Vacuum Electron Sources Research Articles

Preface for the Proceedings of the 14th International Vacuum Electron Sources Conference

Preface for the Proceedings of the 14th International Vacuum Electron Sources Conference

Nanoscale heterojunctions of InGaN/GaN photocathodes for electron sources

The design of the photocathode is crucial for generating high-quality electron beam in electron sources. In this study, InGaN/GaN heterojunction nanowire photocathodes were proposed, and the photoemission theoretical model was developed. The results demonstrate that the built-in electric field along the axis enables the heterojunction nanowire photocathode to achieve higher collection efficiency across the response spectrum compared to In0.5Ga0.5N nanowire photocathodes. The variation of incidence angle results in distinct peaks in quantum efficiency and collection efficiency for the heterojunction nanowire photocathode. Meanwhile, the “additional electric field” has the potential to decrease the number of laterally emitted electrons from the nanowires, consequently reducing the shielding effect of adjacent nanowires. With an incidence angle of 15° and an “additional electric field” of 2 V/μm, the collection efficiency of photoelectrons at the collection end can be maximized to 47.6 %. This indicates that the heterojunction nanowire array photocathode has potential for use in high-performance vacuum electron sources.

Emission surface improvement of In0.5Ga0.5N tilted nanowire array photocathode for vacuum electron sources

Photocathode has received increasing attention in scientific devices that require high-quality electron beams. A theoretical photoemission model for enhancing the performance of In0.5Ga0.5N tilted nanowire array photocathodes with negative electron affinity surface is proposed. Finite Element and finite difference methods are employed in simulation experiments to study the In0.5Ga0.5N tilted nanowire array photocathode. The results demonstrate that the photoelectric conversion performance of the In0.5Ga0.5N tilted nanowire array photocathode can be significantly enhanced through the combined impact of oblique incident light and an additional electric field. Specifically, when θ = 20°, β = −40°, and Eout = 4 V/μm, the total quantum efficiency and collection efficiency are 2.79 times and 4.27 times higher than compared to vertical nanowires without external assistance, respectively.

NEA surface AlGaN heterojunction tilted nanowire array photocathode for vacuum electron sources

As the core component of the electron accelerator, the electron source is responsible for providing high-quality electron beams. GaN has a wide bandgap and must be driven by an ultraviolet laser, making it a strong candidate for photocathode electron source research. We established a theoretical model of photoemission for field-assisted negative electron affinity (NEA) AlGaN heterojunction tilted nanowire array photocathodes. The top-down built-in electric field facilitated the transport of photogenerated carriers within the nanowire. The inclination of the nanowires effectively improved the quantum efficiency of the photocathode. With the critical angle of incidence of ultraviolet, the quantum efficiency was up to 81.2%, and the collection efficiency was up to 51%. This work could provide theoretical support for the design and fabrication of high-quality photocathode electron sources.

Research on reflection-mode InxGa1-xN thin film photocathode

The ternary nitride alloy InxGa1-xN, characterized by tunable band gap, can be used in photoelectric devices to achieve a wide spectrum response of UV-infrared light. A complete photoemission theoretical model of reflective InxGa1-xN thin film vacuum photocathode is established based on the “Spicer's model” and “Andachi model”. The simulation experiment of InxGa1-xN thin film photocathode is constructed by COMSOL Multiphysics software based on the finite element method. By analyzing the optical absorption and the distribution of photogenerated carrier concentration of photocathode, it is found that the photoconversion performance of photocathode can be enhanced by optimizing the thickness of emission layer and the angle of polarized light. The peak quantum efficiency and peak spectral responsivity of In0.5Ga0.5N photocathode with a thickness of 400 nm are 63.13% and 240.83 mA/W, respectively. The quantum efficiency of In0.5Ga0.5N photocathode with an incidence angle of 50° of TM mode polarized light can be increased to 65.99% compared with that with normal incidence. Therefore, the simulation results of this paper can provide reference for the design of wide spectrum photodetectors and vacuum electron sources.

