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Nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] pyramid tip cathode has been considered as a next-generation high peak current electron source for dielectric laser accelerators as well as other high peak current particle accelerator applications. In this work, we study non-linear photoemission from an (N)UNCD pyramid tip cathode using an ultrafast laser with the pulse length of 150 fs with the central wavelength of 800 nm in the peak intensity range of 10 9 – 10 10 W / cm 2. We demonstrated that as the incident laser intensity increases, the current emitted from the nano-tip first increases as a power function with an exponent of about 5 and then starts to roll over to an exponent of 3. This roll over is attributed to the Coulomb interaction between electrons emitted from the tip also known as the space charge. We also measured the photoemission electron energy spectra that show electrons with energies as high as ∼ 10 eV. Based on the shape of the electron energy spectra, we conclude that the high-energy electrons are thermally emitted electrons due to ultrafast laser heating at the tip of the (N)UNCD pyramid tip cathode. [ABSTRACT FROM AUTHOR]

The quantum efficiency and mean transverse energy of electrons emitted from a cathode determine the quality of beams generated from photoinjectors. The nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, is a new class of robust semiconductor photocathodes, which has been considered in photoinjectors for high peak current extraction. In this work, we measure the spectral response in quantum efficiency, photoemission energy spectra, and mean transverse energy of the (N)UNCD photocathode using a photoemission electron microscope. The observed quantum efficiency was comparable to that of copper photocathodes. Photoemission spectra showed the evidence of scattering of electrons before emission. This relaxation of electrons due to scattering is also observed in the spectral response of the mean transverse energy. The mean transverse energy is limited to ∼ 70 meV at the threshold. We attribute this to the physical and chemical roughness of the (N)UNCD photocathode and, hence, smoother films will be required to further reduce the mean transverse energy obtained from the (N)UNCD photocathode. [ABSTRACT FROM AUTHOR]

In this paper, we study the effect of the actual, locally resolved, field emission area on electron emission characteristics of uniform planar conductive nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) field emitters. High resolution imaging experiments were carried out in a field emission microscope with a specialty imaging anode screen such that electron emission micrographs were taken concurrently with measurements of I-V characteristics. An automated image processing algorithm was applied to process the extensive imaging data sets and calculate the emission area per image. It was routinely found that field emission from as-grown planar (N)UNCD films was always confined to a counted number of discrete emitting centers across the surface, which varied in size and electron emissivity. It was established that the actual field emission area critically depends on the applied electric field and that the field emission area and overall electron emissivity improve with the sp 2 -fraction present in the film, irrespective of the original substrate roughness or morphology. Most importantly, when as-measured I-E characteristics were normalized by the electric field-dependent emission area, the resulting j-E curves demonstrated a strong kink and departed from the Fowler-Nordheim law, finally saturating at a value on the order of 100 mA/cm 2 . This value was nearly identical for all studied films regardless of substrate. It was concluded that the saturation value is specific to the intrinsic fundamental properties of (N)UNCD.

Diamond has an ultrawide bandgap with excellent physical properties, such as high critical electric field, excellent thermal conductivity, high carrier mobility, etc. Diamond with a hydrogen-terminated (H-terminated) surface has a negative electron affinity (NEA) and can easily produce surface electrons from valence or trapped electrons via optical absorption, thermal heating energy or carrier transport in a PN junction. The NEA of the H-terminated surface enables surface electrons to emit with high efficiency into the vacuum without encountering additional barriers and promotes further development and application of diamond-based emitting devices. This article reviews the electron emission properties of H-terminated diamond surfaces exhibiting NEA characteristics. The electron emission is induced by different physical mechanisms. Recent advancements in electron-emitting devices based on diamond are also summarized. Finally, the current challenges and future development opportunities are discussed to further develop the relevant applications of diamond-based electron-emitting devices. [ABSTRACT FROM AUTHOR]

