A study of pulsed high voltage driven hollow-cathode electron beam sources through synchronous optical trigger

In this study,a pulsed,high voltage driven hollow-cathode electron beam sources through an optical trigger is designed with characteristics of simple structure,low cost,and easy triggering.To validate the new design,the characteristics of hollow-cathode discharge and electron beam characterization under pulsed high voltage drive are studied experimentally and discussed by discharge characteristics and analyses of waveform details,respectively.The validation experiments indicate that the pulsed high voltage supply significantly improves the frequency and stability of the discharge,which provides a new solution for the realization of a high-frequency,high-energy electron beam source.The peak current amplitude in the high-energy electron beam increases from 6.2 A to 79.6 A,which indicates the pulsed power mode significantly improves the electron beam performance.Besides,increasing the capacitance significantly affects the highcurrent,lower-energy electron beam more than the high-energy electron beam.

Surface modification of Cu-25Cr alloy induced by high current pulsed electron beam

A Cu-25Cr alloy prepared by vacuum induction melting method was treated by the high current pulsed electron beam （HCPEB） with pulse numbers ranging from 1 to 100. Surface morphologies and microstructures of the alloy before and after the treatment were investigated by scanning electron microscopy and X-ray diffraction. The results show that significant surface modification can be induced by HCPEB with the pulse number reaching 10. Craters with typical morphologies on the Cu-25Cr alloy surface are formed due to the dynamic thermal field induced by the HCPEB. Micro-cracks, as a unique feature, are well revealed in the irradiated Cu-25Cr specimens and attributed to quasi-static thermal stresses accumulated along the specimen surface. The amount of cracks is found to increase with the pulse number and a preference of these cracks to Cr phases rather than Cu phases is also noted. Another characteristic produced by the HCPEB is the fine Cr spheroids, which are determined to be due to occurrence of liquid phase separation in the Cu-25Cr alloy. In addition, an examination on surface roughness of all specimens reveals that more pulses will produce a roughened surface, as a result of compromising the above features.

Compounded Surface Modification of ZK60 Mg Alloy by High Current Pulsed Electron Beam+Micro-plasma Oxidation

In this study, compounded surface modification technology-high current pulsed electron beam （HCPEB） ＋ micro-plasma oxidation （MPO） was applied to treat ZK60 Mg alloys. The characteristics of the microstructure of ZK60 Mg alloy after single MPO and HCPEB＋MPO compounded treatment were investigated by SEM. The results showed that the density of the ceramic layer of HCPEB＋MPO-treated ZK60 Mg alloy was improved and defects were reduced compared to that under MPO treatment alone. Surface modified layer of ZK60 Mg alloys treated by HCPEB＋MPO was divided into three zones, namely the top loose ceramic zone, middle compact zone and inside HCPEB-induced melted zone. Corrosion resistance of ZK60 Mg alloy before and after the compounded surface modification was measured in a solution of 3.5% NaCl by potentiodynamic polarization curves. It was found that the corrosion current density of ZK60 Mg alloys could be reduced by about three orders of magnitude, from 311μA/cm^2 of the original sample to 0.2μA/cm^2 of the HCPEB＋MPO-treated sample. This indicates the great application potential of the HCPEB＋MPO compounded surface modification technology in improving the corrosion resistance of ZK60 Mg alloys in the future.

Improvement of Wear Resistance of Magnesium Alloy AZ91HP by High Current Pulsed Electron Beam Treatment

Surface modification of magnesium alloy AZ91HP (9wt%Al, 0.5wt%Zn, 0.5wt%Mn, Mg remaining percentage) by high current pulsed electron beam (HCPEB) treatment was studied in this paper. The secondary phase MgnAln is nearly completely dissolved and as a result, a super-saturated solid solution forms on the re-melted surface. The microhardness is increased both in and far beyond the heat-affected zone (HAZ), reaching about 250um. Measurements on sliding wear have shown that the wear resistance of the treated samples was improved by a factor of about 2.4 as compared to the as-received sample. It is also found that the sliding wear resistance can be further improved by surface alloying with TiN.