Progress of terahertz vacuum electronic radiation sources

Progress of terahertz vacuum electronic radiation sources

Preparation, superior field emission properties and first principles calculation of electronic structure of SiC nanowire arrays on Si substrate

It is a crucial item to develop an available, efficient and stable cathode material for the application of flat-panel display, vacuum micro-electronic device, electron source and related area/equipment. In this work, oriented single-crystalline SiC nanowires (SiCNW) arrays were successfully fabricated onto the Si (100) substrate according to an easy Ni-catalyst-assisted chemical vapor deposition (CVD) process. The systematical characterization results suggested that product growth was modulated via the vapor-liquid-solid (VLS) mechanism. As the promising member in the candidate cathodes, SiCNW arrays with superior current emission stability (~ ± 6.1%) displayed decreased turn-on field (1.26 V/μm) along with threshold field (1.83 V/μm) at the optimal spacing between anode and cathode (about 500 μm), suggesting their great application potential as field emitters in the future. The positive effect of the stacking faults on the electronic structure and field emission (FE) properties were investigated through first principles calculation using Vienna ab initio Simulation package (VASP) of density functional theory (DFT). Furthermore, a reasonable synergetic electronic transmission mechanism based on peculiar morphology and electronic band structure was proposed to explain the superior field emission performances.

Ab Initio Quantum-Mechanical Predictions of Semiconducting Photocathode Materials.

Ab initio Quantum-Mechanical methods are well-established tools for material characterization and discovery in many technological areas. Recently, state-of-the-art approaches based on density-functional theory and many-body perturbation theory were successfully applied to semiconducting alkali antimonides and tellurides, which are currently employed as photocathodes in particle accelerator facilities. The results of these studies have unveiled the potential of ab initio methods to complement experimental and technical efforts for the development of new, more efficient materials for vacuum electron sources. Concomitantly, these findings have revealed the need for theory to go beyond the status quo in order to face the challenges of modeling such complex systems and their properties in operando conditions. In this review, we summarize recent progress in the application of ab initio many-body methods to investigate photocathode materials, analyzing the merits and the limitations of the standard approaches with respect to the confronted scientific questions. In particular, we emphasize the necessary trade-off between computational accuracy and feasibility that is intrinsic to these studies, and propose possible routes to optimize it. We finally discuss novel schemes for computationally-aided material discovery that are suitable for the development of ultra-bright electron sources toward the incoming era of artificial intelligence.

Electronic structure and optical properties of Na2KSb and NaK2Sb from first-principles many-body theory

In the search for novel materials for vacuum electron sources, multi-alkali antimonides and in particular sodium-potassium-antimonides have been recently regarded as especially promising due to their favorable electronic and optical properties. In the framework of density-functional theory and many-body perturbation theory, we investigate the electronic structure and the dielectric response of two representative members of this family, namely Na2KSb and NaK2Sb. We find that both materials have a direct gap, which is on the order of 1.5 eV in Na2KSb and 1.0 eV in NaK2Sb. In either system, valence and conduction bands are dominated by Sb states with p- and s-character, respectively. The imaginary part of the dielectric function, computed upon explicit inclusion of electron–hole interactions to characterize the optical response of the materials, exhibits maxima starting from the near-infrared region, extending up to the visible and the ultraviolet band. With our analysis, we clarify that the lowest-energy excitations are non-excitonic in nature and that their binding energy is on the order of 100 meV. Our results confirm the potential of Na2KSb and NaK2Sb as photoemissive materials for vacuum electron sources, photomultipliers, and imaging devices.

Electronic structure of cesium-based photocathode materials from density functional theory: performance of PBE, SCAN, and HSE06 functionals

The development of novel materials for vacuum electron sources in particle accelerators is an active field of research that can greatly benefit from the results of ab initio calculations for the characterization of the electronic structure of target systems. As state-of-the-art many-body perturbation theory calculations are too expensive for large-scale material screening, density functional theory offers the best compromise between accuracy and computational feasibility. The quality of the obtained results, however, crucially depends on the choice of the exchange–correlation potential, v xc. To address this essential point, we systematically analyze the performance of three popular approximations of v xc [PBE, strongly constrained and appropriately normed (SCAN), and HSE06] on the structural and electronic properties of bulk Cs3Sb and Cs2Te as representative materials of Cs-based semiconductors employed in photocathode applications. Among the adopted approximations, PBE shows expectedly the largest discrepancies from the target: the unit cell volume is overestimated compared to the experimental value, while the band gap is severely underestimated. On the other hand, both SCAN and HSE06 perform remarkably well in reproducing both structural and electronic properties. Spin–orbit coupling, which mainly impacts the valence region of both materials inducing a band splitting and, consequently, a band-gap reduction of the order of 0.2 eV, is equally captured by all functionals. Our results indicate SCAN as the best trade-off between accuracy and computational costs, outperforming the considerably more expensive HSE06.

