Search

A new electron accelerator has been designed and is now under construction at AIST, Tsukuba, Japan. The accelerator will be used to produce a pulsed neutron beam for materials analysis. The design maximizes the yield of slow neutrons with high pulse temporal resolution as described in a previous publication [K. Kino et al., Nuclear Inst. and Methods in Physics Research, A 927 (2019) 407–418]. The accelerator is composed of a thermal cathode type electron gun with S-Band buncher/accelerator cavity followed by three, 2.9 m long S-Band acceleration cavities. High power RF is supplied by three klystron units, with klystron No.1 supplying both the electron gun and the 1st cavity, and klystrons No.2 and No.3 connected to the 2nd and 3rd cavities respectively. The design parameters of the accelerator are; pulse rate: 100 Hz, pulse length: 10 μs, electron current: 250 mA, electron energy: 40 MeV. Operating at these parameters a maximum beam power of around 10 kW will be delivered to the water-cooled Ta target for neutron production. In this report we will describe the design and construction of the electron accelerator. [ABSTRACT FROM AUTHOR]

The efficiency of using electron beam welding in producing permanent joints of structural materials of all thicknesses and shapes is well-known. The work for the first time considers the possibility of using electron beam welding for the production of high-precision parts of optical system in cathode unit, providing the necessary parameters and their joining with electron beam gun insulator in future. The use of electron beam welding at the final stage of manufacturing cathode unit opens the opportunity of minimizing the number of further technological operations. It is noted that to create the welding technology, in which the operation of welding would become a final assembly operation, the development of new designs of welded joints and schemes for assembly of cathode unit are required. Due to a correct design of welding units and compliance with the accuracy of assembling for welding, it became possible to preserve geometric dimensions after electron beam welding and to provide the operational reliability of the design as a whole. 5 Ref., 6 Figures. [ABSTRACT FROM AUTHOR]

In this article, the design and simulation of an electron injector based on a thermionic RF-modulated electron gun for particle accelerator applications is presented. The electron gun is based on a gridded thermionic cathode with the geometry based on a Pierce-type configuration. Both theory and numerical simulation were used to explore the relationship between the bunch length and the charge. The reasons for the pulse widening were also analyzed. The beam dynamics simulations showed that a minimum pulselength of 106 ps could be achieved with a bunch charge of 33 pC when the driving RF frequency was 1.5 GHz. The average transverse emittance was about 17 mm·mrad from the particle-in-cell simulations. Operating at a higher RF frequency did not significantly reduce the micro pulselength. [ABSTRACT FROM AUTHOR]

We propose an unconventional structure of an electron gun to produce an annular beam for the frequency-tunable gyrotrons. The proposed gun takes advantage of the nonadiabatic electron motion during the acceleration of electrons by placing an emitter on the concave region of the cathode with a relatively weak electric field. The commonly employed adiabatic theory fails to predict the simulated transverse velocity spread, which suggests that the nonadiabatic effect plays an important role during the acceleration of electrons. Simulations are carried out by the EGUN code and verified with the commercial Computer Simulation Technology (CST) Particle Studio. From the change in the kinetic energy and the magnetic moment, we can define a section of the nonadiabatic electron motion. The rapid change in the beam-quality parameters within the nonadiabatic section is associated with the significant change in the electric field near the cathode. By considering the nonadiabatic electron motion, simulations predict appropriate parameters, namely, a pitch factor of 1.5 and a transverse velocity spread of 2.8%, over a wide range of the magnetic field (7.4–8.0 T) and the beam voltage (12–22 kV) with a high structural tolerance on the cathode geometry. The promising results enable the development of frequency-tunable gyrotrons. [ABSTRACT FROM AUTHOR]

We report the design and experimental demonstration of a frequency tunable terahertz gyrotron at 527 GHz built for an 800-MHz dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR) spectrometer. The gyrotron is designed at the second harmonic (ω = 2ωc) of the electron cyclotron frequency. It produces up to 9.3-W continuous microwave (CW) power at 527.2-GHz frequency using a diode type electron gun operating at V = 16.65 kV, Ib = 110 mA in a TE11,2,1 mode, corresponding to an efficiency of ~0.5%. The gyrotron is tunable within ~0.4 GHz by combining voltage and magnetic field tuning. The gyrotron has an internal mode converter that produces a Gaussian-like beam that couples to the HE11 mode of an internal 12-mm i.d. corrugated waveguide periscope assembly leading up to the output window. An external corrugated waveguide transmission line system is built including a corrugated taper from 12- to 16-mm i.d. waveguide followed by 3 m of the 16-mm i.d. waveguide The microwave beam profile is measured using a pyroelectric camera showing ~84% HE11 mode content. [ABSTRACT FROM AUTHOR]

