High Current Pulsed Electron Research Articles

A comprehensive study of pulsed high-current secondary electron emission cathode

A comprehensive study of pulsed high-current secondary electron emission cathode

A Tutorial on the One-Dimensional Theory of Electron-Beam Space-Charge Effect and Steady-State Virtual Cathode

The space-charge effects of pulsed high-current electron beams are very important to high-power particle beam accelerators and high-power microwave devices. The related physical phenomena have been studied for decades, and a large number of informative publications can be found in numerous scientific journals over many years. This review article is aimed at systematically summarizing most of the previous findings in a logical manner. Using a normalized one-dimensional mathematical model, analytical solutions have been obtained for the space-charge-limited current of both planar diode and drifting space. In addition, in the case of a beam current higher than the space-charge-limited current, the virtual cathode behavior and beam current reflection are quantitively studied. Furthermore, the criteria of steady-state virtual cathode formation are investigated, which leads to the physical understanding of the unstable nature of the virtual cathode. This review article is expected to serve as an integrated source of related information for young researchers and students working on high-power microwaves and pulsed particle beams.

Modification of stainless steel based on synergistic of low-energy high-intensity ion implantation and high-current electron beam impact on the surface layer

The results of experiments on low-energy implantation of AISI 321 stainless steel by nitrogen ions are presented. The treatment was carried out by a pulsed beam of nitrogen ions obtained using a ballistic ion focusing system. The surface modification occurs with the formation of a two-layer structure, which is typical for ion-plasma nitriding processes of stainless steels. The thickness of the modified layer can reach 27 μm after 1 hour of ion-plasma treatment. The influence of subsequent modification of the ion-doped layer by the action on the surface of the pulsed high-current electron beam of microsecond duration is studied. The work presents the results of the studying the regularities of changes in the depth distribution of dopants, microstructure and phase composition of the modified and matrix layers by optical metallography, diffraction analysis and transmission electron microscopy.

ИССЛЕДОВАНИЕ ФАКТОРОВ, СПОСОБСТВУЮЩИХ НЕCАНКЦИОНИРОВАННОМУ ИНИЦИИРОВАНИЮ ЭНЕРГОНАСЫЩЕННЫХ МАТЕРИАЛОВ

The results of experimental studies on the initiation of compositions containing energy-saturated materials by high-current electron beams and electrodischarge plasma are presented. Experimental studies simulate the effects on energy-saturated materials of electromagnetic fields of natural and man-made origin. The threshold voltage of the electrical breakdown of the compositions during a high-voltage electric discharge was determined and the possibility of initiation of the compositions by high-current electron beams of nanosecond duration using a pulsed high-current electron accelerator GKVI-300 was studied. Four compositions were selected as objects of study: a composition based on potassium picrate, a composition containing ammonium perchlorate, a composition based on lead minium with zirconium and colloxylin additives, and ballistite composition SBK-3. It is shown in the work that the initiation of compositions by a high-current electron beam is potentially possible only for compositions with low ignition temperatures, ignition of compositions with high ignition temperatures is possible due to the introduction of a powdery nanoscale additive, particles of which have specific heat less than particles of an energy-saturated material, and ignition of ammonium perchlorate is possible only if the particles of the additive have lower thermal conductivity than the particles of perchlorate ammonium.

Heat Transfer in Surface Layer of a T15k6 Heterogeneous Hard Alloy under Pulsed High-Energy Irradiation

A one-dimensional model of a hard alloy is proposed, which takes into account the heterogeneity of the physical properties of a composite material by representing it as a sequence of 2 μm TiC and WC carbide particles. The influence of the energy density of pulsed high-current electron beams on the melted layer depth and microstructure is investigated. A comparison of the penetration depths of the hard alloy obtained by calculation and experimentally shows their good agreement. It is found out that with an increase in the beam energy density due to melting of the carbides a (Ti, W)C supersaturated tungsten solid solution is formed and a WC→W2C transformation occurs during crystallization.