The recently demonstrated concept of the plasma photocathode, whereby a high-brightness bunch is initialized by laser ionization within a plasma wakefield acceleration bubble, is informally referred to as Trojan Horse wakefield acceleration. In a similar vein, the dielectric Trojan Horse concept incorporates a dielectric-lined waveguide to support a charged particle beam-driven accelerating mode and uses laser initiated ionization of neutral gas within the waveguide to generate a witness beam. One of the advantages of the dielectric Trojan Horse concept is the reduced requirements in terms of timing precision due to operation at a lower frequency. In this paper, we present experimental results on the generation and characterization of a four-bunch drive train for resonant excitation of wakefields in a cylindrical dielectric waveguide conducted at the Argonne Wakefield Accelerator facility. The results lay the foundation for the demonstration of a plasma photocathode scheme within a dielectric wakefield accelerating structure. Modifications to improve capture efficiency with improved beam transmission are suggested as well. [ABSTRACT FROM AUTHOR]

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Multiple electron emission mechanisms often contribute in electron devices, motivating theoretical studies characterizing the transitions between them. Previous studies unified thermionic and field emission, defined by the Richardson-Laue-Dushman (RLD) and Fowler–Nordheim (FN) equations, respectively, with the Child-Langmuir (CL) law for vacuum space-charge limited current (SCLC); another study unified FN and CL with the Mott-Gurney (MG) law for collisional SCLC. However, thermionic emission, which introduces a nonzero injection velocity, may also occur in gas, motivating this analysis to unify RLD, FN, CL, and MG. We exactly calculate the current density as a function of applied voltage over a range of injection velocity (i.e., temperature), mobility, and gap distance. This exact solution approaches RLD, FN, and generalized CL (GCL) and MG (GMG) for nonzero injection velocity under appropriate limits. For nonzero initial velocity, GMG approaches zero for sufficiently small applied voltage and mobility, making these gaps always space-charge limited by either GMG at low voltage or GCL at high voltage. The third-order nexus between FN, GMG, and GCL changes negligibly from the zero initial velocity calculation over ten orders of magnitude of applied voltage. These results provide a closed form solution for GMG and guidance on thermionic emission in a collisional gap. [ABSTRACT FROM AUTHOR]

The interest in the field electron emission cathode nanomaterials is on the rise due to the wide applications, such as electron sources, miniature X-ray devices, display materials, etc. In particular, nanodiamond (ND) film is regarded as an ideal next-generation cathode emitter in the field emission devices, due to the low or negative electron affinity, small grain size, high mechanical hardness, low work function, and high reliability. Increasing efforts are conducted on the investigation of the emission structures, manufacturing cost, and field emission properties improvement of the ND films. This review aims to summarize the recent research, highlight the new findings, and provide a roadmap for future developments in the area of ND film electron field emitter. Specially, the optimizing methods of large-scale, high-quality, and cost-effective synthesis of ND films are discussed to achieve more stable surface structure and optimal physical properties. Additionally, the mainstream strategies applied to produce high field emission performance of ND films are analyzed in detail, including regulating the grain size/boundary, hybrid phase carbon content, and doping element/type of ND films; meanwhile, the problems existing in the related research and the outlook in this area are also discussed. [ABSTRACT FROM AUTHOR]

Fiber Optic Network is an advanced and modern system technology, which is used in sending pulses of laser light inside a glass of fiber over long distances, widely used in every environment with various sorts of applications in a different field. It is well-known that the main material of fiber optics is glass, therefore it is typical that the temperature can affect the glass during the thermal expansion. This effect will be applied to the properties of the optical components such as refractive index, radius curvature of the fiber optics layers, and also there is an effect on the data transfer through the fiber optics network units. In this paper, the effect of temperature degree on the optical signal and the functions of the fiber optic network will be simulated, measured, and analyzed. The result will be discussed and the conclusion will show the serious points of thermal effects on the optical signal of a fiber-optic network. [ABSTRACT FROM AUTHOR]