Improvement of wear resistance of AZ31 and AZ91HP by high current pulsed electron beam treatment

The surface modification of magnesium alloys (AZ31 and AZ91HP) was studied by a high current pulsed electron beam(HCPEB). The results show that the cross-sectional microhardness of treated samples increases not only in the heat affected zone(HAZ), but also beyond HAZ, reaching over 250μm. This is due to the action of quasi-static thermal stress and the shock thermal stress wave with materials, which result in its fast deformation on the surface layer and so increases microhardness. For the AZ91HP alloy, a nearly complete dissolution of the intermetallic phase Mg_ 17Al_ 12 is observed, and a super-saturated solid solution forms on the re-melted surface, which is due to the solute trapping effect during the fast solidification process. Measurements on sliding wear show that wear resistance is improved by approximately 5.6 and 2.4 times for the AZ31 and AZ91HP respectively, as compared with as-received samples.

Characterizing modified surface layer of 316L stainless steel treated by high current pulsed electron beam

High current pulsed electron beam(HCPEB) is now developing as a useful tool for surface modification of materials.When concentrated electron flux transferring its energy into the surface layer of target material within a short pulse time,coupled thermal and stress processes would lead to the formation of metastalbe microstructure with improved properties.In the present work,HCPEB treatment of 316L stainless steel(SS) was carried out and the microstructural changes in modified surface layer were characterized with optical microscopy,X-ray diffractometry and electron backscatter diffractometry(EBSD) techniques.The corrosion resistance of modified surface was measured in a 5wt.% salt solution.The evolution regularity of surface craters and grain refinement effect,as well as the preferred orientation of(111) crystal plane occurring in the HCPEB treatment under different working parameters were discussed along with their influence on corrosion resistance.

HIGH-CURRENT PULSED ELECTRON BEAM: RAPID SURFACE ALLOYING AND WEAR RESISTANCE IMPROVEMENT

The present paper reports the rapid surface alloying induced by the bombardment of high-current pulsed electron beam. Two kinds of substrate materials were examined to show this effect. The first sample was a pure Al metal pre-coated with fine carbon powders prior to the bombardment, and the second alloy is the D2-Crl2MolVl mould steel pre-coated with Cr, Ti, and TiN powders. The surface elements diffuse about several micrometers into the substrate materials only after several bombardments. Tribological behaviors of these samples were characterized and significant improvement in wear resistance was found. Finally, a TEM analysis reveals the presence of stress waves generated by coupled thermal and stress fields, which was considered as the main cause of the enhanced properties.

Numerical simulation of thermal-mechanical process of Al-Si-Pb alloy treated by high current pulsed electron beam

The modified microstructure of Al-Si-Pb alloys irradiated by high current electron beam (HCPEB) reveals three distinct regions: a molten zone, an overlapped zone of heat-affected and quasistatic thermal stress-affected zone, and a transition zone followed by the substrate. The hardness and wear properties of the alloys were significantly improved. To better understand these changes in microstructure and properties, the physical model for the simulation of temperature and quasistatic stress fields was established. Based on experimental investigation and physical models, the temperature field and stress field were simulated for Al-Si-Pb alloy. The starting melting position, largest crater depth, melting layer thickness, and quasistatic stress distribution were obtained. These results reveal the mechanism of crater formation on the surface and improvement of hardness and wear resistance.

Microstructure and properties of Nb/Ta multilayer films irradiated by a high current pulse electron beam

Nb/Ta multilayer films deposited on Ti6A14V substrate with Nb and Ta monolayer thicknesses of 30 nm, 120 nm, and 240 nm were irradiated by a high current pulse electron beam （HCPEB） to prepare Nb-Ta alloyed layers. The mi- crostructure and the composition of the outmost surface of melted alloyed layers were investigated using a transmission electron microscope （TEM） equipped with an X-ray energy dispersive spectrometer （EDS） attachment. The Ta content of the alloyed surface layer prepared from the monolayer of thickness 30 nm, 120 nm, and 240 nm was- 27.7 at.%, 6.37 at.%, and 0 at.%, respectively. It was found that the Ta content in the alloyed layer plays a dominant role in the microstructure of the films. The hardness and the wear rate of the alloyed layers decrease with the increasing content of Ta in the surface laver.