Generation of focused high-current electron beam with millisecond pulse duration by a forevacuum plasma-cathode electron source based on cathodic arc

We describe our work on the generation of low-energy (up to 10 keV) pulsed high-current electron beams with pulse duration up to 7 ms, compressed (focused) by a magnetic field in the forevacuum pressure range (3–30 Pa). We show that the focusing system, consisting of two coaxially arranged coils, provides effective compression of the beam. In the forevacuum pressure range, the gas pressure and type of gas affect the focusing of the beam at fixed emission current, accelerating voltage and magnetic field. Increased pressure leads first to increased beam current density, but beyond a certain optimal value further pressure increase leads to a decrease in the current density of the electron beam. The use of gas with greater ionization cross-section results in stronger compression of the beam at lower gas pressure, but also leads to decreased maximal operating gas pressure of the plasma-cathode electron beam source. Variation of magnetic fields provides control of the beam current density and the position of the beam crossover (focus). A low-energy electron beam with current density up to 15 A·cm−2 and pulse duration up to 7 ms is obtained in the forevacuum pressure range. The use of the focused beam for evaporation of high-temperature ceramics is demonstrated.

SMALL-SIZE DIRECT-ACTION ELECTRON ACCELERATOR WITH A HIGH-EFFICIENCY NANOSECOND PLASMA-CURRENT SWITCH

The paper presents the results of a study undertaken to determine operating modes for a small-size direct-action electron accelerator with a high-efficiency nanosecond plasma-current switch (PCS). The investigations have shown that using PCS as a base it is possible to develop small-size nanosecond pulsed high-current electron accelerators with a voltage pulse sharpening coefficient of about 12, beam (flow) electron energy of 300…400 keV and current of 100 kA for pulse duration of 30 ns. The ways for improving the PCS and accelerator operating parameter stability, increasing the switching current and maximum accessible switching frequency are proposed.

Three-electrodes open discharge in low-pressure deuterium: transition from the overvoltage regime into low-voltage one

Strongly overvoltage open discharges in a narrow gap (2-3 mm) between the solid cathode and the grid anode are used for generation of the pulsed high-current electron beams with the energy up to several tens of keV. Strongly overvoltage (SO) regime is unstable and the discharge tends to transit into low-voltage (LV) regime with a high-current. We studied this transition by example of the three-electrodes open discharge in D2 at low pressure (about 0.5-2 Torr) being powered by stepwise voltage with amplitude up to 25 kV. The existence of the auxiliary third electrode allows one to get more stable operation of the open discharge. The physical properties of the SO and LV regimes and transition between them have been explored in detail with the usage of the fast multi-frame camera synchronized with the current and voltage of the open discharge.

Stability of electron beam generation by forevacuum-pressure plasma-cathode electron beam source based on a cathodic arc

We describe our experimental research on the stable generation of large-radius pulsed high-current electron beams using a forevacuum-pressure plasma-cathode electron beam source with cathodic arc plasma generation. We find that increased gas pressure and the use of gas with a higher ionization cross-section considerably decrease the stable emission current due to loss of electric strength of the acceleration gap (breakdown). Increased pulse duration results in a decrease in the emission current, but the influence of the gas species and its pressure remains for all pulse durations. There is an extremum in the dependence of emission current on the acceleration gap length and the optimum gap length depends on the gas pressure and the kind of gas. The experiment results can be explained by the effect of back-streaming ion flow, which charges dielectric inclusions on the emission electrode (anode grid) and thereby amplifies the electric field.

On transition from diffuse mode to the constricted one with high-current cathode spot in overvoltage open discharge in D2

So called “open discharges” in a narrow gap between the solid cathode and grid anode are widely used for generation of the pulsed high-current electron beams with energy up to 100 keV. The need to get high-energy e-beams leads to the necessity in using of strong overvoltage of the short gas gap with the reduced electric field of the order of 105 Td or higher. The discharge under strong overvoltage is unstable and tends to transit into high-current regime with low voltage. In the case of the open discharge in D2 at low pressure (about 0.5-2 Torr) and powered by stepwise voltage with amplitude up to 25 kV we revealed that this discharge exhibits two diffuse regimes which follow one by one and finally transits into the constricted mode with formation of high-current spots on the cathode. The physical properties of these gas discharge regimes have been explored in detail with the usage of the fast multi-frame camera synchronized with the current and voltage of discharge. Our findings promote more insight into physics of the overvoltage open discharge generating the e-beams with energy up to 25 keV.