We present a theoretical model describing photoelectron transport dynamics in stepwise-doped GaAs photocathodes. Built-in electric field caused by the doping structure is analyzed, and the time-evolution of electron concentration in the active layer induced by a femtosecond laser pulse is solved. The predictions of the model show excellent agreement with the experimental data measured with pump-probe transient reflectometry, demonstrating the capability of the theoretical model in predicting photoelectron behaviors in real devices. Comparisons are also made between this stepwise doping model and the conventional gradient doping model with a continuous doping profile, thereby providing the first quantitative evaluation of the effectiveness and the limitation of the gradient doping model in describing actual stepwise-doped devices. [ABSTRACT FROM AUTHOR]

Accelerator scientists have high demands on photocathodes possessing high quantum efficiency (QE) and long operational lifetime. p-GaN, as a new photocathode type, has recently gained more and more interest because of its ability to form a negative electron affinity (NEA) surface. Being activated with a thin layer of cesium, p-GaN:Cs photocathodes promise higher QE and better stability than the known photocathodes. In our study, p-GaN samples grown on sapphire or silicon were wet chemically cleaned and transferred into an ultra-high vacuum (UHV) chamber, where they underwent a subsequent thermal cleaning. The cleaned p-GaN samples were activated with cesium to obtain p-GaN:Cs photocathodes, and their performance was monitored with respect to their quality, especially their QE and storage lifetime. The surface topography and morphology were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) spectroscopy. We have shown that p-GaN could be efficiently reactivated with cesium several times. This paper systematically compares the influence of wet chemical cleaning as well as thermal cleaning at various temperatures on the QE, storage lifetime and surface morphology of p-GaN. As expected, the cleaning strongly influences the cathodes' quality. We show that high QE and long storage lifetime are achievable at lower cleaning temperatures in our UHV chamber. [ABSTRACT FROM AUTHOR]

Diamond films with different grain sizes in the range of ~ 9 nm to ~ 50 μm have been deposited on silicon substrates using a homemade microwave plasma chemical vapor deposition reactor by varying the deposition parameters. The surface morphologies have been examined by scanning electron microscope and atomic force microscope, which show the secondary nucleation intensity and surface defects of the diamond films increase with the decrease of the diamond grain size. Although X-ray diffraction spectra show the absence of graphitic carbon features, the Raman and X-ray photoelectron spectroscopy show the sp2/sp3-bonded carbon ratios increase with the decrease of the diamond grains. The CH4 percentage in plasma during deposition plays a crucial role in the formation of diamond films with different grain sizes and sp2 contents, which in turn determines the electron field emission behavior of the corresponding diamond films. The smaller the grain size of the diamond, the higher is the grain boundary density, which can provide more electron emission sites and form conductive networks for electron transport. The ultra-nanocrystalline diamond film shows needle-like cluster structures and optimum electrical performance. The corresponding electron field emission behavior can be turned on at a field of 6.71 V/μm and attain a current density of 16.28 μA/cm2 at an applied field of 11.31 V/μm. [ABSTRACT FROM AUTHOR]

The multipurpose detector complex of the Tien Shan mountain station of LPI provides the means to carry out studies in the various fields of experimental geophysics. The detectors of cosmic ray particles present at the station can be used for continuous monitoring of different types of background radiation: of the current flux of (1–100) GeV cosmic ray hadrons, of thermal neutrons in the surrounding environment, and of the gamma-rays with energy (30–3000) keV. Special system of the high altitude detectors permits to register the electrons accelerated up to several tens and hundreds of MeV by the atmospheric electric fields in thunderclouds, as well as the gamma radiation generated by these particles. A combination of diverse radiation receivers can be used for simultaneous detection of the time profiles of lightning emission in the different wave ranges of electromagnetic spectrum, starting from near ultraviolet and up to (1–30) MHz radio-waves, as well as of the electromagnetic pulses propagating over the global earth-ionosphere waveguide in the VLF, (1–10) kHz, and SLF, (1–10) Hz, frequency ranges. An underground complex of the neutron, gamma-ray, and acoustic detectors placed in a borehole at an up to 300-m depth below the surface of the ground is used for detection of the various signals of geophysical origin, including those which could be triggered by interaction of penetrative cosmic ray particles with the matter of the earth's crust. Perspectives of geophysical experiments at the Tien Shan station are discussed here together with application opportunity of their results to various problems of the space physics, of the physics of atmospheric discharge and lightning development, of environmental monitoring, and of the monitoring of seismic processes for the purpose of timely earthquake forecast. [ABSTRACT FROM AUTHOR]