Microstructure and property modifications in surface layers of a Mg-4Sm-2Al-0.5Mn alloy induced by pulsed electron beam treatments

In this work,surface modification of a Mg-4Sm-2Al-0.5Mn alloy with high current pulse electron beam(HCPEB)under different number of pulses were investigated.The evolution in microstructure,composition and phase components and properties in the surface layer before and after HCPEB treatment were characterized.It was found that the Al 11 Sm 3 and Al 2 Sm phases in the surface layer were gradually dissolved during HCPEB treatment,leading to the formation of a chemical homogeneous melted layers.Besides,deformation bands were formed in the treated layer due to the thermal stress generated during treatment.After 15 pulses treatment,the surface hardness increases to the maximum value of about 62.2 HV,about 61.2%higher than that of the untreated state.Electrochemical results show that the 15 pulses treated sample presents the best corrosion resistance in the 3.5wt%NaCl water solution by showing the highest corrosion potential(E_(corr))of-1.339V SEC and the lowest corrosion current density(I_(corr))of 1.48×10^(-6)A·cm^(-2).The results prove that the surface properties of the Mg-4Sm-2Al-0.5Mn alloy can be significantly improved by the HCPEB treatments under proper conditions.

Development of a high current short pulse electron gun

Dielectric wall accelerator(DWA), towards high gradient acceleration field(30 MeV/m–100 MeV/m), is under development at Institute of Modern Physics. A prototype was designed and constructed to prove the principle. This needs a short pulse high current electron source to match the acceleration field generated by the Blumlein-type pulse forming lines(PFLs). In this paper, we report the design and test of a new type short pulse high current electron gun based on principle of vacuum arc discharge. Electron beams of 100 mA with pulse width of 10 ns were obtained.

A Pulsed Electromagnet for Laser Wakefield Electron Acceleration Experiments

Laser Wakefield plasma acceleration of electrons to energies above 10 GeV, may be possible in the new high power Laser beam facilities. The design of an Electron Spectrometer with an electro-magnet with adjustable magnetic field is proposed for the characterization of electron energy spectrum with a precision better than 10% for the entire energy range from 0.5 GeV to 38 GeV. The expected precision in the measurement of the electron energy is calculated as a function of the magnetic field, of the electron energy and of the magnet length. To outline the advantages offered by a pulsed electromagnet with high magnetic fields, the mass and the electric power lost in the coils of a 4 m long electromagnet with continuous current and Iron yoke are calculated.

Enhanced Surface Properties Induced by High Current Pulsed Electron Beam (HCPEB) Treatment

High current pulsed electron beam (HCPEB) has been developed as a useful tool for surface treatment of materials.In the present work,the fundamental principle of HCPEB source was described along with the device configuration and working parameters.Through the different kinds of HCPEB surface treatment experiments conducted,the enhanced surface properties induced by HPCEB treatment were illustrated and explained with their microstructure characterization results.

Corrosion Resistance of AZ91 Mg Alloy Modified by High-Current Pulsed Electron Beam

The high-current pulsed electron beam(HCPEB) treatment with current density 6 J/cm^2 was applied on AZ91 Mg alloy to improve its corrosion resistance. Results showed that the net-like Mg_(17)Al_(12) disappeared on the surface of AZ91 Mg alloy after irradiation by HCPEB, which was instead of supersaturated Al element on the surface. Nevertheless, the application of HCPEB also led to the formation of crater-like and groove-like structures as well as micro-cracks on the surface of AZ91 Mg alloy. After HCPEB treatment by 3, 5 and 10 pulses, the AZ91 Mg alloy exhibited better corrosion resistance.However, the increasing amount of micro-cracks reduced the anti-corrosive properties of AZ91 Mg alloy as the pulse increased to 20 and 30.

Deformation mechanism and microstructures on polycrystalline aluminum induced by high-current pulsed electron beam

In this paper, we present an experimental investigation of deformation twinning in polycrystalline aluminum exposed to high-current pulsed electron beam (HCPEB) irradiation. The residual tensile stress with about 10 2 MPa was introduced in the irradiated surface layer. The feature characteristic irradiated with various numbers of pulses was investigated. The formation of a large number of twin bands on the surface irradiated with multiple pulses was determined. The experimental observations indicated that the deformation twinning was indeed triggered during HCPEB irradiation. It is suggested that high value of stress and strain rate induced by rapid heating and cooling due to HCPEB irradiation may cause the shifting of whole atomic planes simultaneously. Additionally, some slipping systems may be suppressed due to the geometric confinement by thinned size of surface layer, which can promote the initiation of deformation twinning.

