Virtual anode as a source of low-frequency oscillations of a high-current electron beam

We have studied the transport of a relativistic electron beam with supercritical current in a cylindrical drift chamber in the presence of an ion flux. A theoretical analysis of the electron-ion flux dynamics was based on the coarse particle PIC method (code SOM). Simultaneous injection of a supercritical electron beam and a weak-current low-energy ion beam may result in the formation of a virtual anode in addition to the electron virtual cathode in a drift chamber. The virtual anode exhibits periodic pulsations. Numerical results obtained for hydrogen and nitrogen ions show that the ratio of the frequencies of these pulsations is inversely proportional to the square root of the ion mass ratio. These oscillations of the virtual anode lead to temporal modulation, at the same frequency, of both electron and ion currents at the drift chamber output.     

Chaotic signal generation in low-voltage vircator with electron source shielded from external magnetic field

The effect of shielding an electron source from a homogeneous external magnetic field of the drift chamber on the nonlinear dynamics of the electron beam with a virtual cathode (VC) and on the characteristics of output microwave radiation in a low-voltage vircator have been numerically simulated within the framework of a two-dimensional model. It is established that the increased degree of shielding of the electron source from the external magnetic field leads to the complication of the VC dynamics in the system and the corresponding chaotization of the output microwave radiation. Physical processes that account for the observed effect of shielding are analyzed.     

Influence of feedback on the complex dynamics of an electron beam with a virtual cathode in a virtode

Results are presented of a numerical simulation of the nonlinear dynamics of a relativistic electron beam with a virtual cathode in a drift relativistic-electron-beam vircator system with and without external feedback. Pis’ma Zh. Tekh. Fiz. 24, 51–57 (March 12, 1998)     

Vircator radiation controlled by axial current-carrying filament

The operation of a vircator system with an axial current-carrying filament was studied by computer modeling. It is demonstrated that, by correctly selecting the value of current in the filament, the electron beam dynamics can be controlled so as to excite the output feeder in an optimum regime, which provides for a fourfold increase in the output power.     

Investigation of the influence of electron-velocity dispersion on the output characteristics of a laboratory breadboard of a low-voltage vircator

Some ways of forming multivelocity electron beams are considered, and experimental dependences (obtained on a laboratory breadboard of a low-voltage vircator) of the integral output power and generation band on the electron-velocity dispersion in a beam are shown.     

High-current relativistic electron beam transport in a drift tube in the absence of an external magnetic field

The current of an intense relativistic electron beam emitted from a metal-dielectric multiedge cathode that passes through an anode grid and is transported in a smooth-wall drift tube in the absence of an external magnetic field has been measured at the collector. In these experiments, the electron flux was transported over a distance twice as large as the drift tube diameter. An optimum cathode and storage electrode configurations have been found that allowed the efficiency of the relativistic Cherenkov microwave oscillator without a guiding magnetic field to be increased up to 10% at a maximum output pulse radiation power of 1.3 GW and a working frequency of 4.03 GHz.     

3D simulation of electron beam squeezed-state generation in a two-section drift tube and analysis of its characteristics

A 3D numerical electromagnetic simulation of a vircator in the mode of electron-beam squeezedstate generation is performed. Detailed numerical investigations are carried out, and the dynamics of charged particles in the squeezed state is analyzed. It is shown that the electron beam density (and, as a consequence, effective beam plasma frequency) can be significantly increased in the squeezed state. Hence, the vircator generation frequency and power can also be increased.     

Effect of external signal on the output power of an oscillator with electron feedback

The effect of an external single-frequency harmonic signal on the output power of an oscillator with electron feedback has been studied by analytical and numerical methods. It is established that an increase in the input signal power leads to sharp growth in the output power of the nonautonomous oscillator. Physical processes that take place in the electron beam with virtual cathode under the action of an external harmonic signal, which leads to velocity modulation in the electron beam entering the interaction space, have been analyzed. The obtained results well agree with the data of previous experimental investigations of the signal gain in a low-voltage vircator.     

Effect of positive ions on the microwave generation in a low-voltage vircator

The effect of vacuum conditions on the characteristics of microwave generation in a nonrelativistic electron beam with a virtual cathode (VC) formed in a retarding field (low-voltage vircator) has been experimentally studied. The ionization of residual gases in the working chamber of a low-voltage vircator leads to displacement of the VC out of the interaction space, which results in breakdown of the microwave generation. The experimental data agree with the results of numerical simulations.     

Output microwave radiation power of low-voltage vircator with external inhomogeneous magnetic field

Dependence of the power of a broadband microwave radiation generated by a low-voltage oscillator with virtual cathode (vircator) on the parameters of an external inhomogeneous magnetic field has been studied by numerical simulations using a two-dimensional model. It is established that there are optimum parameters of the generator (configuration of the external magnetic field, electron beam current) for which the output radiation power is maximum. A relationship between the optimum conditions of virtual cathode formation in the electron beam and the microwave generation regime is established.     