We developed a small-size electron gun capable of producing electrons with kinetic energy less than few tens of eV to investigate the slowing down and transport mechanisms of electrons in hydrogen negative ion source plasmas. The maximum extractable beam current density reached 36 μA/cm2 for 1 eV beam energy in a preliminary experiment. Although the present electron current density is still insufficient compared with our target value, 1 mA/cm2, we have found some hints to realize larger beam current density from the electron gun through this study. The measured beam profile along the electron beam axis has shown that the electron beam could travel approximately 7 mm from the electron gun in vacuum. The Particle-In-Cell (PIC) simulation explained the measured beam profile well and indicated that the electron beam has an energy spread as small as 0.1 eV compared to the 1 eV mean energy. The PIC simulation showed a discrepancy from the measurement in the dependence of the electron beam current on the beam extraction voltage of the electron gun. It implies that we should introduce a more realistic filament structure inside the electron gun in the PIC simulation in order to study the transport of low energy electrons more precisely. [ABSTRACT FROM AUTHOR]

We developed a new prototype micro-multifocus X-ray source based on electron beam scanning (MMFEBS) for multi-view stationary Micro Computed Tomography (multi-view stationary Micro-CT). This micro-multifocus X-ray source includes an electron gun, electron beam deflection coils, a dynamic condensor, a flat panel transmission target and dynamic vacuum system. The electron beam is controlled by the deflection coils and sweeps back and forth on the flat panel transmission target. It hits a series of accurately-defined positions on the target, resulting in two-dimensional X-ray multi-focus array. A dynamic condensor is utilized to overcome the electron beam defocusing caused by the deflection. It guarantees the same high spatial resolution (better than $4~\mu \text{m}$) at any deflection angle. The MMFEBS, together with a rotational stage and a flat panel detector, produces a multi-view stationary Micro-CT which can obtain multi-view X-ray projection data for CT images in static condition. Micron resolution CT images obtained in our experiments showed that the MMFEBS has high resolution and good repeatability. [ABSTRACT FROM AUTHOR]

A low-energy electron beam profile monitor has been developed and tested. It has been used to optimize the focusing of continuous electron beam of current up to 5 mkA and kinetic energy less than 20 eV. The device realizes the pin-hole camera method. The beam-sampling part of monitor is based on Nipkow disc principle, which allows investigating the beam cross section for one complete turnover. This made it possible to accelerate the measurement process in comparison with devices using the reciprocating movement of a collector with a hole. The monitor enables the determination of two-dimensional beam profile and measurement of beam current. The monitor was used to study the parameters of a low-energy electron beam gun designed to measure the work function of electrons from a metal by the Anderson method. The scheme is given and the operation of the experimental setup is described. The electrical signals received in the process of scanning together with synchronization pulses through the PCI-1802L adapter are transmitted to a personal computer, where a specially developed program collects and processes the received data. The measurement results are displayed on the computer monitor as a family of line graphs, 3D images and tables for each measurement cycle (disk turnover). The design incorporates low material and labor costs and minimal amount of devoted electronics. [ABSTRACT FROM AUTHOR]

Electron beam deposition is a process that involves the use of an electron beam as a source of energy and wire as a raw material for the fabrication of metal products with complex shape in a vacuum and is part of the promising additive technologies group. In this paper a methodology for determining the electron-beam deposition regimes of a wire by the example of corrosion-resistant austenitic steel ANSI 316L is proposed. The proposed calculation methodology connects the main parameters of the process: the electron beam current, the addition wire feed rate, the movement speed of the electron gun and the dimensions of the deposited rollers. The investigation results of the microstructure and properties of the deposited metal are presented. Examples of the obtained cylindrical and conical products are adduced. [ABSTRACT FROM AUTHOR]

In this article, a nonlinear self-consistent, multimode analysis, and particle-in-cell (PIC) simulation of a uniformly dielectric-loaded (UDL) millimeter-wave gyrotron traveling-wave tube (gyro-TWT) amplifier has been discussed. A single-anode magnetron injection electron gun is designed to produce a relativistic electron beam of 100 kV and 5 A with a unity velocity ratio and 5% of axial velocity spread. The desired operating mode TE01 is fed by converting the TE10 rectangular waveguide mode through the Y-shaped power transducer. Both the multimode theory and PIC simulation results are agreed and predicted the saturated RF power of ~140 and ~145 kW at 92 GHz, respectively. A saturated gain of ~51 dB, a 3-dB bandwidth of ~5%, and a power conversion efficiency of ~28% are obtained. To collect the spent electrons, a collector is designed by using electron optics, E-GUN. The maximum heat wall loading at the collector surface due to a spent beam is calculated as 0.255 kW/cm2. [ABSTRACT FROM AUTHOR]

We report the enhancement of carbon nanotube (CNT) field emitters by growing them directly on stripe-patterned alloy substrates. For a gate voltage of 1300 V, CNT field emitters grown on full and stripe-patterned alloy substrates achieved respective emission currents of 0.22 and 2.32 mA, corresponding to tenfold performance enhancement. Performance enhancement is attributed to the reduced screening effect (as a result of the spacing between strips) during field emission and a damage-free patterning process. To verify the ability of the stripe-patterned CNT field emitters to generate X-rays, we designed and assembled an electron gun through brazing. With the open type X-ray system, an X-ray image of human teeth was successfully obtained, verifying the potential of the stipe-patterned CNT emitters in X-ray applications. [ABSTRACT FROM AUTHOR]

