Electron Beam Formation Research Articles

Hollow Cathode Electron Beam Formation and Effects on X-Ray Emission in Capillary Discharges

Electron beams produced by the hollow cathode effect are studied in a compact gas filled 24-kV, 5-kA capillary discharge. The characteristics and role of electron beams on X-ray production are studied in order to better understand their impact on X-ray production. X-ray plasma emission is analyzed with a spectrometer, while wideband diodes and Faraday cup probe both electron beams and X-rays. Electron beams of >5 keV are observed early on the discharge and two types of electron beams are identified: a first beam of high energy and low current (with a speed of $\sim 5 \times 10^{7}$ m/s and a current density of $\sim 4\times 10^{\mathrm {-2}}$ A/cm $^{2})$ enhanced by larger cathode apertures, and a second beam of low energy and high current enhanced by smaller cathode apertures. The use of a simple configuration of external magnets prevents X-ray fluorescence production, caused by electron beams, from reaching electronic detectors hence confusion when evaluating X-ray yield from the plasma source is avoided. This is particularly important in extreme ultraviolet lithography and soft X-ray production applications. Furthermore, the presence of the external magnets does not modify line emission, which is detected by means of spectrometry. Therefore, the use of external magnets provides a way to discriminate X-ray diode signals produced by source emitted X-rays and X-ray fluorescence from electron beams.

The generation of accelerated electrons in the nanosecond discharge in the short interelectrode gap

Electrical, optical and polarization characteristics of nanosecond discharges in the short interelectrode gap have been experimentally researched. The conditions and mechanisms of accelerated electrons formation are established. The estimates of energy, the length of accelerated electrons run and electron beam generation efficiency in the investigated discharge are obtained. The formation of electron beams is confirmed by the studies of polarization of spontaneous discharge irradiation.

Mechanisms of plasma disruption and runaway electron losses in the TEXTOR tokamak

Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron (RE) beams and their losses are proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode-number magneto-hydro-dynamics (MHD) modes. It is hypothesized that the RE beam is formed in the central plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of plasmas with the safety factor $q(0)<1$ the most stable RE beams are formed by the outermost intact magnetic surface located between the magnetic surface $q=1$ and the closest low-order rational surface $q=m/n>1~(q=5/4,q=4/3,\dots )$. The thermal quench (TQ) time caused by the fast electron transport in a stochastic magnetic field is calculated using the collisional transport model. The current quench (CQ) stage is due to the particle transport in a stochastic magnetic field. The RE beam current is modelled as a sum of a toroidally symmetric part and a small-amplitude helical current with a predominant $m/n=1/1$ component. The REs are lost due to two effects: (i) by outward drift of electrons in a toroidal electric field until they touch the wall and (ii) by the formation of a stochastic layer of REs at the beam edge. Such a stochastic layer for high-energy REs is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to the wall. The effect of external resonant magnetic perturbations on RE loss is discussed. A possible cause of the sudden MHD signals accompanied by RE bursts is explained by the redistribution of runaway current during the resonant interaction of high-energetic electron orbits with the $m/n=1/1$ MHD mode.

Mechanism of runaway electron beam formation during plasma disruptions in tokamaks

A new physical mechanism of formation of runaway electron beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between q = 1 and the closest low–order rational magnetic surfaces [q = 5/4 or q = 4/3,…]. It results in that runaway electron beam current has a helical nature with a predominant m/n = 1/1 component. The thermal quench and current quench times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms for the decay of the runaway electron current owing to an outward drift electron orbits and resonance interaction of high–energy electrons with the m/n = 1/1 MHD mode are discussed.

Suppression of the Oscillatory Modes of a Space Charge in the Magnetron Injection Guns of Technological Gyrotrons

We present the results of based on the PIC method numerical simulation of the dynamic processes of trapping of electrons into the adiabatic trap of a technological gyrotron for different configurations of the electric and magnetic fields in the electron beam formation region. The electrode geometry providing a low reflection coefficient of the magnetic mirror to suppress oscillatory modes in the space-charge cloud and ensure the stability of the electron beam with a high fraction of oscillatory energy in such a system has been found.

Alfvénic instabilities driven by runaways in fusion plasmas

Runaway particles can be produced in plasmas with large electric fields. Here, we address the possibility that such runaway ions and electrons excite Alfvénic instabilities. The magnetic perturbation induced by these modes can enhance the loss of runaways. This may have important implications for the runaway electron beam formation in tokamak disruptions.