An S-band vircator with premodulated electron beam based on a compact generator with inductive energy storage

An experimental S-band vircator with premodulated electron beam based on a compact generator with inductive energy storage is described. The vircator radiation power was 300 MW at a power efficiency of about 5% and a pulse duration of 50 ns. The oscillation frequency, determined by the electrodynamic system parameters, remained unchanged during the pulse.     

Experimental study of the effect of external signal on microwave oscillations in a nonrelativistic electron beam with virtual cathode

The effect of an external harmonic signal on the characteristics of microwave generation in a non-relativistic electron beam with virtual cathode (VC) formed in a static retarding electric field (low-voltage vircator system) has been experimentally studied. A significant increase in the vircator generation power is observed when the frequency of the external signal is close to the frequency of VC oscillations. At large detunings, a broadband chaotic generation is observed.     

Microwave generation power in a nonrelativistic electron beam with virtual cathode in a retarding electric field

The power of microwave generation in a nonrelativistic electron beam with virtual cathode formed in a static retarding electric field (low-voltage vircator system) has been studied experimentally and by means of numerical simulation within the framework of a one-dimensional theory. The limits of applicability of the one-dimensional theory have been experimentally determined.     

Studying impurities in the plasma jet of a closed-electron-drift thruster

A new approach to the real-time diagnostics of the erosion of walls in the discharge chamber of a plasma accelerator (thruster) with closed electron drift (CED) is described. The proposed method is based on the measurement of intensities of (i) the spectral lines of atoms sputtered from the discharge chamber walls and (ii) the reference lines of the neutral working gas component. The rate of erosion of the dielectric walls in a discharge chamber of a CED thruster has been measured for the discharge voltage increased from 300 to 700 V. An experimental relationship is established that takes into account changes in the velocities and charge states of the sputtered species and allows the proposed method to be used for various discharge voltages.     

Experimental study of the effect of external noise-like signal on the dynamics of vircator with magnetron injection gun

The effect of an external noise-like signal on the operation of a low-voltage vircator with magnetron injection gun has been experimentally studied. The results show that the noise-like input signal allows the output characteristics of the low-voltage vircator to be controlled so as to drive it to operate at either maximum output power or maximum generation bandwidth.     

A relativistic backward wave oscillator with a modulating resonance reflector

A high-efficiency relativistic microwave source based on a backward wave oscillator (BWO) with a resonance reflector has been studied by experimental and numerical methods. The BWO is capable of generating 12-ns pulses with a carrier frequency of 9.93 GHz at an output radiation power of 0.75 ± 0.11 GW. For the BWO pumped by a high-current electron beam from a SINUS-6K accelerator, the regime with relatively high pulse-to-pulse stability is characterized by a 40% efficiency of power conversion.     

Relativistic surface-wave generators based on two-dimensional periodic structures

In order to increase the integral output radiation power of relativistic surface-wave generators, it is suggested to use two-dimensional (2D) periodic slow-wave systems. Numerical simulations of dynamics of a new variant of these generators showed that additional wave beams, which appear in the 2D structure and propagate in the transverse direction, are capable of synchronizing radiation from a wide sheet electron beam. This circumstance makes it possible to implement surface-wave generators operating in the millimeter wavelength range at a gigawatt power level—e.g., based on the ELMI high-current accelerator at the Institute of Nuclear Physics (Novosibirsk).     

Relativistic Cherenkov microwave oscillator without a guiding magnetic field

A relativistic Cherenkov microwave oscillator without a guiding magnetic field has been designed, constructed, and tested in which a continuous cylindrical electron beam propagates in a short (L ≈ 3λ, λ being the radiation wavelength) resonant slow-wave structure. The electron beam is energy-modulated at the input of the interaction space, which provides conditions for the energy exchange at a wave phase velocity exceeding the particle velocity. The effective beam-wave coupling is provided by a nearly homogeneous profile of the longitudinal electric field component of the synchronous wave in the interaction space cross section. The efficiency of power conversion from high-current electron beam to electromagnetic radiation at E₀₁ mode is about 8% at a maximum output pulse radiation power of 1.2 ± 0.3 GW and a working frequency of 4.05 GHz.     

Scenario for the establishment of an electron beam “squeezed state” in a magnetically insulated vircator

A 2.5-dimensional particle-in-cell code was used to simulate different scenarios for the establishment of an electron beam “squeezed state” in a magnetically insulated vircator. Vircators with and without anode foils were compared. It was found that the squeezed state is established in both cases but the dynamics of establishment differ. Pis’ma Zh. Tekh. Fiz. 23, 29–33 (November 26, 1997)     

Experimental investigation of electron beam in the squeezed state

The electron beam transported in a two-section drift tube of a SINUS-7 setup has been studied. A high-density electron beam in the “squeezed” state has been obtained with a relativistic factor γ_b below that corresponding to the limiting current (γ > Γ^1/3).