The cathode power supply is one of the most important ancillary devices of gyrotrons. Some interesting transient phenomena about the cathode voltage and the cathode current were found in the gyrotron operation in the electron cyclotron (EC) resonance heating system of the Experimental Advanced Superconducting Tokamak. The cathode voltage drops to about 10% of the original value in about $90~\mu \text{s}$ in the overcurrent case, which is much longer than the $25~\mu \text{s}$ in the normal case. In order to explain these phenomena, an equivalent circuit model of the magnetron injection gun was proposed. The equivalent circuit is composed of parallel resistors and capacitors, and it can explain the test results very well. Using the equivalent circuit model to analyze gyrotrons can be an effective means of developing and operating an EC heating system. [ABSTRACT FROM AUTHOR]

The porosity of the anodic films grown on aluminium substrates depends on various operating conditions related to the anodization electrolyte and to the applied electrical parameters, as well as to the substrate itself. In the present study, three different aluminium substrates were studied and anodized: AA 1050 (rolled; thickness = 1 mm), AA 2024 T3 (rolled; 1 mm), and AA 2024 T3 (machined; 3 mm). For each type of anodic film, the porosity, as well as its changes during anodization, was accurately characterized using both field‐emission gun scanning electron microscopy (FEG‐SEM) and a reanodization technique. Moreover, for the first time, the corresponding tortuosity was quantified for all studied substrates. Results for rolled AA 2024 T3 and for machined AA 2024 T3 especially showed significant differences in tortuosity values, contributing towards clarifying, in part, their different wettability characteristics or anticorrosion behaviour, so far not clearly explained. [ABSTRACT FROM AUTHOR]

A complete virtual simulation experiment scheme for the micro-focus field emission electron gun based on the X-ray tube is proposed. A computer simulation software OPERA-3D is used to model the structure of the electron gun, and the effects of the gate shape, grid voltage, grid-focus distance and focus shape on the beam current and spot are studied. The optimized simulation results meet the design requirements. Through this simulation experiment, students can understand the basic principles of electron emission and the theory of electronic optics deeply, laying the foundation for studying related professional courses in the future. [ABSTRACT FROM AUTHOR]

Tantalum pentoxide (Ta2O5) thin films were synthesized using thermionic vacuum arc (TVA) technology. TVA is an original deposition method using a combination of anodic arc and electron gun system for the growth of thin films from solid precursors under vacuum of 10-6 Torr. The properties of the deposited Ta2O5 thin films were investigated in terms of wettability, refractive index, morphology, and structure. The surface free energy was determined by means of surface energy evaluation system indicating a hydrophilic character and the refractive index was measured by Filmetrics F20 device. The morphology was determined from bright field transmission electron microscopy (TEM) image performed by Philips CM 120 ST TEM system. It exhibits nanoparticles of 3-6 nm diameter smoothly distributed. Selected area electron diffraction pattern revealed the contrast fringes given by complex polycrystalline particles included in the amorphous film. The measured fringes could be indexed using monoclinic structure of Ta2O5. [ABSTRACT FROM AUTHOR]

We have designed a femtosecond electron gun suitable for ultrafast electron diffraction experiments, operating in the 30–100 kV regime. The concept is based on recompression of chirped expanding electron pulses emitted from a direct current photogun using a novel dispersion-corrected reflectron concept. We show, using detailed numerical simulations, that our design is capable of producing electron pulses containing 200 000 electrons with a full width at half maximum pulse duration of 130 fs, a root mean squared (rms) pulse radius of 140 μm, and transverse coherence length of 1.5 nm at 100 kV. Our analysis includes the bunch properties at the sample, as well as interactions of the main pulse of high charge density with diffracted electrons. Since our design employs only static electron optics, we believe that it will be easier to implement than concepts based on radio frequency compression. [ABSTRACT FROM AUTHOR]

An optical emission scheme was demonstrated, in which a high-refractive-index waveguide is excited by a traveling electron beam in a vacuum environment. The waveguide was made of Si–SiO2 layers. The velocity of light propagating in the waveguide was slowed down to 1/3 of that in free space due to the high refractive index of Si. The light penetrated partly into the vacuum in the form of a surface wave. The electron beam was emitted from an electron gun and propagated along the surface of the waveguide. When the velocity of the electron coincided with that of the light, optical emission was observed. This emission is a type of Cherenkov radiation and is not conventional cathode luminescence from the waveguide materials because Si and SiO2 are transparent to light at the emitted wavelength. This type of emission was observed in an optical wavelength range from 1.2 to 1.6 μm with an electron acceleration voltage of 32–42 kV. The characteristics of the emitted light, such as the polarization direction and the relation between the acceleration voltage of the electron beam and the optical wavelength, coincided well with the theoretical results. The coherent length of an electron wave in the vacuum was confirmed to be equal to the electron spacing, as found by measuring the spectral profile of the emitted light. [ABSTRACT FROM AUTHOR]

