory (Berkeley, USA). It is revealed that the ion background plays a fundamental role in the formation of a high potential relief in the cross section of a plasma lens. It is established that, in the volume of the plasma lens, a stratified electron structure appears that is governed by the nonuniform distribution of the external potential over the fixing electrodes and the insulating magnetic field. The stratification is very pronounced because of the finite sizes of the cylindrical fixing electrodes of the lens. It is shown that the presence of such a structure limits the maximum compression ratio for an ion beam to values that agree with those observed experimentally.     

Nonlinear dispersion in a plasma with a relativistic electron beam

The nonlinear interaction of a relativistic electron beam with a plasma is investigated numerically on the basis of the extended notions of the physical quantities that enter the linear dispersion relation. Extending the notions of the wave frequency, wavenumber, and wave phase velocity to the nonlinear stage of an instability makes it possible to analyze the evolution of the Cherenkov and plasma resonances and to study how they affect the saturation of the wave amplitude. A model of the beam-plasma instability in which the growth rate is calculated from the corresponding linear hydrodynamic formula on the basis of the results obtained using a numerical kinetic model makes it possible to establish the applicability range of the hydrodynamic approximation for beams with different energies.