A 20 keV electron gun system for the electron irradiation experiments

An electron gun consisting of cathode, focusing electrode, control electrode and anode has been designed and fabricated for the electron irradiation experiments. This electron gun can provide electrons of any energy over the range 1–20 keV, with current upto 50 μA. This electron gun and a Faraday cup are mounted in the cylindrical chamber. The samples are fixed on the Faraday cup and irradiated with electrons at a pressure ∼10⁻⁷ mbar. The special features of this electron gun system are that, at any electron energy above 1 keV, the electron beam diameter can be varied from 5 to 120 mm on the Faraday cup mounted at a distance of 200 mm from the anode in the chamber. The variation in the electron current over the beam spot of 120 mm diameter is less than 15% and the beam current stability is better than 5%. This system is being used for studying the irradiation effects of 1–20 keV energy electrons on the space quality materials in which the irradiation time may vary from a few tens of seconds to hours.     

聚焦离子束的束径测量

报道了聚焦离子束装置的束径测量方法和典型测量结果。偏转离子束使其扫过金属栅网的边缘 ,由法拉第筒接收到的离子流强度变化所形成的S形电流曲线可确定束径大小。在数据处理中 ,采用了快速傅立叶变换滤波的办法来消除微小电流测量结果中的噪声 ,从而可以对S形电流曲线进行微分 ,得到束流密度空间分布。在改变透镜光阑口径而维持靶室离子流强度不变的测量模式中 ,证实了离子间库仑相互作用增大束径的效应     

Beam quality test technology and devices of electron beam welding

The electron beam four-dimensional quality test system was developed targeted at the 5-100 mA electron beam current of a high-voltage electron beam welding machine. The system includes the control module, sensor module, driver module, and analysis software, which is based on the complex programmable logic device (CPLD). The quality test system obtains the data of quasi-instantaneous power density distribution (QIPDD) of the electron beam by controlling the beam periodically, scanning the surface of a Faraday cup. Fixing the Faraday cup at an X- and Y-axis coordinate position under constant conditions, the three-dimensional graphic of QIPDD of the beam about the data of a single cross-section was showed by using computer reconstruction technology. Several QIPDD of the electron beam cross-section at different working distance in axial direction reconstructed the four-dimensional graphics of the beam.     

An improved design of Faraday cup detector to reduce the escape of secondary electrons in plasma focus device by COMSOL

Faraday cup (FC) is a beamline diagnostic which is used to measure the total charge of pulsed beams. The reliable operation of an FC depends on its ability to recapture the ejected electrons when energetic particles strike with interior cup(IC) of an FC. In this paper COMSOL multi-physics software is used to investigate the performance of two new proposed FCs. The FC with A = 40 mm and B = 98 mm had better efficiency in contrast with traditional FC geometries.     

Beam quality test technology and devices of electron beam welding

The electron beam four-dimensional quality test system was developed targeted at the 5–100 mA electron beam current of a high-voltage electron beam welding machine. The system includes the control module, sensor module, driver module, and analysis software, which is based on the complex programmable logic device (CPLD). The quality test system obtains the data of quasi-instantaneous power density distribution (QIPDD) of the electron beam by controlling the beam periodically, scanning the surface of a Faraday cup. Fixing the Faraday cup at an X- and Y-axis coordinate position under constant conditions, the three-dimensional graphic of QIPDD of the beam about the data of a single cross-section was showed by using computer reconstruction technology. Several QIPDD of the electron beam cross-section at different working distance in axial direction reconstructed the four-dimensional graphics of the beam.     

An improved design of Faraday cup detector to reduce the escape of secondary electrons in plasma focus device by COMSOL

Faraday cup (FC) is a beamline diagnostic which is used to measure the total charge of pulsed beams. The reliable operation of an FC depends on its ability to recapture the ejected electrons when energetic particles strike with interior cup(IC) of an FC. In this paper COMSOL multi-physics software is used to investigate the performance of two new proposed FCs. The FC with A = 40 mm and B = 98 mm had better efficiency in contrast with traditional FC geometries.     

