Nonlinear dynamics of electron beam with virtual cathode in external inhomogeneous magnetic field

Nonlinear dynamics of virtual cathode (VC) formed in electron beam in a strongly inhomogeneous external magnetic field generated by a ring magnet has been numerically simulated using a two-dimensional model. It is established that the field characteristics strongly influence the VC dynamics. Physical processes responsible for this strong dependence are considered. The output power of generation in a model system with VC is determined as a function of the field parameters.     

The current of an annular electron beam with virtual cathode in a drift tube

An analysis based on the laws of conservation of energy and the z components of the field and particle momentum shows that a thin magnetized annular electron beam in a homogeneous drift tube behind a virtual cathode under stationary conditions occurs in a “squeezed” state corresponding to a slow left-hand branch of the current characteristic with a relativistic factor in the interval 1≤γ≤Γ^1/3. The beam current I ₁ behind the virtual cathode in a homogeneous drift tube can vary from zero up to a limiting value I _lim, while the injection currents (I ₂) and the current of electrons reflected from the virtual cathode (I ₃) for every stationary state are single-valued functions of I ₁ and fall within the intervals I _F/2≤I ₂≤I _lim and 0≤I ₃≤I _F/2, respectively, where I _F is the Fedosov current.     

The electron beam with virtual cathode in a two-section drift tube

An analysis based on the law of conservation of the z component of field and particle momentum in a thin annular magnetized monoenergetic electron beam in a two-section drift tube composed of tube segments with different radii (R ₁<R ₂) allowed the critical injected beam current to be determined for which a virtual cathode, formed in the wider tube at the joint section, starts traveling through the narrower tube toward the beam injection region. A region behind the traveling virtual cathode features a “squeezed” single-flux state of the beam (corresponding to the “slow” left branch of the current characteristic, high charge density, but low relativistic factor). When the injected current decreases below a critical transition level (I _in<I _Tr), the virtual cathode returns to the initial position and restores the double-flux electron beam. This current is smaller than (I _lim1+I _lim2)/2, and, depending on R ₂, varies from the limiting transport current for the narrower section (I _lim1) up to I _F/2, where I _F is the Fedosov current for this tube section.     

The current of an annular electron beam behind virtual cathode

The output current I _out of a magnetized annular electron beam transported in a homogeneous drift tube of a SINUS-7 setup has been studied as a function of the injected beam current I _inj. The electron beam was injected via an anode inset of smaller radius and had a current above the Fedosov limit for the given drift tube (I _inj ≥ I _F). It is established that, to within the experimental uncertainty, a virtual cathode (VC) is formed when the injected beam current exceeds the corresponding limiting I _lim. In this state, the current transmitted behind VC is approximately equal to the limiting value (I _out ≈ I _lim).     

Bicoherent wavelet analysis of the structure formation in an electron beam with virtual cathode

The supercritical electron beam structure formation in a diode gap with inhomogeneous ion background density are analyzed by the bicoherent wavelet transformation method. By studying the wavelet bicoherency of the spatiotemporal data about oscillations in the system, it is possible to effectively reveal and analyze local spatial structures formed in the electron beam.     

The effect of the conductivity of drift chamber walls on the dynamics of a relativistic electron beam with a virtual cathode

The effect of conductivity of walls of a drift chamber of the axial vircator on the behavior of a relativistic electron beam with a supercritical current was investigated. The dynamics of a relativistic electron beam is shown to be characterized by the formation of a virtual cathode of complex structure with two or three potential minima in the azimuthal direction, which rotate around the drift space axis. It is established that variation in the conductivity of drift chamber walls leads to stepwise switching of the generation frequency and a sharp change in the output power. Dependences of the output radiation power of the investigated vircator system on the conductivity of drift chamber walls for two characteristic regimes of the dynamics of a relativistic electron beam were obtained.     

The virtual cathode velocity during electron beam transport in a drift tube

Technical Physics Letters - A solution corresponding to a thin annular electron beam in a homogeneous drift tube featuring a uniformly moving virtual cathode (VC) has been obtained. The dependence...     

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.     

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.     

Current density distribution in a large cross-section beam in an electron accelerator with a multiaperture plasma cathode

An electron accelerator with a multiaperture plasma cathode with grid stabilization of the boundary of emission plasma based on a low-pressure arc discharge generating a large cross-section (750 × 150 mm²) beam with its ejection into the atmosphere or high-pressure gas through the exit foil window is described. A comparatively simple method for decreasing the nonuniformity of the current density distribution over the beam cross section using a mask with variable-diameter holes is proposed and experimentally tested. In this case, the higher the plasma concentration in the emission region, the smaller the diameter of the holes in the mask. When this mask was used, the nonuniformity of the current density distribution was decreased from ±15 and ±10% to ±10 and ±5% on the long and short sides of the beam cross section, respectively.     

Reflex triode with virtual cathode and controlled internal TEM-waveguide feedback

A reflex triode with virtual cathode (VC) was studied in which a controlled-length TEM-waveguide feedback was introduced between the region of generation (VC localization) and a diode region. At a certain feedback length, the microwave output power of the reflex triode increases by a factor of 1.6 as compared to the same system without feedback.     