Passing an electron beam through an aperture can serve to reduce the beam current or change the transverse beam profile. For a sufficiently intense beam, space charge will drive a radial expansion of the beam, which may cause the current passing through the aperture to decrease even though the current arriving at the aperture is increasing. When a gridded electron gun is used, this may be expressed by stating that the transconductance of the apertured gun is negative. Here, we explain this effect and explore some of the key factors governing when it can occur and influencing its strength. [ABSTRACT FROM AUTHOR]

The calculation of the current in an electron gun structure is one of the main investigations involved in the electron gun physics understanding. In particular, various simulation codes exist but often present some important discrepancies with experiments. Moreover, those differences cannot be reduced because of the lack of physical information in these codes. We present a simple physical three-dimensional model, valid for all kinds of gun geometries. This model presents a better precision than all the other simulation codes and models encountered and allows the real understanding of the electron gun physics. It is based only on the calculation of the Laplace electric field at the cathode, the use of the classical Child–Langmuir’s current density, and a geometrical correction to this law. Finally, the intensity versus voltage characteristic curve can be precisely described with only a few physical parameters. Indeed, we have showed that only the shape of the electric field at the cathode without beam, and a distance of an equivalent infinite planar diode gap, govern mainly the electron gun current generation. [ABSTRACT FROM AUTHOR]

This paper presents the development of an L-band continuous-wave (CW) two-cell buncher at Tsinghua University for a very-high-frequency electron gun photoinjector. The dynamic simulations indicate the VHF injector generally employs 2-stage compression by the buncher and the first acceleration structure. The optimized voltage for the buncher is about 340 kV, with a shunt impedance of 7.93 M Ω and an RF power dissipation of 14.4 kW. Multipole components due to the waveguide coupler slot are reduced with a four-port design. Multiphysics simulations demonstrate that the maximum temperature rise is 35 °C and the rf heating induced frequency shift is -650 kHz. The resonant frequency, quality factor, and field balance are consistent with the design after tuning during the cold test. About 14 kW rf power has been successfully fed into the buncher during offline tests. After that, the buncher was mounted on the test beamline. The first phase of the beam commissioning has been completed. Unfortunately, the 1.3 GHz solid-state power source of the test beamline could only deliver 6.5 kW rf power, limiting the buncher to operate at 230 kV voltage. Nevertheless, the buncher operated stably during the beam commissioning, successfully compressing 50 pC bunches to 1.15 mm rms and 100 pC bunches to 1.44 mm rms, and maintaining good beam quality. [ABSTRACT FROM AUTHOR]

Time-resolved electron diffraction harbors great promise for resolving the fastest chemical processes with atomic level detail. The main obstacles to achieving this real-time view of a chemical reaction are associated with delivering short electron pulses with sufficient electron density to the sample. In this article, the propagation dynamics of femtosecond electron packets in the drift region of a photoelectron gun are investigated with an N-body numerical simulation and mean-field model. It is found that space-charge effects can broaden the electron pulse to many times its original length and generate many eV of kinetic energy bandwidth in only a few nanoseconds. There is excellent agreement between the N-body simulation and the mean-field model for both space-charge induced temporal and kinetic energy distribution broadening. The numerical simulation also shows that the redistribution of electrons inside the packet results in changes to the pulse envelope and the development of a spatially linear axial velocity distribution. These results are important for (or have the potential to impact on) the interpretation of time-resolved electron diffraction experiments and can be used in the design of photoelectron guns and streak tubes with temporal resolution of several hundred femtoseconds. [ABSTRACT FROM AUTHOR]

The electron pulse broadening and energy spread, caused by space charge effects, in a photoelectron gun are studied analytically using a fluid model. The model is applicable in both the photocathode-to-mesh region and the postanode electron drift region. It is found that space charge effects in the photocathode-to-mesh region are generally unimportant even for subpicosecond pulses. However, because of the long drift distance, electron pulse broadening due to space charge effects in the drift region is usually significant and could be much larger than the initial electron pulse duration for a subpicosecond electron pulse. Space charge effects can also lead to a considerable electron energy spread in the drift region. Temporal broadening is calculated for an initial electron pulse as short as 50 fs with different electron densities, final electron energies, and drift distances. The results can be used to design electron guns producing subpicosecond pulses for streak cameras as well as for time resolved electron diffraction. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

With the growing demand from scientific projects such as the X-ray free electron laser (XFEL), ultrafast electron diffraction/microscopy (UED/UEM) and electron ion collider (EIC), the semiconductor photocathode, which is a key technique for a high brightness electron source, has been widely studied in China. Several fabrication systems have been designed and constructed in different institutes and the vacuum of most systems is in the low 10−8 Pa level to grow a high QE and long lifetime photocathode. The QE, dark lifetime/bunch lifetime, spectral response and QE map of photocathodes with different kinds of materials, such as bialkali (K2CsSb, K2NaSb, etc.), Cs2Te and GaAs, have been investigated. These photocathodes will be used to deliver electron beams in a high voltage DC gun, a normal conducting RF gun, and an SRF gun. The emission physics of the semiconductor photocathode and intrinsic emittance reduction are also studied.