电子束焊接束流诊断用法拉第筒传感器研制

针对中高压电子束焊机5~100mA的电子束流诊断,完成了法拉第筒传感器的研制。该传感器由法拉第筒、微弱信号放大电路、增益控制电路构成。法拉第筒电极入射孔采用双孔结构,有效的阻止了二次电子和背散射电子的逸出,提高了电子束流的收集效率。通过控制微弱信号放大电路的增益将整个束流强度范围内的信号放大到伏级。在Ua=150 kV,Ib=10mA,If=360mA,H=315 mm条件下采集功率密度分布数据,得到束流密度的二维分布图形。由于受电子束焊机特性的影响,束流功率密度分布呈非对称性的近高斯分布。     

Tribo-electrification of particles under sudden change of fluid flow in the junction between branch pipe and straight pipe

An experimental investigation of the tribo-electrification of glass beads fed by an ejector has been conducted by measuring both the current generated at the pipe wall and the specific-charge measured by a Faraday cage under sudden change of fluid flow in the junction between a stainless-steel branch pipe and a stainless-steel straight pipe. In measuring a current per unit mass, for D_p,50 ? 206 μm at the branch pipe section the current has a positive value as expected by the contact potential difference between glass beads and stainless-steel. On the other hand, for D_p,50 ? 105 μm at the branch and straight pipe section, the current has a negative value couldn’t be explained solely by the contact potential difference. In measuring a specific-charge by the Faraday cage, the specific-charge has a negative highest value at the ejector. The negative specific-charge decreases along the particle flow direction. Therefore, an “unusual” charge-transfer, which couldn’t be explained solely by the contact potential difference, was confirmed also by the Faraday cage. Although the charge-transfer between the beads and the inclined stainless plate with high impact speed has been examined, the sign of the current is positive for all data. It was found that the “unusual” charge-transfer in this study couldn’t be caused by the high speed impaction. An negative current in air by using a stainless steel needle detected at the ejector for D_p,50 = 51 μm while an positive current in air detected at the branch and straight pipe. The reason is suggested that the ion balance in the air does not keep between the ejector and the branch pipe due to both the adsorption of some negative ions on the pipe wall and the decrement of negative charge of particles. Therefore the “unusual” charge-transfer consists of not only the ionization caused by the self-discharge but also an adsorption of ions on the inner wall of the pipe.     

A high-charge and short-pulse RF photocathode gun for wake-field acceleration

In this paper we present a design report on 1- cell, L-Band RF photocathode gun which is capable of generating and accelerating electron beams with peak currents> 10 kA. We address several critical issues of high-current RF photoinjectors such as longitudinal space charge effect, and transverse emittance growth. Unlike conventional short electron pulse generation, this design does not require magnetic pulse compression. Based on numerical simulations using SUPERFISH and PARMELA, this design will produce 100 nC beam at 18 MeV with r.m.s. bunch length 1.25 mm and normalized transverse emittance 108 mm mrad. Applications of this source beam for wake-field acceleration are also discussed.     

pA级弱电子流的获取及测量研究

介绍了利用电子加速器进行内带电地面模拟试验研究的关键部分——弱束流的获取及测量方法研究。电子束在通过微孔板散射后得到了弱束流,并用法拉第筒进行了测量,得到了地面模拟试验需要的pA级的弱束流。     

The temporal structure of a runaway electron beam generated in air at atmospheric pressure

Supershort avalanche electron beams (SAEBs) generated in air at atmospheric pressure have been studied with picosecond time resolution. It is established that an SAEB has a complicated structure that depends on the interelectrode gap width and cathode design. In a gas-filled diode with a small gap width, an SAEB current pulse with a full width at half maximum (FWHM) of ??25 ps has been observed behind a collimator with a hole diameter of 1 mm. As the gap width is increased or decreased relative to the optimum value that corresponds to the maximum beam current, the SAEB current pulse shape changes and pulses with two peaks are more likely detected. The two-peak SAEB current pulse shape is retained behind aluminum foil with a thickness of 60 and 110 ??m.     

