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In EXL-50 plasma currents over 100 kA are non-inductively generated and maintained solely by electron cyclotron heating (ECH) power with an efficiency of ∼1 A W−1. These currents are carried by energetic electrons (EEs) in the energy range from several tens of keV up to several hundreds of keV which also account for almost all pressure in plasma. This EE component can be viewed as a large number collection of various periodic orbits of energetic particles. Based on this picture we have developed a method for particle orbit description of the EE component in a typical plasma at I P = 121 kA as analysis target. We use a fluid description as a bridge to describe successfully the EE component as a collection of various passing and trapped orbits in the approximation of monochromatic particle energy with good matching to the flux loop signals. The description has revealed characteristics of passing and trapped particles. Passing particles carry almost all toroidal current, while they account for only 20 % of total particle number of the EE component. While net current carried by trapped particles is a very small fraction, they account for a major fraction in number and carry a large positive current outside the last closed flux surface (LCFS) and a large negative current inside. As a result, trapped particles redistribute the current from inside of the LCFS to outside both radially and vertically, generating a large vertically elongated cross section in current as well as number density profiles. There is a ridge-like structure along the LCFS in the current density profile, with no such structure in the number density profile. The results suggest that forward passing particles are more advantageous in confinement than backward passing particles. This advantage increases with particle energy and contributes to the current generation observed in EXL-50 experiments.

To study controlled evolution of nonautonomous matter-wave breathers and rogue waves in spinor Bose–Einstein condensates with spatiotemporal modulation, we focus on a system of three coupled Gross–Pitaevskii equations with spacetime-dependent external potentials and temporally modulated gain-loss distributions. With different external potentials and gain-loss distributions, various solutions for controlled nonautonomous matter-wave breathers and rogue waves are derived by the Darboux transformation method, such as breathers and rogue waves on arched and constant backgrounds which have the periodic and parabolic trajectories. Effects of the gain-loss distribution and linear potential on the breathers and rogue waves are studied. Nonautonomous two-breathers on the arched and constant backgrounds are also derived.

The fully non-inductive spherical tokamak EXL-50, built and operated by the ENN private limited company, has routinely achieved high current drive efficiency of ∼ 1 A/W in only ECRH powered experiments. We have numerically investigated the effectiveness of multiple electron cyclotron resonance (ECR) harmonics in generating such a high efficiency of electron cyclotron current drive (ECCD) in non-inductive plasma start-up. The Fokker-Planck equation is numerically solved to obtain the electron distribution function, under the steady state of relativistic nonlinear Coulomb collision and quasi-linear diffusion operators, for calculating plasma current driven by the injected EC waves. Multi-pass absorption simulations, done with the CQL3D code for extra-ordinary EC waves, demonstrate over 1 A/W efficiency in current for a relatively low density (∼ 2 × 1018 m −3), and low temperature (∼ 100 eV) plasma, consistent with the experimental results observed on EXL-50. Systematic scanning of different ECR harmonics in simulation has revealed that the multi-harmonic resonance interaction in EXL-50 plays a pivotal role in generating the energetic electron tail responsible for the current.

As a new spherical tokamak designed to simplify the engineering requirements of a possible future fusion power source, the EXL-50 experiment features a low aspect ratio vacuum vessel (VV), encircling a central post assembly containing the toroidal field coil conductors without a central solenoid. Multiple electron cyclotron resonance heating (ECRH) resonances are located within the VV to improve current drive effectiveness. Copious energetic electrons are produced and measured with hard x-ray detectors, carry the bulk of the plasma current ranging from 50–150 kA, which is maintained for more than 1 s duration. It is observed that over one ampere current can be sustained per watt of ECRH power issued from the 28 GHz gyrotrons. The plasma current reaches I p > 80 kA for high density (>5 × 1018 m−2) discharge with 150 kW ECRH. An analysis was carried out combining reconstructed multi-fluid equilibrium, guiding-center orbits of energetic electrons, and resonant heating mechanisms. It is verified that in EXL-50 a broadly distributed current of energetic electrons creates a smaller closed magnetic-flux surface of low aspect ratio that in turn confines the thermal plasma electrons and ions and participate in maintaining the equilibrium force balance.

Railway wheels in service experience rolling contact fatigue loading, but also need to resist frictional heating on braking, yielding temperatures up to 500 °C. The combination of mechanical and thermal loads leads to changes in the mechanical properties of the material. The focus of this study is to investigate the effect of annealing on local microstructure and residual stresses in railway wheel pearlitic steel (medium carbon steels, ~0.55 wt.% C) using synchroton X-ray Laue micro-diffraction. It is found that the local residual stress releases to a large extent after annealing at 500 °C. The stress formation and relief mechanisms and their relationship to the local microstructure are discussed.

Although experiments have demonstrated a feasibility of the electron-cyclotron-wave (ECW) non-inductive current start-up, the physical mechanism of the current start-up remains unclear, particularly the role of the preferential confinement of collisionless tail electron currents ( j TPC ) and vertical charge-separation currents ( j VTF ) . Therefore, j TPC and j VTF were observed using limiter probes during the ECW non-inductive current start-up on an ENN XunLong-50 (EXL-50) spherical torus to understand the significance of these values. It was found that j TPC increased and j VTF decreased during start-up, indicating a dominant role played by j TPC in this process. Furthermore, it was found that the product of the gas pressure and vertical magnetic field has a limit for the ECW non-inductive current start-up. Finally, we found that the ECW non-inductive current start-up can be realized only for sufficiently low vertical fields and gas pressures in EXL-50.