Advanced technical solution for the design of a low perveance electron gun with a high quality beam dedicated to high power Ka-band klystrons is presented in this paper. The proposed electron gun can be used to feed linear accelerating structures at 36 GHz with an estimated input power of 20 MW, thus achieving an effective accelerating electric field in the (100–150) MV/m range. Additionally, in the framework of the Compact Light XLS project, a short Ka-band accelerating structure providing an integrated voltage of at least 15 MV, has been proposed for bunch-phase linearization. For the klystron, a very small beam dimension is needed and the presented electron gun responds to this requirement. An estimate of the rotational velocity at beam edge indicates that the diamagnetic field due to rotational currents are small compared to the longitudinal volume. A detailed analysis of how this has been achieved, including compression of the beam, rotation in the magnetic field, and analysis of the subsequently generated diamagnetic field has been discussed.

• The surface modification of 40CrMn steel is carried out by continuous scanning electron beam process. • The method of ring scanning surface treatment by continuous electron beam is proposed. • Explore the effects of process parameters on surface morphology, microstructure, microhardness and wear resistance. • The mechanical properties of 40CrMn steel were improved significantly after electron beam treatment. Continuous scanning electron beam process (CSEBP) was conducted on 40CrMn steel to enhance its mechanical properties. The influence of electron beam process parameters on the surface microstructures and mechanical properties of 40CrMn steel was investigated. The result shows that the cross-section of the sample after CSEBP contained three districts as follows: melted layer, heated affected zone and substrate. At a beam current of 5 mA, the surface microhardness increased from 309.7 HV to 692.8 HV, which was in 2.34 times of the substrate hardness. Additionally, the surface microhardness decreased nonlinearly with the increase of electron gun moving speed. At a moving speed of 120 mm/min, the surface microhardness reached a maximum value of 666.2 HV. At a moving speed of 300 mm/min, the surface roughness reduced from 1.754 μm to 0.801 μm. The wear loss of the samples after scanning electron beam treatment decreased significantly compared with the initial samples. [ABSTRACT FROM AUTHOR]

A novel hybrid photo injector is being commissioned at the Schlesinger center for compact accelerators in Ariel University. The hybrid gun is composed of a standing wave section (3.5 cells) and a traveling wave section (9 cells) in a single structure, and therefore has a much larger inner surface than a typical 1.5 cell electron gun. This gun will be driving a THz Free Electron Laser (FEL), operating at the super-radiance regime, and therefore requires extraordinary beam properties. First operation of the RF gun was in January 2018. High-power RF field entering a metallic cavity can severely damage the RF structure or the RF window. For this reason, in order to achieve the desired high-field gradients in the RF gun, a careful and extensive conditioning work on the gun is required. This is done by applying slowly increasing power levels into the structure, while monitoring the response constantly. To increase the efficiency and safety aspects of the conditioning work, we developed in-house routines and interlocks, controlled by a dedicated software. Methods and information of diagnostics elements in the RF system are presented, and the necessary requirements to simplify the conditioning work are outlined. [ABSTRACT FROM AUTHOR]

In this study, NiAl–Al 2 O 3 -WC nanocomposite has been synthesized from the Al–NiO–W–C powder mixture by mechanical alloying and subsequent heat treatment. Phase development and structural changes were studied by X-ray diffraction technique, field-emission gun scanning electron microscopy, and transmission electron microscopy. XRD results showed that NiO was disappeared after 20 h milling and NiAl and W 2 C phases were formed. By the increase of milling time, no phase development was observed. So for access to desired compounds, 40 h milled powder thermally treated at 1200 °C for 3 h, and XRD and TEM results confirmed the formation of NiAl-WC-Al 2 O 3 nanocomposite. [ABSTRACT FROM AUTHOR]

The interaction between the supersonic molecular beam (SMB) and electron beam is a key issue in the design of electron ionization mass spectrometry with SMB (EI-MS with SMB). In this paper, one EI ion source prototype was designed based on an electron gun, whose filament was far away from the interaction range and the influence of the filament field could be neglected. The interaction was studied by one high-speed camera and one Faraday cup. The ion flux as well as the variation with stagnation pressure and axial distance was studied. The experimental results were consistent with the simple simulated results about the ionization and photon emission process in the interaction. It was found that higher stagnation pressure was necessary for higher electron energy to acquire the maximum ion flux. The optimized parameters for the EI ion source including the axial position, stagnation pressure, and electron energy were reported. These results were meaningful for the design and optimization of the ion source. [ABSTRACT FROM AUTHOR]