Measurements of energy spread and intensity distributions of the laser photoionized neodymium ions

Energy spread of the laser photoionized neodymium ions were measured by retarding-potential method. The obtained energy spread derived from the derivative ion charge ratio spectra corresponded to the potential of the laser photoionized region between ion extraction electrodes. Ion beam with the central ion energy between 300 and 1400 eV were then produced, and their lateral intensity distributions were measured with a multi-channel Faraday cup. The results show that the collimated beams of the laser photoionized ions at these ion energies were obtained.     

Low emittance ion beams by laser interaction

Laser ion sources offer the possibility to get ion beam utilizable to improve particle accelerators. Pulsed lasers at intensities of the order of 10⁸ W/cm² and of ns pulse duration interact with solid matter in vacuum to produce plasma of high temperature and density. The charge state distribution of the plasma generates high electric fields which accelerate ions along the normal to the target surface. The energy of emitted ions has a Maxwell Boltzmann distribution, which depends on the ion charge state. To increase the ion energy, a post-acceleration system can be employed by means of high voltage power supplies of about 100 kV. The post-acceleration system results in a good method to obtain high ion currents by an inexpensive system and the final ion beams find interesting applications in the field of the ion implantations, scientific applications and industrial use. In this work we compare the electromagnetic and geometric properties, like the emittance, of the beams delivered by a Cu and Y target. The characterization of the plasma was performed by a Faraday cup for the electromagnetic characteristics and a pepper pot system for the geometric ones. At 60 kV accelerating voltage and 5.5 mA output current the normalized beam emittance resulted in 0.22 π mm mrad for the Cu target, while under the same accelerating voltage but with 7.4 mA output current, a lower normalized beam emittance value was reached for the Y target. It resulted in 0.14 π mm mrad. The brightness of the beams was of 114 and 378 mA (π mm mrad)⁻² for the Cu and Y targets, respectively.     

Strengthening and repair of RC beams with fiber reinforced concrete

The use of a jacket made of fiber reinforced concrete with tensile hardening behavior for strengthening RC beams is investigated by means of full-scale tests on 4.55 m long beams. A 40 mm jacket of this material was directly applied to the beam surface. Both the strengthening and the repair of RC beams were studied. In particular, in the latter case the beam was initially damaged and eventually repaired. A numerical analysis is also performed in order to better understand the reinforcement behavior. The experimental and numerical results show the effectiveness of the proposed technique both at ultimate and serviceability limit states.     

Surface treatment of DZ4 directionally solidified nickel-based superalloy by high current pulsed electron beam

Surface treatment of DZ4 directionally solidified nickel-based superalloy was carried out by using high current pulsed electron beam (HCPEB) technique. Microstructure changes in modified surface layer were characterized together with their corrosion property. It is found that the treated surface becomes rougher with increasing the number of HCPEB pulse due to the formation of craters occurring preferentially in the interdendritic areas. The thickness of remelted layer increased slightly when more HCPEB pulses applied and reached about 3 μm after 10 HCPEB pulses. The corrosion resistance of modified surfaces exhibited an effective improvement when measured in 0.5 mol/L sulfuric acid solution. The factors that influence the corrosion resistance of DZ4 alloy were discussed by considering the surface purification effect featured by HCPEB surface treatment.     