Blue photoluminescence from nanocrystalline porous silicon structure fabricated by high-current pulsed electron beam irradiation

N-type Si(111) wafers have been processed by high-current pulsed electron beam(HCPEB) treatment with an increasing number of irradiation(1, 10 and 20pulses). The results of this work show that a highly porous nanostructure was formed after irradiation. Moreover, the high-density Si nanocrystals(Si-ncs) about 3 nm were distributed on the surface of Si wafers and exhibited3.02 e V Photoluminescence(PL) emission in blue band.The PL intensity increases with the increase in the Si-ncs' density in accordance with the quantum confinement model, which can be ascribed to the different pulse time of HCPEB treatment. The possible formation mechanisms of micropores and Si-ncs are discussed.

Preliminary Research on Primary Energy Source of High Current Electron Beam Accelerator Based on Spiral Pulse Forming Line

In this paper,the primary energy source of high current electron beam accelerator based on spiral pulse forming line is investigated.It consists of the constant-current power supply,the high voltage pulse capacitor,the field distortion switch,and the protection system.The primary energy source can discharge to the primary winding of the transformer with high voltage pulses whose amplitude of voltage is 40kV,current is 80kA,pulse width is 8μs and repetition frequency is less than 5Hz.The primary energy source is applied to a high current electron beam accelerator, and is featured by its compactness,stability and reliability.

Surface alloying of Al films/Ti substrate based on high-current pulsed electron beams irradiation

Ti–Al surface alloy was fabricated using a cyclic pulsed liquid-phase mixing of predeposited 100 nm Al film with a-Ti substrate by low-energy high-current electron beam. Electron probe micro-analysis（EPMA）,grazing incidence X-ray diffraction analysis（GIXRD）,transmission electron microscopy（TEM）, and nanoindentation were used to investigate the characterization of Ti–Al surface alloy. The experimental results show that the thickness of alloy layer is ＊3 lm, and the content of Al in the ＊1 lm thickness surface layer is ＊60 at%. The tetragonal TiAl and TiAl2intermetallics were synthesized at the top surface, which have nanocrystalline structure.The main phase formed in the ＊2.5 lm thick surface is TiAl, and there are few TiAl2and Ti3Al phase for the alloy.Dislocation is enhanced in the alloyed layer. The nanohardness of Ti–Al surface alloy increased significantly compared with a-Ti substrate due to the nanostructure and enhanced dislocation. Since the e-beam remelted repeatedly, the Ti–Al surface alloy mixed sufficiently with Ti substrate. Moreover, there is no obvious boundary between the alloyed layer and substrate.

高能束表面改性技术在航空领域的应用

高能束表面改性适用于各种金属和合金,能够显著提升材料表面硬度、耐磨、耐蚀等性能指标,是航空部件实现性能提升的有效手段之一。本文总结了6种高能束表面改性技术的基本原理、设备构成和改性应用,其中激光相变硬化通过马氏体相变强化金属材料表面;激光熔覆通过选择不同粉末实现表面修复和表面性能提升,重点在于控制裂纹缺陷;激光冲击强化可有效解决航空发动机部件高周疲劳断裂问题;强流脉冲电子束和强流脉冲离子束一方面需要提高设备的性能和运行稳定性,另一方面要针对航空部件应用开展深入研究;而离子束辅助沉积则可以通过制备固体润滑涂层实现对微动磨损的有效防护。最后,提出对高能束表面改性机理深入研究、发展专业化智能化装备和实现多种束源复合与集成的发展方向。

一种小尺寸轫致辐射转换靶设计

以减小直线感应加速器X射线光源横向尺寸为目标,开展轫致辐射转换靶的设计。对聚焦打靶过程中电子束运动轨迹进行分析,指出同一个电子束轨迹分布,既可以描述为电子束在某纵向位置处具有一定的横向展宽,也可以描述为电子束保持较小横向尺寸时的轴向分布展宽,由此提出在束腰附近放置多个小靶片实现聚焦电子束有效阻挡的小尺寸多层靶概念设计。采用EGS4程序对X射线产额进行计算,发现靶厚度在一定范围内改变时X射线产额变化较小,基于这一规律完成了小尺寸多层靶的结构设计。进一步考察了一个设计应用实例,当聚焦电子束最小包络直径3 mm、会聚角100 mrad时,对比大尺寸靶,采用小尺寸多层靶可以获得等效直径减小约50%、产额减小约10%的X射线光源。该设计方法有望在相同的电子束品质和聚焦条件下,获得横向尺寸小于电子束最小束包络直径的X射线光源,具有一定的应用价值。