The electron beam-current density at a target greatly affects imaging intensity and contrast when using a low-energy nano-focus X-ray source. This paper proposes a new electron gun comprising a 10-keV nano-focus X-ray source with a single-pole magnetic lens to improve its effective brightness and beam-current density. After analyzing the effect of the electric field on the extended Schottky emission, we first optimized the anode with high effective brightness and a fine virtual spot with enough intensity for nano-focus X-ray imaging. High effective beam-current density was then obtained by optimizing the position, geometry and electrical supplies of the single-pole magnetic lens. The effective brightness of this electron gun is 1.70 × 108 A•cm−2•sr−1, and the nano-focus X-ray source has a spot size of 41 nm when its tube current is 2 μ A. This electron gun achieves higher beam current density (1.6 × 105A•cm−2), and its collimated landing beam efficiently transports the beam current in the X-ray source. [ABSTRACT FROM AUTHOR]

The accelerator components and radiation production systems at the PBP-CMU Electron Linac Laboratory of the Plasma and Beam Physics Research Facility are currently being developed and upgraded to make it possible for generation of the mid-infrared free-electron laser (MIR FEL) and the coherent intense terahertz (THz) radiation via transition radiation and superradiant undulator radiation. To generate a high brightness radiation, a high quality electron beam is crucially required. One of the important keys for this issue is the good performance and reliable electron source. Development of a new thermionic radio-frequency (RF) electron gun is foreseen at our facility. The design of the new RF gun was conducted with the aim to reduce transverse emittance while keeping the longitudinal phase space distribution that is suitable for the bunch compression with the alpha magnet. The present design of the PBP-CMU RF electron gun, which has 1-1/2 cell TM 010 reentrant cylindrical standing-wave resonant cavities and a side-coupling cavity, was used as the starting model for this work. The shape and dimensions of the new RF gun were optimized and the electromagnetic field distribution was simulated by using the program CST Microwave Studio 2016. Electron beam dynamic simulations were done with program ASTRA. The initial particle distribution from thermionic emission process was created by using the self-developed MATLAB script that provides the thermal emittance values corresponding well to the theoretical values. The results of this work suggest that by adding an external resonant cavity with a rounded pill-box shape to the present RF gun model can reduce the transverse emittance. This additional cavity has an optimal length of 25 mm and is connected to the present RF gun at the position 13 mm downstream the second cavity exit. The RF wave is coupled from the second cavity to the external cavity via the second side-coupling cavity. The entire structure finally has a resonant frequency of 2856.03 MHz, an unloaded quality factor of 15623 and a shunt impedance of 1.18 M Ω. The maximum electric field values in the first, the second and the external cavity are 32.8, 65.0 and 29.8 MV/m, respectively. The results from beam dynamic simulations show that this new design RF gun should be able to produce electron bunches with a charge of 170 pC per bunch, a maximum kinetic energy of 2.72 MeV and an energy spread of 540 keV. The simulated horizontal and vertical normalized emittances of 90% beam were respectively reduced by 11.6% and 4.5% compared to the values obtained from the original PBP-CMU RF gun. Furthermore, it was found that the transverse emittance is conserved at the distance longer than 60 cm downstream the external cavity exit. [ABSTRACT FROM AUTHOR]

A converging sheet electron beam with a cross section of 0.05 × 2 mm2 and a current density of 200 A/cm2 formed by an electron gun is modeled using the synthesis and analysis methods with partial and complete magnetic shielding of the cathode and a linear compression of 10 and 15. The cross-sectional deformation of the beam in a focusing magnetic field is analyzed based on the three-dimensional computer model of electron-optical systems with a sheet electron beam. The possibility of generation of a low-perveance flux with small deformation in the transit channel of a comb slow-wave structure up to 30 mm in length is demonstrated. [ABSTRACT FROM AUTHOR]

The results of experimental studies into the application of anisotropic pyrolytic graphite (APG) as grid structures in power microwave devices with a dispenser cathode are detailed. The dependences of emission characteristics of molybdenum, hafnium, and APG in diodes and electron guns on the electron-flow power dissipated at the studied samples and on cathode temperature are presented. It is demonstrated that the power dissipation capacity of APG grid structures (without parasitic thermionic emission) is up to 20 and 9 times higher than that of molybdenum and hafnium grids, respectively. [ABSTRACT FROM AUTHOR]