Modelling of chemical reactions in plasma

The paper is devoted to theoretical investigation of interaction of pulsed high current electron beam with gas substance. As a result of the interaction the formation of chemical active plasma can be observed. One of the key parameter for theoretical analyze of the process is the electron distribution function. Within the framework of the Boltzmann approach we obtained the dynamical equation for electron distribution function depending on the electron energy, coordinate and time.

The Special Features of the Phase Formation and Distribution in the Titanium Nickelide Surface Layers Treated by Electron Beams

The special features inherent in the formation of the structural-phase states on the surface of titanium nickelide samples subjected to multiple low-energy high-current variable energy density pulsed electron beam irradiation are investigated. The parameters of the electron beam operated in the fivefold surface melting regime are as follows: pulse duration τ = 150 μs, current I = 70 А, and energy densities Е 1, Е 2, and E 3 = 15, 20, and 30 J/сm2, respectively. The surface layer structure was examined by methods of the x-ray diffraction analysis and transmission electron microscopy. It is found that in the irradiated TiNi samples with Е ≤ 20 J/сm2, the layer containing a martensite phase lies at a certain depth below the surface rather than on the surface. In the irradiated TiNi sample with E 3 = 30 J/сm2, the subsurface region is in a two-phase state (B2 + B19′), with the В19′ phase being predominant. It appears that the lower is the energy density, the smaller is the amount of the martensite phase.

On shock initiation in brisant explosives by a high-current electron beam

The feasibility of shock initiation in thin pentaerythritol tetranitrate (PETN) single crystals under the action of a pulsed high-current electron beam (0.25 MeV, 20 ns, 15 J/cm2) is shown experimentally. The real-time dynamic characteristics of crystal glow arising under the action of the electron beam and glow due to subsequent shock-wave-induced transformations are presented. A shock wave results from beam energy absorption and initiation of an exothermal chemical reaction in the irradiated layer.

Current characteristics of quasi-planar vacuum diodes with explosive-emission cathodes made of various materials at a high-voltage pulse duration of a few nanoseconds

The currents of 5-ns pulsed high-current electron beams produced in a planar vacuum diode with explosive-emission cathodes made of various materials with no external magnetic field at an average electric field strength in the gap of about 300 kV/cm have been measured and time-integrated observation of the optical luminescence of the cathode surface have been performed. Cathodes with a ceramic bushing and spring metal contacts, with ceramic plates set in a magnetic iron matrix, with blades made of stamped exfoliated graphite (Graflex), with blades made of foil fiberglass plastic, and a composite cathode made of crystalline boron and copper powders were tested. The current carried by one emission center has been estimated to range between 5 and 20 A for various cathodes. For the metal-dielectric cathode, the velocity of expansion of the cathode plasma over the ceramic surface has been estimated as 2·107 cm/s. The lifetimes of the cathodes at a pulse repetition rate of 50 Hz have been investigated.

Electric breakdown and explosive decomposition of PETN monocrystals initiated by electron beam

The glowing, fracture, and explosive decomposition of pentaerythritol tetranitrate (PETN) monocrystals initiated by a nanosecond pulsed high-current electron beam (HEB) have been experimentally studied. It is established that crystallographically oriented electric (strimer) discharges are formed within the electron-beam action range in the crystal in a broad range of energy densities (0.1–75.0 J/cm2). Threshold energy density for the electric breakdown, fracture, and explosive decomposition of PETN monocrystals under HEB action have been determined. In contrast to pressed PETN samples, monocrystals exhibit incomplete explosion with expansion of the majority of unreacted substance.