High-efficient electron collection capability of graded Al compositional GaN nanowire arrays cathode

Based on the purpose of solving the “secondary absorption” of adjacent nanowires and the lateral emission in the GaN nanowire arrays (NWAs) cathode, an exponential-doping and graded Al compositional GaN NWAs photocathode is proposed, which could generate internal electric field to increase the quantum efficiency (QE) of top surface, and the introduction of an external electric field promote the side-emission electrons to shift toward the collecting side. The QE and collection efficiency (CE) of exponential-doping and graded compositional GaN NWAs under different array structure parameters, incident angles and external electric field intensities are analyzed. The results show that although the collection ratio of emitted electrons in the exponential-doping GaN NWAs is higher, the graded Al compositional photocathode with a stronger built-in electric field can obtain better CE under the application of an external electric field, and the peak value can reach 33.2% in a specific structure. External electric field has a more significant effect on the CE of uniform-doping GaN NWAs. The solutions provided in this study can make the GaN NWAs photocathode more suitable for the strict requirements of vacuum electron sources

Effects of frequency-dependent surface impedance on the vacuum electronic terahertz sources

When the working frequency of vacuum electronic device reaches the terahertz frequency, the ohmic loss has a significant impact on the vacuum electronic device. To study the effect of the ohmic loss on the working characteristic of the vacuum electronic terahertz devices, this paper implements the frequency-dependent surface impedance boundary condition (SIBC) in the 3 dimensional particle in cell code UNIPIC-3D. Conformal mesh is adopted in the code to overcome the staircase error in traditional particle in cell method. By using the surface impedance boundary, we eliminate the need to study the field inside the lossy dielectric objects which require extremely small grid cells for numerical stability. In comparison with constant parameter SIBC, the dispersive SIBC is applicable over a very large frequency bandwidth and over a large range of conductivities. The correctness of the implementation is verified by simulating the lossy resonant cavity and circular waveguide, the simulated power loss is comparable with the theoretical predication. High power vacuum electronic devices of terahertz regime are attracting extensive interests due to their potential applications in science and technologies. The impulse-wave relativistic surface wave oscillator (SWO) and low-voltage continuous-wave planar grating backward wave oscillator (BWO) both made of copper are numerically studied by using UNIPIC-3D and dispersive surface impedance boundary condition. Numerical results show that the strongest field is very close to the slow wave structure where the beam-wave interaction occurs and that terahertz wave generates both in these two devices. The distributed wall loss has a considerable effect on the devices: the output power has a significant decrease and the startup time becomes longer, but the working frequencies of the two devices keep unchanged. To improve the efficiency of relativistic SWO, a resonant reflector is proposed between the diode and the slow wave structure. Numerical results show that the working frequency of the device with a resonant reflector keeps unchanged as the original one, but the output power increases to 60 MW from 41 MW of the original one when the ohmic loss is considered.

Effective carbon nanotubes/graphene hybrid films for electron field emission application

In this study, carbon nanotubes (CNTs) are combined with graphene to form effective CNTs/graphene hybrid films for electron field emission by simple and facile electrophoretic deposition approach. The hybrid films exhibit lower turn-on and threshold field as well as larger field enhancement factor compared to those of pure CNTs and pure graphene films. A pulse high voltage is applied to achieve a high emission current from the hybrid films. A fairly high emission current of 80 mA (160 mA/cm2) is obtained, which is greatly larger than the maximum current achieved under continuous high voltage. The pulsewidth affects the field emission properties of the hybrid films due to the current heating effect and should be strictly limited. The hybrid films also show good long-term stability under pulse high voltage. Furthermore, a Preliminary study is presented using the CNTs/graphene hybrid films as cold cathode of a X-ray source. These results demonstrate that CNTs/graphene hybrid films can be a potential candidate for vacuum electron source.