This paper deals with optimum design and development of a thermionic electron gun to meet specified beam requirements within defined electric and geometric constraints for a Ka-band traveling wave tube (TWT) for space applications. The electron gun design is based on the Pierce method and carried out according to the iterative process indicated by Vaughan. The design of a periodic permanent magnet (PPM) beam focusing system for the stability of the beam is also required. A sensitivity analysis, by varying electric parameters and geometric parameters, is presented and taken into account as a fundamental role to the aim of optimizing the design of the Pierce gun. A cathode current value of 550 mA at a cathode voltage value of 20 KV with a beam diameter value of less than 0.55 mm, as a result of the analysis, is imposed as a goal design parameter, to obtain a TWT output power value more than 500 W. The simulated results show very good agreement with the beam tester experimental results. The body current measured value, in excellent agreement with the simulated one, shows the effectiveness of the design procedure. Data from the experimental beam tester demonstrate 98% beam transmission to the collector. [ABSTRACT FROM AUTHOR]

Glycine is an amino acid that has already been detected in space. It is relevant to estimate its resistance under cosmic radiation. In this way, a sublimate of glycine in α-form on KBr substrate was exposed in the laboratory to a 1.0 keV electron beam. The radiolysis study was performed at 40 K, 80 K, and 300 K sample temperatures. These temperatures were chosen to cover characteristics of the outer space environment. The evolution of glycine compaction and degradation was monitored in real time by infrared spectroscopy (Fourier-transform infrared) by investigation in the spectral ranges of 3500–2100, 1650–1200, and 950–750 cm−1. The compaction cross-section increases as the glycine temperature decreases. The glycine film thickness layer of ∼160 nm was depleted completely after ∼15 min at 300 K under irradiation with ∼1.4 μA beam current on the target, whereas the glycine depletion at 40 K and 80 K occurred after about 4 h under similar conditions. The destruction cross-section at room temperature is found to be (13.8 ± 0.2) × 10−17 cm2, that is, about 20 times higher than the values for glycine depletion at lower temperatures (<80 K). Emerging and vanishing peak absorbance related to OCN− and CO bands was observed in 2230–2100 cm−1 during the radiolysis at 40 K and 80 K. The same new IR bands appear in the range of 1600–1500, 1480–1370, and 1350–1200 cm−1 after total glycine depletion for all temperature configurations. A strong N-H deformation band growing at 1510 cm−1 was observed only at 300 K. Finally, the destruction cross-section associated to tholin decay at room temperature is estimated to be (1.30 ± 0.05) × 10−17 cm2. In addition, a correlation between the formation cross-sections for daughter and granddaughter molecules at 300 K is also obtained from the experimental data. [ABSTRACT FROM AUTHOR]

Numerical simulation of physical processes in the electron-optical system of a DUET accelerator was carried out using the ERA-DD code. The calculations were made on adaptive quasi-structured grids developed by the authors. A mathematical model for the emission plasma surface deformable when solving the problem is proposed. In this model, the problem is considered in a two-dimensional axisymmetric approximation and the front of the electron entrance to the computational domain is represented as a set of circular arcs connected by necks. In order to increase the accuracy of the calculations, it is proposed to divide the multi-scale extended domain into two subdomains and alternately solve self-consistent problems in the subdomains using the Schwarz alternating method. The beams are simulated by the method of current tubes, and the electric field potential is calculated by the finite volume method. The obtained characteristics of the beam are compared with experimental data. It is shown that for the operating parameters of the beam source, its losses on the accelerator components are minimal and can be caused primarily by the imperfect alignment of the holes in the mask and the support grid, as well as by deviations of electron beams generated by the structures located on the periphery of the emission electrode. [ABSTRACT FROM AUTHOR]

A 352.2 MHz klystron with an output power of 100 kW CW is under development at CSIR-CEERI for charged particle accelerator application. The paper presents various design results achieved through simulation of the electron gun, radio frequency (RF) interaction structure and collector for the device. Based on the design results, a beam stick tube has been developed and hot tested for which the test results are given. A study for the design of RF interaction structure of this klystron has been carried out to get the optimized design parameters using 1-D code AJDISK and MAGIC (2D/3D). The intermediate RF cavities have been developed and characterized for their designed frequencies and the results of cold RF measurements are given. [ABSTRACT FROM AUTHOR]

The Hybrid Illinois Device for Research and Applications (HIDRA) is a classical stellarator designed for conducting plasma material interaction experiments and developing novel Plasma Facing Components (PFCs). Notably, the testing of two open-channel liquid lithium PFCs is imminent. Determining the shape of the plasma and its magnetic structure inside HIDRA is essential to carry out these tests. For this, electron traces were captured to build up the images of the HIDRA magnetic flux surfaces for several magnetic configurations, following the same procedure previously employed on the WEGA stellarator coupling an electron gun with a fluorescent detector. The FIELDLINES code has then been used to generate computational surfaces matching the experimental results. The obtained surfaces were found to be subject to a similar n = 1 error field as the one observed on WEGA, suggesting that the origin of this error field is inherent to the HIDRA vacuum vessel. Also, the effect of adding a vertical field was investigated, demonstrating the ability to radially shift the magnetic axis and move to a regime free of low-order rational resonances. This additional control over the HIDRA plasma and magnetic structure allows more freedom in setting up the PFC tests in the limiter and divertor regions. [ABSTRACT FROM AUTHOR]