Low-energy high-current electron beam heating of target with second-phase microinclusions

The temperature field generated by microsecond pulsed low-energy high-current electron beam (LEHCEB) in the surface layer of a stainless-steel target containing second-phase (manganese sulfide, MnS) microinclusions has been numerically simulated. The results of calculations show that the temperature is nonuniformly distributed over the target surface. By the end of the LEHCEB pulse, the temperature in the regions of MnS inclusions significantly exceeds that of the steel matrix. This nonuniformity is related to (i) markedly greater thermal conductivity of steel compared to that of MnS and (ii) the pulsed character of the electron-beam-induced heating of the target surface. It is also established that LEHCEB-induced melting begins at the inclusion-steel interface and then involves the inclusion and spreads over the entire irradiated surface. The dependence of the characteristics of the irradiation-induced temperature field on the parameters of the pulsed electron beam has been studied.

Surface doping of VT6 alloy with zirconium by pulsed electron-beam mixing of predeposited multilayer Zr/Ti film

We have studied characteristics of the surface doping of VT6 alloy (Ti-6Al-4V) with zirconium, which was effected in order to reduce the concentrations of Al and V at the surface. The doping was performed by liquid-phase mixing of a [Zr(20 nm)/Ti(20 nm)]12 multilayer film with a total thickness of 480 nm and the substrate (VT6 alloy) under the action of low-energy (∼20 keV) pulsed high-current electron beam (2.5 μs, 3.5 J/cm2). It is established that the pulsed beam-induced melting leads to the homogeneous mixing of all Ti/Zr nanolayers and the diffusion of Zr into substrate to a depth of ∼0.5 μm. As a result, the surface layer with a thickness of ∼0.5 μm is free of Al and V atoms and has a single-phase submicrocrystalline structure of α-Ti70Zr30 solid solution. Subsequent vacuum annealing leads to a decrease in the average grain size in the nearsurface layer to 90 nm and to an increase in the nanohardness of the doped layer.

Effect of pulsed plasma and high-current electron beam treatments on the structure and properties of nickel-based coatings

Using plasma-detonation technology (PDT), nickel-based powder (PGAN-33, PG-10N-01 and PG-19N-01) coatings 80–300 μm thick were deposited on a low-carbon St-3 steel substrate. Coatings were then subjected to additional treatment either by a high-current electron beam (HCEB) or by a high-velocity pulsed plasma jet until melting. The structure was analyzed using x-ray diffraction and conversion Mossbauer spectroscopy in the transmission and scattering modes. The morphology and elemental composition were studied using scanning electron microscopy and microanalysis. The micro-and nanohardness, volumetric wear, and corrosion resistance of the grown coatings were measured. It was found that, as the HCEB and PDT energy density increase, the phase composition changes, phase redistribution occurs, and new phases are formed. The last process is caused by mass transfer from the erosion plasmatron electrode and from a gas plasma jet, and interdiffusion of coating and substrate elements. After the exposure to concentrated energy fluxes, the coating surface roughness decreased. It is shown that physicochemical and mechanical characteristics of modified coatings improve in the case of optimum treatment conditions.

Surface cracking of soda lime glass under pulsed high-current electron radiation

Electron beam radiation has been widely used to modify the surface properties of materials such as metals, ceramics, and glasses. However, a few investigations of surface topology of glasses after electron irradiation can be found. In contrast to the surface cracking by bending, indentation, and thermally induced stress in soda lime glasses a 2 μs pulsed high-current electron beam was used to modify the surfaces of soda lime glass. Surface topology of irradiated samples was studied by using traditional optical microscopy and atomic force microscopy. Parallel to and perpendicular to surface cracks were observed. The depth of crack can be obtained by electron penetration, Newton's ring and AFM. The stress to produce the crack by electron radiation was calculated using three obtained depths. The observed surface crack is explained in terms of radiation-induced thermal stress and high local electric field-induced by deposited charges from pulsed electrons.

Experimental study of the converging density wave excitation in a cylindrical anode of a high-current diode

Density waves excited in a cylindrical anode irradiated by a short pulsed high-current electron beam in a relativistic vacuum diode have been experimentally studied. The electron trajectories are almost perpendicular to the side surface of a cylindrical anode coaxial with the cathode. As a result of the discharge, the surface layer of the continuous anode exhibits no visible changes, whereas an inner region of the anode is evaporated. The mechanism of the isentropic wave excitation is elucidated by experiments with a coaxial anode comprising a tube tightly fit to the central rod. According to the most probable explanation, the electron beam induces collapse in the subsurface layer of the anode.