Gyrotron Technology

The article presents a microwave vacuum tube called gyrotron. Its applications, construction and principle of operation are briefly described. It is also discussed the issue of an appropriate electron beam generation and formation.

Development of an electron-beam welding unit for small-size pieces

Requirements for equipment for electron-beam welding of small-size pieces are presented. Features of physical processes of the formation of low-power electron beams in process installations are shown. Mathematical simulation methods were used for the analysis of the operation of a three-electrode electron gun and for the choice of operation modes of power sources. The design and characteristics of the developed inverter power source for the electron-beam gun are described. The design of an electron-beam welding unit for small-size pieces is given.

Super-filament formation of a relativistic Gaussian electron beam in a dense collisional plasma

Energy flux transport of a finite high current relativistic electron beam with the Gaussian profile in both the transverse and axial directions in a dense collisional background plasma is studied by means of three-dimensional particle-in-cell simulations. Simulation results reveal the development of a needle-like super-filament formation in both homogeneous as well as inhomogeneous collisional background plasmas. However, in the case of an inhomogeneous background plasma, the beam suffers severe filamentation due to the lower plasma density encountered in the early stage, and only the head of the super-filament survives and travels further inside the plasma. This may not be desirable for the fast ignition fusion scheme.

Stochastic particle acceleration by helical turbulence in solar flares

Flaring release of magnetic energy in solar corona is only possible if the magnetic field deviates from a potential one. We show that the linear magnetohydrodynamic (MHD) modes excited on top of the non-potential magnetic field possess a non-zero kinetic helicity. Accordingly, this necessarily results in a noticeable kinetic helicity of the turbulence, composed of these linear modes with various scales and random phases, generated at the flare site by the primary energy release, which may be important for many applications. In particular, a non-zero turbulence helicity has a potentially strong effect on the particle acceleration because the helical component of the turbulence induces a mean regular large-scale (DC) electric field capable of directly accelerating the charged particles in addition to the commonly considered stochastic turbulent electric field. In this paper, we derive the kinetic helicity density of the linear MHD modes excited on top of a twisted large-scale magnetic field, estimate the corresponding turbulence helicity and take its effect on stochastic particle acceleration by the turbulence into consideration; in particular, we compare this induced mean electric field with the electron and estimated effective ion Dreicer fields. We have discovered that this, so far missing but highly important, ingredient of the turbulence at the flare site can be responsible for the thermal-to-non-thermal energy partition in flares by controlling the process of particle extraction from the thermal pool and formation of the seed particle population to be then stochastically accelerated to higher energies. In addition, it is naturally consistent with such puzzling flare manifestations as spatial separation of electron and proton emission sites, electron beam formation, and enrichment of the accelerated particle population by 3He and other rare ions.

Study on the Radial-Sheet-Beam Electron Optical System

A new kind of radial-sheet-beam electron optical systems in W-band radial-sheet-beam traveling-wave tubes was described in this paper. A cylindrical cathode with an arc emitting surface was adopted to produce a 2-kV 2-A radial sheet electron beam. The effects of cathode size and anode size on the space-charge-limited current were studied. The electron-beam formation and transmission were discussed under the condition of the space-charge-limited current, and the radial Brillouin focusing magnetic field was obtained. This kind of electron optical systems was simulated by the particle-in-cell method to check up the design. The simulation results presented in this paper show that the beam transmission attained 100% over a distance of 35 mm with a fill factor of ~ 0.55.

Simulation of the formation of a runaway electron beam in an overvolted gas gap breakdown

The paper reports on numerical simulation to inquire into the breakdown of a gas-filled diode in a highly inhomogeneous electric field. It is shown that early in the breakdown a runaway electron beam (RAEB) is formed in the diode and this strongly affects the rate of breakdown development. The energy gained by RAEB electrons corresponds to the electron energy gained under the same conditions in vacuum. The properties of the emission surface of the cathode determine the instant at which the beam is formed during subnanosecond voltage pulse rise time and hence the beam current and the energy spectrum of runaway electrons.