• We present the largest database to date for the elemental crystalline work functions. • Facet-dependent work function is inversely correlated to the number of broken bonds at a surface. • Values of work function weighted by the Wulff shape are comparable to polycrystalline values. • The average work function of metals can be modelled from atomic parameters using Gauss's law. The work function is a fundamental electronic property of a solid that varies with the facets of a crystalline surface. It is a crucial parameter in spectroscopy as well as materials design, especially for technologies such as thermionic electron guns and Schottky barriers. In this work, we present the largest database of calculated work functions for elemental crystals to date. This database contains the anisotropic work functions of more than 100 polymorphs of about 72 elements and up to a maximum Miller index of two and three for non-cubic and cubic crystals, respectively. The database has been rigorously validated against previous experimental and computational data where available. We also propose a weighted work function based on the Wulff shape that can be compared to measurements from polycrystalline specimens, and show that this weighted work function can be modeled empirically using simple atomic parameters. Furthermore, for the first time, we were able to analyze simple bond breaking rules for metallic systems beyond a maximum Miller index of one, allowing for a more generalized investigation of work function anisotropy. [ABSTRACT FROM AUTHOR]

Abstract We report on a novel way of performing stimulated electron energy-loss and energy-gain spectroscopy (sEELS/sEEGS) experiments that does not require a pulsed gun. In this scheme, a regular scanning transmission electron microscope (STEM) equipped with a conventional continuous electron gun is fitted with a modified EELS detector and a light injector in the object chamber. The modification of the EELS detector allows one to expose the EELS camera during tunable time intervals that can be synchronized with nanosecond laser pulses hitting the sample, therefore allowing us to collect only those electrons that have interacted with the sample under light irradiation. Using ∼ 5 ns laser pulses of ∼ 2 eV photon energy on various plasmonic silver samples, we obtain evidence of sEELS/sEEGS through the emergence of up to two loss and gain peaks in the spectra at ± 2 and ± 4 eV. Because this approach does not involve any modification of the gun, our method retains the original performances of the microscope in terms of energy resolution and spectral imaging with and without light injection. Compared to pulsed-gun techniques, our method is mainly limited to a perturbative regime (typically no more that one gain event per incident electron), which allows us to observe resonant effects, in particular when the plasmon energy of a silver nanostructure matches the laser photon energy. In this situation, EELS and EEGS signals are enhanced in proportion to n + 1 and n , respectively, where n is the average plasmon population due to the external illumination. The n term is associated with stimulated loss and gain processes, and the term of 1 corresponds to conventional (spontaneous) loss. The EELS part of the spectrum is therefore an incoherent superposition of spontaneous and stimulated EEL events. This is confirmed by a proper quantum-mechanical description of the electron/light/plasmon system incorporating light–plasmon and plasmon–electron interactions, as well as inelastic plasmon decay. [ABSTRACT FROM AUTHOR]

Measuring the charge on a nearby space object during close proximity, servicing, and rendezvous and docking operations without requiring physical touch remains challenging despite decades worth of data on spacecraft charging and its risks. This paper proposes the means to identify the charge on a closely neighboring space object and its elemental composition by examining the X-ray spectrum generated by energetic electrons impacting the target. In particular, deconvolution of the bremsstrahlung X-ray continuum provides an estimate of the landing energy of the electrons. Knowing the initial electron energy, the potential difference between the source and the target is determined. Additionally, characteristic X-rays emitted during the process of energetic electron–matter impact allows the relative abundance of elements in the target to be determined. Spatial separations in the order of tens of meters are required between an electron gun and the corresponding detector to maximize the collection of the bremsstrahlung spectrum. This could be achieved with either the single-craft and deployable booms or through the use of two spacecraft. Electron beam energies of 40 kV are found to generate sufficient levels of X-rays for potential determination at over 10 m from the target and to determine the landing energy of the beam to within 0.14% using commercial X-ray detectors. [ABSTRACT FROM AUTHOR]

High-voltage electron guns can commonly be damaged by surface flashover, which can occur across the insulator surface of the gun as a result of electron emission around the triple junction structure. Using secondary electron emission avalanche (SEEA) theory as a basis, the triple junction in a 90 kV electron gun is optimized in this work to avoid damage from the effects of surface discharges. A metal shielding structure is used to reduce the surface flashover caused by electron emission and ensure that the electron gun provides reliable high voltage performance. The aim of the proposed design method is to reduce the field strength that occurs within the electron gun structure. Simultaneously, it is shown that SEEA theory can provide a reasonable explanation for and a solution to the flashover phenomenon that occurs on the insulator surface. In addition, the metal shielding structure used to address the high voltage vacuum discharge phenomenon proves to be an effective design method. Experiments performed on the electron gun for a 90 kV X-ray tube demonstrate that the proposed design method has a favorable effect on control of the flashover phenomenon. [ABSTRACT FROM AUTHOR]