Criteria for Determining Maximum Theoretical Oscillating Frequency of Extended Interaction Oscillators for Terahertz Applications

Extended interaction oscillators (EIOs) are high-frequency vacuum-electronic sources, capable to generate millimeter-wave to terahertz (THz) radiations. They are considered to be potential sources of high-power submillimeter wavelengths. Different slow-wave structures and beam geometries are used for EIOs. This paper presents a quantitative figure of merit, the critical unloaded oscillating frequency ( ${f}_{\textsf {cr}} $ ) for any specific geometry of EIO. This figure is calculated and tested for $2\pi $ standing-wave modes (a common mode for EIOs) of two different slow-wave structures (SWSs), one double-ridge SWS driven by a sheet electron beam and one ring-loaded waveguide driven by a cylindrical beam. The calculated ${f}_{\textsf {cr}}\text{s}$ are compared with particle-in-cell (PIC) results, showing an acceptable agreement. The derived ${f}_{\textsf {cr}}$ is calculated three to four orders of magnitude faster than the PIC solver. Generality of the method, its clear physical interpretation and computational rapidity, makes it a convenient approach to evaluate the high-frequency behavior of any specified EIO geometry. This allows to investigate the changes in geometry to attain higher frequencies at THz spectrum.

Developing a Load-Line Concept to Study the Extended Interaction Oscillators

Extended interaction oscillators (EIOs) are millimeter-wave to terahertz vacuum electronic sources, based on the Smith-Purcell (Cerenkov) interaction between an electron beam of low to moderate voltage and a resonant slow-wave structure. They have been analyzed using the power-balance and Particle in cell (PIC) methods. In this paper, based on power-balance method, a load-line concept is introduced to investigate the EIOs. By this approach, the analysis of EIOs is much faster than the PIC method, and a wider quantitative and qualitative insight into the interaction process is obtained compared with the common power-balance method. To achieve the optimum RF power from an EIO, a region on the defined “beam-line” curve is assigned. A “frequency-shift line” curve is also derived to obtain the electronic tuning of the oscillator. In addition, the correct steady state in the multivalued solution conditions can be selected, which is not evident in the power-balance approach. A 94-GHz sheet-beam EIO is analyzed and optimized in this way, and the results are compared with those of a PIC solver.

Efficient power combiner for THz radiation

Most dangerous explosive materials, both toxic and radioactive, contain nitrogen salts with resonant absorption lines in the frequency range 0.3-10 THz. Therefore, there has been growing interest in remotely detecting such materials by observing the spectrum of reflected signals when the suspicious material is interrogated by THz radiation. Practical portable THz sources available today generate only 20–40 mW output power. This power level is too low to interrogate suspicious material from a safe distance, especially if the material is concealed. Hence, there is a need for sources that can provide greater power in the THz spectrum. Generating and extracting high output power from THz sources is complicated and inefficient. The efficiency of vacuum electronic microwave sources is very low when scaled to the THz range and THz sources based on scaling down semiconductor laser sources have low efficiency as well, resulting in the well known “THz gap.” The reason for such low efficiencies for both source types is material losses in the THz band. In this article an efficient power combiner is described that is based on scaling to higher frequencies a microwave combiner that increases the output power in the THz range of interest in simulation studies. The proposed power combiner not only combines the THz power output from several sources, but can also form a Gaussian wavebeam output. A minimum conversion efficiency of 89% with cophased inputs in a lossy copper power combiner and maximum efficiency of 100% in a Perfect Electric Conductor (PEC)-made power combiner were achieved in simulations. Also, it is shown that the TE01 output mode is a reasonable option for THz applications due to the fact that conductive loss decreases for this mode as frequency increases.