New multipurpose experimental complex of the Tien Shan mountain cosmic ray station of LPI incorporates a set of detector subsystems for simultaneous investigation of the various components of extensive air showers (EAS) which arise from interaction of the (1–100) PeV cosmic ray particle in the atmosphere. A wide-spread system of the charged particles detectors is used for the measurement of the local density of EAS electrons, and for estimation of the main EAS parameters by spatial distribution of this density. The radio-antennas and Cherenkov detectors provide an alternative way for investigation of the EAS electron component. The ionization-neutron calorimeter and the neutron monitor give information on EAS hadrons with the energy above 0.1 TeV, while the low-threshold neutron and gamma detectors can be applied for registration of the prolonged flux of thermalized neutrons and soft gamma rays after EAS passage. The underground muon detector is used for investigation of the muonic component of cosmic rays in an exclusively wide range of muon energies, starting from 5 GeV, and up to tens and thousands of TeV, and with estimation possibility of the energy of muon. The multi-component technique practiced now at the Tien Shan station permits to study effectively those aspects of high energy cosmic ray interaction which have been never considered in previous experiments. In particular, it is possible now to adequately register the flow of various EAS components in the very region of EAS core which opens a real opportunity to solve the long-standing problem of the 3-PeV knee in the energy spectrum of cosmic rays. Some examples of the physical results gained lately at the Tien Shan detector complex are presented here for performance illustration of the newly elaborated methods of EAS investigation. [ABSTRACT FROM AUTHOR]

Purposeful investigation of radiation fluxes strongly delayed in relation to the main particle front of extensive air shower (EAS) was undertaken at the Tien Shan Mountain Cosmic Ray Station. It was found that the passage of the EAS can be accompanied by the delayed thermal neutrons and by the soft (30–50) keV gamma rays, mostly concentrated within a region of about (5–10) m around the shower axis, where the integral radiation fluence can vary in the limits of (10 - 4 - 1) cm - 2 for neutrons, and of (0.1 - 1000) cm - 2 for gamma rays. The dependence of signal multiplicity on the shower size N e has a power shape both for the neutron and gamma ray components, with a sharp increase of its power index around the value of N e ≈ 10 6 , which corresponds to the position of the 3 × 10 15 eV knee in the primary cosmic ray spectrum. The total duration of detectable radiation signal after the EAS passage can be of some tens of milliseconds in the case of neutron component, and up to a few whole seconds for gamma rays. The delayed accompaniment of low-energy radiation particles can be an effective probe to study the interaction of the hadronic component of EAS. [ABSTRACT FROM AUTHOR]

The emission area is one of the main parameters of field emission systems. A variety of formal determination techniques for the emission area is studied. Using the model hemisphere-on-cylindrical-post constructed with the COMSOL Multiphysics software package, the relation between well-known estimates of the emission area is shown. The corresponding fractions of the emission currents are computed. It is demonstrated that the formal emission area (the ratio of the total current to the current density at the vertex) ensures approximately 75% of the emission current. The effect of abnormal behavior on the effective emission area is obtained (cutoff from the trend line of the current–voltage characteristics) for varying voltage level in standard Fowler–Nordheim coordinates. An experimental assessment of the area with application of modified Fowler–Nordheim coordinates ln(I/U2–η/6) vs 1/U is proposed. The method is applied for analysis of field emission data of the multipoint nanocomposite emitter with carbon nanotubes. [ABSTRACT FROM AUTHOR]