Virtual cathode formation in annular electron beam in an external magnetic field

The conditions of virtual cathode (VC) formation in an annular electron beam in the presence of an external focusing magnetic field have been studied by means of numerical simulation within the framework of a two-dimensional model. The dependences of a critical beam current (at which the VC is formed) on the magnetic field and of the optimum field (at which the critical current is minimal) on the beam geometry are determined. The optimum magnetic field depends on the Brillouin field value.     

Chaotic oscillator with electron feedback and magnetic field at the cathode

The influence of a magnetic field at the cathode on the operation of a low-voltage vircator with electron feedback has been experimentally studied. It is shown that this magnetic field can significantly improve the characteristics and parameters of the vircator, provided that the field lines do not coincide with electron trajectories in the system.     

Runaway electron beam generation by a plasma cathode in atmospheric air discharge

Electron beam generation in atmospheric air discharge with a flat cathode has been experimentally studied. The discharge was excited by voltage pulses of negative polarity with an amplitude of 220 kV, a full width at half maximum of 2 ns, and a leading front width of 0.7 ns. The electron beam was monitored using luminescence of a layer of ZnS-CdS:Ag phosphor placed behind a 20-μm-thick foil anode. It was found that the intensity and homogeneity of luminescence of the phosphor layer increased when a dielectric tube with a length smaller than half of the interelectrode distance was placed in the near-cathode part of the air gap. It is concluded that a plasma cathode is formed within the volume confined by the tube and electrons emitted from this region are accelerated in the open part of the gap. In addition, the dielectric tube decreases the divergence of the electron beam.     

Influence of nonuniformity of the ion background on the oscillation frequency of a virtual cathode

The oscillation frequency of a virtual cathode in a diode gap is investigated as a function of the density of the anode ion layer. It is shown that when the densities of the ion layer are high, a stable electron bunch forms in the flux, which leads to an increase in the oscillation frequency. Pis’ma Zh. Tekh. Fiz. 24, 74–80 (November 12, 1998)     

Effect of cathode emissivity on runaway electron beam formation in gas-filled diode with inhomogeneous electric field

Mechanisms of the formation of runaway electron beams at the initial stage of breakdown in a gasfilled diode with sharply inhomogeneous electric field have been studied by numerical simulations using a specially developed PIC/MC code. It is established that a beam of runaway electrons can be generated either immediately at the cathode or at a discharge plasma boundary. The obtained dependence of the runaway electron beam current on the state of the cathode surface (emissivity) has the characteristic shape.     

Metal-insulator cathode in an rf electron gun

The possibility of using metal-insulator cathodes in rf guns of 10 cm linear resonant electron accelerators has been studied experimentally for the first time. Results of calculations and the development of a prototype cathode are described. Results of an experimental investigation of the operating regimes of an rf gun are presented. The gun delivered a beam having a particle energy greater than 300 keV, a pulse length of 40–50 ns, and a pulsed current of 3.5–4.5 A. Pis’ma Zh. Tekh. Fiz. 24, 36–39 (October 12, 1998)     

On the beam parameters of an electron gun with a plasma emitter

The brightness of a focused electron beam generated by a gun with a plasma emitter is measured. Measurements are carried out for a beam with a power up to 4 kW and an electron energy of 60 keV focused at a distance of 0.52 m from the focusing lens. For the first time, it is shown that the brightness of the focused beam of a gun with a plasma emitter is not inferior to the brightness of beams in guns with a hot cathode.     

铁电阴极几何参数对二极管电子发射的影响

主要讨论了触发电场的脉宽和阴极几何参数对铁电二极管电子发射的影响,通过对不同脉宽激励下的电子发射试验发现触发脉冲的最佳工作宽度范围是150-250ns。对不同几何参数铁电阴极片在正脉冲激励下的电子发射研究指出:一般实验条件下(触发电压小于4kV/mm),触发梯度电场相同时,阴极越厚,条栅电极越细密,发射电流密度越大;极化反转发射中前沿发射方式和后沿发射方式可以共时存在。     

The electron beam gun with thermionic hairpin-like cathode for welding and surface modifications

An axial thermionic electron beam emitter assembly with a special geometry of the cathode along with particular spacing of the electrodes has been used to produce a stable, sharp and high power density image at an acceleration voltage of 10 kV only. A hairpin-like tungsten wire, with diameter of 0.7 mm having semi-spherical emitting area at the crown with an angle of 45 degree at the vertex was used as a cathode. A direct heating method was used to heat the cathode. The emission current of the gun is in accordance with the Langmuir relation. An electromagnetic coil was used for focusing the beam at the target. A two dimensional programmable movement was applied to control the work site in the x-y direction. Focusing of the beam has been achieved up to 1 mm in diameter at an acceleration voltage of 10 kV.Thermionic efficiency of the gun is 4 mA W⁻¹ and the power density measured is ∼10⁵ W cm⁻².The gun was used for welding and surface modification of different materials including refractory metals.