Facile synthesis and enhanced field emission properties of Cu nanoparticles decorated graphene-based emitters

Cu nanoparticles (Cu NPs) were decorated on the surface of graphene films to form composite emitters. The graphene films were deposited by electrophoretic deposition and subsequently coated with Cu NPs via electrochemical reduction. The Cu NPs decorated graphene films exhibited lower turn-on field, lower threshold field and larger field enhancement factor compared with those of the pristine films. The Cu NPs decorated graphene films also showed good emission stability. The field emission properties of Cu NPs decorated graphene films were tunable through adjusting decoration time. This graphene-based composite could be used as a possible candidate for vacuum electron source.

Controlled electron emission and vacuum breakdown with nanosecond pulses

Vacuum electron sources exploiting field emission are generally operated in direct current (DC) mode. The development of nanosecond and sub-nanosecond pulsed power supplies facilitates the emission of compact bunches of electrons of high density. The breakdown level is taken as the highest value of the voltage avoiding the thermo-emission instability. The effect of such ultra-fast pulses on the breakdown voltage and the emitted electron current is discussed as a result of the thermo-emission modelling applied to a significant protrusion. It is found that pulsing very rapidly the vacuum breakdown occurs at higher voltage values than for the DC case, because it rises faster than the heat diffusion. In addition, the electron emission current increases significantly regardless of the theoretical approach is used. A comparative study of this theoretical work is discussed for several different forms of the protrusion (elliptic and hyperbolic) and different metals (hence varying the melting point), particularly refractory (tungsten) versus conductor (titanium). Pulsed mode operation can provide an increase on breakdown voltage (up to 18%) and a significant increase (up to 330%) of the electron extracted current due to its high non-linear dependency with the voltage, for the case for the case with a hyperbolic protrusion.

Sub-band gap photo-enhanced secondary electron emission from high-purity single-crystal chemical-vapor-deposited diamond

Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distribution as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ∼0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum electron sources, particle detectors, and other electronic devices.

Analysis on dispersion characteristics of rectangular metal grating based on spoof surface plasmons

The unique optical and physical properties of surface plasmon polaritons (SPP) has brought about a series of novel phenomena such as SPP-enhanced transmission, local resonance, etc., and SPP has become a research hotspot around the world. In this paper, the dispersion characteristics and modes of rectangular metal grating based on spoof surface plasmons (SSP) are studied theoretically and numerically. The electromagnetic fields of SSP which are below and above the grating surface are presented using eigenmode expansion method and under periodic boundary conditions, besides the fact that the SSP dispersion relations are obtained by matching the boundary conditions of electromagnetic fields both for rectangular metal grating with roofed metal plate and that without roofed metal plate. Results for these two different cases are given according to numerical calculation and it is found that the roofed metal plate can introduce an additional fast wave mode which is beyond the light line in the dispersion diagram. And the results of analytical SSP dispersion are verified by electromagnetic simulations based on the finite difference method and finite integration method. The dependence of the dispersion characteristics and mode distributions on various parameters of metal grating is studied theoretically. It is shown that the dispersion relations obtained by eigenmode expansion method agree well with the results of electromagnetic simulations. The phase velocity of SSP on the grating surface can be decreased by increasing metal grating depth or decreasing grating period. The bandwidth of electron beam-SSP interaction can be extended by increasing grating period ratio. The influence of the distance between the roofed metal plate and the grating surface on the SSP dispersion is studied and is found that the role of roofed metal plate is insensitive to the slow wave SSP mode. The SSP dispersion and modes for the 3-D metal grating which are extended from the above 2-D SSP dispersion are also given. The SSP symmetric modes and anti-symmetric modes manifest themself alternately in the dispersion diagram on the 3-D grating surface. Compared with the 2-D SSP bound mode without roofed metal plate, it is found that in the 3-D grating structure the slow wave SSP modes and fast wave SSP modes coexist. The 3-D SSP mode with various grating lateral width is studied, and the competition and degeneracy of modes are analyzed particularly. The SSP mode intervals can be enlarged by decreasing the lateral width of the grating, which is optimum for avoiding mode competitions. Studies on dispersion and modes of the 2-D and 3-D metal grating structures based on SSP will lay the foundations for further studies of electron beam-SSP interaction, and development of the novel terahertz vacuum electronic source with high-efficiency and wide-bandwidth.