Spectra of the most energetic hadrons in cores of extensive air showers (EAS) were obtained for the first time in the HADRON hybrid experiment in Tien Shan mountains. The spectra in question exhibit a scaling violation, which requires invoking nontrivial effects in order to explain this circumstance. In the present article, unusual data on pp-interaction cross sections from recent experiments at the Large Hadron Collider are analyzed with this aim in view. A comparison with data at lower energies shows that the elastic cross section for pp interaction grows faster than the respective inelastic cross section. It would be natural to expect that a similar or a stronger effect is present in nucleus-nucleus interactions in cosmic rays. A possible change in the properties of nuclear cascades in the Earth’s atmosphere provides the possibility of discussing a new interpretation of the knee in the EAS spectrum and some exotic phenomena observed in cosmic-ray experiments. This interpretation is proposed for the first time. Particular attention is given to unique data from the HADRON hybrid experiment. [ABSTRACT FROM AUTHOR]

Low temperature (350 °C) grown conductive nanocrystalline diamond (NCD) films were realized by lithium diffusion from Cr-coated lithium niobate substrates (Cr/LNO). The NCD/Cr/LNO films showed a low resistivity of 0.01 WΩcm and excellent field electron emission characteristics, viz. a low turn-on field of 2.3 V/μm, a high-current density of 11.0 mA/cm2 (at 4.9 V/m), a large field enhancement factor of 1670, and a life-time stability of 445 min (at 3.0 mA/cm2). The low temperature deposition process combined with the excellent electrical characteristics offers a new prospective for applications based on temperature sensitive materials. [ABSTRACT FROM AUTHOR]

The phenomenon of gamma-ray families featuring halos that is observed in an experiment with x-ray emulsion chambers (XREC) in the Pamir experiment and in other XREC experiments is explained. The experimental properties of halos are analyzed via a comparison with the results of their simulation. It is shown that gamma-ray families featuring halos are predominantly produced (more than 96% of them) by protons and heliumnuclei. This makes it possible to employ the experimental properties of halos to estimate the fraction of protons and helium nuclei in the mass composition of primary cosmic radiation. [ABSTRACT FROM AUTHOR]

We demonstrate that photoemission properties of p-type GaAs can be altered by surface acoustic waves (SAWs) generated on the GaAs surface due to dynamical piezoelectric fields of SAWs. Multiphysics simulations indicate that charge-carrier recombination is greatly reduced, and electron effective lifetime in p-doped GaAs may increase by a factor of 10× to 20×. It implies a significant increase, by a factor of 2× to 3×, of quantum efficiency (QE) for GaAs photoemission applications, like GaAs photocathodes. Conditions of different SAW wavelengths, swept SAW intensities, and varied incident photon energies were investigated. Essential steps in SAW device fabrication on a GaAs substrate are demonstrated, including deposition of an additional layer of ZnO for piezoelectric effect enhancement, measurements of current–voltage (I–V) characteristics of the SAW device, and ability to survive high-temperature annealing. Results obtained and reported in this study provide the potential and basis for future studies on building SAW-enhanced photocathodes, as well as other GaAs photoelectric applications. [ABSTRACT FROM AUTHOR]

The present paper extends the prior findings on self-induced heating of solid state field emission devices. It was found that a vacuum diode (base pressure  ∼10−9 Torr), that makes use of graphite-rich polycrystalline diamond as cathode material, can switch from a diode regime to a resistor regime to a glow discharge plasma regime without any external perturbation, i.e. all transitions are self-induced. Combined results of in situ field emission microscopy, ex situ electron microscopy and Raman spectroscopy suggest that it is the nanodiamond cathode of the diode heated to about 3000 K which causes self-induced material evaporation, ionization and eventually micro-plasma formation. Our results confirm that field emission, commonly called cold emission, is a very complex phenomenon that can cause severe thermal load. Thermal load and material runaway could be the major factors causing vacuum diode deterioration, i.e. progressive increase in turn-on field, decrease in field enhancement factor, and eventual failure. [ABSTRACT FROM AUTHOR]