Generation of subnanosecond electron beams in air at atmospheric pressure

Optimum conditions for the generation of runaway electron beams with maximum current amplitudes and densities in nanosecond pulsed discharges in air at atmospheric pressure are determined. A supershort avalanche electron beam (SAEB) with a current amplitude of ∼30 A, a current density of ∼20 A/cm², and a pulse full width at half maximum (FWHM) of ∼100 ps has been observed behind the output foil of an air-filled diode. It is shown that the position of the SAEB current maximum relative to the voltage pulse front exhibits a time shift that varies when the small-size collector is moved over the foil surface.     

A transparent vacuum window for high-intensity pulsed beams

The HiRadMat (High-Radiation to Materials) facility [1] will allow testing of accelerator components, in particular those of the Large Hadron Collider (LHC) at CERN, under the impact of high-intensity pulsed beams. To reach this intensity range, the beam will be focused on a focal point where the target to be tested is located. A 60 mm aperture vacuum window will separate the vacuum of the beam line which is kept under high vacuum 10⁻⁸ mbar, from the test area which is at atmospheric pressure. This window has to resist collapse due to beam passage. The high-intensity of the beam means that typical materials used for standard vacuum windows (such as stainless steel, aluminium and titanium alloy) cannot endure the energy deposition induced by the beam passage. Therefore, a vacuum window has been designed to maintain the differential pressure whilst resisting collapse due to the beam impact on the window. In this paper, we will present calculations of the energy transfer from beam to window, the design of the window and associated mechanical calculations.     

Ellipsometer measurements using focused and masked beams

While optical spectroscopic measurements using ellipsometry may be made in air and are non-destructive, the relatively large (> 2 mm) spot size has limited their use to surface regions greater than 2 mm in lateral extent. Recent developments in focusing instruments have made spot sizes on the order of 20 to 25 μm possible. The work to be presented explores the use of the 25 μm spot size to probe non-uniform nanostructured thin films. Measurements were performed on a highly non-uniform film (0 to 2 μm in thickness across 4 mm in lateral dimension) using such a 25 μm spot. Further reduction of the spot size is possible using mechanical masking with a slit. Measurements have been made to the range of a few microns in width. The practical resolution limits of beam masking may be decreased by increasing incident light intensity, improving slit alignment, and improving detection methods.     

Deep-level studies in GaN layers grown by epitaxial lateral overgrowth

The electrical properties, deep-level spectra, microcathodoluminescence (MCL) spectra and diffusion lengths of minority charge carriers were measured in GaN films grown by the epitaxial lateral overgrowth (ELOG) technique. The results are compared to the properties of GaN layers grown in a standard fashion without masking of the initial template. MCL and electron beam induced current (EBIC) imaging of the laterally overgrown regions revealed the presence of dark spots with density of 1-5 × 10⁶ cm^− 2 that are associated with individual dislocations. The concentration of deep electron and hole traps was found to be much higher in the standard material than in the ELOG material. Diffusion lengths of minority carriers determined from EBIC signal profiling gave values of 0.8-1 μm along the bright regions and 0.4-0.5 μm in the dark regions of the ELOG samples. Similar measurements on metal organic chemical vapor deposition templates gave a diffusion length of 0.4-0.5 μm, close to the diffusion length in the dark stripes of the ELOG samples.     

Recent developments in dynamic transmission electron microscopy

One of the current major driving forces behind instrument development in transmission electron microscopy (TEM) is the ability to observe materials processes as they occur in situ within the microscope. For many processes, such as nucleation and growth, phase transformations and mechanical response under extreme conditions, the beam current in even the most advanced field emission TEM is insufficient to acquire images with the temporal resolution (∼1 μs to 1 ns) needed to observe the fundamental interactions taking place. The dynamic transmission electron microscope (DTEM) avoids this problem by using a short pulse laser to create an electron pulse of the required duration through photoemission which contains enough electrons to form a complete high resolution image. The current state-of-the-art in high time resolution electron microscopy in this paper describes the development of the electron optics and detection schemes necessary to fully utilize these electron pulses for materials science. In addition, developments for future instrumentation and the experiments that are expected to be realized shortly will also be discussed.