Features of accelerated electron beam formation in LHCD experiments on FT-2 tokamak

In experiments with lower hybrid current drive (LHCD) on the FT-2 tokamak, lower hybrid (LH) waves have been successfully used for the first time to ensure effective additional heating of plasma electrons from 450 to 600 eV (I Pl = 32 kA, Δt RF = 14 ms, P RF = 100 kW, F = 920 MHz). Several factors influencing the efficiency of plasma heating have been discovered. In particular, significant growth of radiation losses in the LHCD regime has been found, which is probably related to an increase in the intensity of synchrotron radiation from accelerated electrons. The increase in this intensity in the 53–156 GHz frequency range was accompanied by short spikes of microwave radiation, which were observed only in a narrower frequency range (53–78 GHz) and apparently resulted from interaction of a runaway electron beam with significant local mirrors of toroidal magnetic field. A model of the additional heating of plasma electrons due to absorption of the microwave radiation generated by a beam of accelerated electrons is proposed.

A paraxial solution to the exterior problem of the theory of 2D and 3D electron beam formation

For several reference 2D and 3D nonparaxial beams considered in coordinate systems associated with the geometry of a flow, a solution determining the configuration of forming electrodes is obtained in the paraxial approximation. The approximate solution is compared with the exact solution if the latter exists. The comparison is performed for extrapolation to the cases when this approach seems the only one possible.

Programmable Sub-nanometer Sculpting of Graphene with Electron Beams

Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without C(s) correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains.

Type II radio bursts: 2. Application of the new analytic formalism

Type II radio bursts drift in frequency as shock waves and coronal mass ejections (CMEs) move through the Sun's corona and the solar wind. This paper applies an extended analytic theoretical model for type II radio bursts to an MHD simulation of the rippled shock front found upstream of the flanks of a CME. The theory treats the acceleration of electrons at the shock, formation of electron beams, growth of Langmuir waves, and conversion of Langmuir energy into radiation. The extended theory is entirely analytic and includes kappa electron velocity distribution functions for the ambient plasma electrons and the shock‐reflected electrons. It also includes the plateauing of the electron beam, which releases energy for the Langmuir waves. This paper presents and discusses our numerical results for synthetic radio source regions and synthetic dynamic spectra, gained by applying our radiation model to an MHD simulation of a shock driven by a CME. The investigation reveals strong emission upstream of the flanks of the shock. A complicated rippled shock geometry develops with embedded “ripples” that stimulate short‐lived “bright spot” radio sources, which lead to complicated substructures in the dynamic spectrum, and more extended sources that usually have a weaker and more diffuse radio emission. The natural development of ripples on the shock provides a natural link between the ripple theory of Knock, Cairns, and colleagues and the bolt‐on model presented here.

Formation of 3D wedge-shaped electron beams

The paper presents the paraxial approximation of the solution to the problem on the electron beam formation as an arbitrary cut from a flow in a cylindrical diode, in particular, with a cross section close to a rounded rectangle with a large half-side ratio (wide beams) in situations when a converging or diverging electron flow acts on rectangular targets. The approximation is obtained with allowance for the cathode singularity.

Magnetospheric substorms and discrete arcs of the polar aurora

A brief review of the results of the research in physics of the Earth’s magnetosphere leading to a substantial modification of the previously developed approaches is presented. The main emphasis is placed on physics of magnetospheric substorms and the nature of auroral arcs. It is shown that the formation of powerful electron beams that produce multiple arcs can be associated with the penetration of cold electrons of an ionospheric origin into the region of field-aligned acceleration of hot magnetospheric electrons.

A Paraxial approximation of the problem of formation of nonparaxial conical and toroidal electron beams of an arbitrary cross section

A paraxial solution is constructed for the problem of calculation of forming electrodes for non-paraxial cuts from converging and diverging flows in a spherical diode and a Meltzer flow with circular paths. The solution is expressed in elementary functions for the case of a cone. For example, computations for a square on a sphere (the first approximation of the approximation of a contour) and a toroid with a circular cross section are presented.

Special features of the formation of electron beam welded joints in pressed strips of a high-strength aluminium alloy of the Al–Zn–Mg–Cu system

The special features of electron beam welding of pressed strips of the V-1963-strength aluminium alloy of the Al–Zn–Mg–Cu system with a thickness of 40 mm are investigated. The results of tests of the welded joints in static tensile loading, bending, and impact toughness of the weld metal are presented. It is shown that the strength of the welded joints equals 0.7–0.8 of the strength of the parent material. The results of investigations of the macro- and microstructure of the welded joints are generalized and it is shown that the welded joints are characterized by the formation of an equiaxed fine-grained structure with the grain size of 5–10 μm.