Significant number of confined energetic electrons have been observed outside of the LCFS (last-closed flux surface) of EXL-50’s solenoid-free electron cyclotron resonance heating (ECRH) sustained plasmas. Several measurement technologies have been applied to verify the key characteristics of energetic electrons for the first time. Experiments reveal that the presence of high-temperature, low-density electrons can carry relatively large quantities of the stored energy. The boundary between the thermal plasma and the energetic fluid is clearly separated and the distance between the two boundaries can reach tens of centimeters (around the size of the minor radius of the thermal plasma). This implies that the Grad-Shafranov equilibrium is not adequate to describe the equilibrium of EXL-50 plasma and a multi-fluid model is required. Particle simulations of full orbits show that energetic electrons can be well confined outside the LCFS. This is consistent with the experimental observations.

This work focuses on chirped solitons in a higher-order nonlinear Schrödinger equation, including cubic-quintic-septic nonlinearity, weak nonlocal nonlinearity, self-frequency shift, and self-steepening effect. For the first time, analytical bright and kink solitons, as well as their corresponding chirping, are obtained. The influence of septic nonlinearity and weak nonlocality on the dynamical behaviors of those nonlinearly chirped solitons is thoroughly addressed. The findings of the study give an experimental basis for nonlinear-managed solitons in optical fibers.

With the rapid development of communication technology, optical fiber communication has become a key research area in communications. When there are two signals in the optical fiber, the transmission of them can be abstracted as a high-order coupled nonlinear Schrödinger system. In this paper, by using the Hirota’s method, we construct the bilinear forms, and study the analytical solution of three solitons in the case of focusing interactions. In addition, by adjusting different wave numbers for phase control, we further discuss the influence of wave numbers on soliton transmissions. It is verified that wave numbers k 11, k 21, k 31, k 22, and k 32 can control the fusion and fission of solitons. The results are beneficial to the study of all-optical switches and fiber lasers in nonlinear optics.

High-performance ultraviolet (UV) detectors with both high responses and fast speeds are hard to make on homogeneous crystal semiconductor materials. Here, the UV response characteristics of mixed-phase MgZnO thin films with different internal structure distributions are studied. The mixed-phase MgZnO-based detector with the given crystal composition has a high response at both deep UV light (96 A W−1 at 240 nm) and near UV light (80 A W−1 at 335 nm). Meanwhile, because of the quasi-tunneling breakdown mechanism within the device, the high-response UV detector also shows a fast response speed (tr = 0.11 μs) and recovery speed (td1 = 26 μs) at deep UV light, which is much faster than both low-response mixed-phase MgZnO-based UV detectors with other structure constitutions and reported high-response UV devices on homogenous crystal materials. The Idark of the device is just 4.27 pA under a 5 V bias voltage, so the signal-to-noise ratio of the device reached 23852 at 5.5 uW cm−2 235 nm UV light. The new quasi-tunneling breakdown mechanism is observed in some mixed-phase MgZnO thin films that contain both c-MgZnO and h-MgZnO parts, which introduce a high response, signal-to-noise ratio, and fast speed into mixed-phase MgZnO-based UV detectors at weak deep UV light.

The investigation of two-dimensional (2D) nonlinear optical materials offers a promising way to construct the high-performance optical devices in fundamental and industrial applications because of their rich distinct optoelectronic properties. Herein, by utilizing the liquid exfoliation method, vanadium disulfide (VS2) nanosheets are prepared and the thickness is measured to be 3.16 nm. In addition, we have fabricated the VS2-based optical device and the nonlinear optical property is characterized with modulation depth of 23.97%. By using VS2 as saturable absorber, a high stable passively mode-locking Er-doped fiber laser is obtained with pulse duration of 169 fs and the largest average output power of 70.5 mW. The slope efficiency is up to 7.9%. In comparison to recent results of mode-locking fiber lasers with 2D materials, the VS2-based fiber laser demonstrates better performance. To the best of our knowledge, this is the first example of using VS2 for generating femtosecond mode-locked laser pulse. Our experimental results not only reveal VS2 ultrafast photonics application, but also advance the high-performance applications for information science and nonlinear optics.

Two-dimensional (2D) MoS2 nanosheets vertically supported on TiO2/PVDF flexible fibers have been successfully synthesized by combining electrospinning with a low-temperature hydrothermal method without acid. The morphology of the 2D MoS2 nanosheets could be controlled by adjusting experimental parameters. The loaded 2D MoS2 nanosheets can not only broaden the light capture range of TiO2, but also greatly inhibit the recombination rate of photogenerated electron-holes pairs. Due to the synergistic effect between MoS2 and TiO2, the photocatalytic rate for levofloxacin hydrochloride is about 40 times higher than that of MoS2 only. Recycle experiments have proved the stability and reusability of TiO2/PVDF@2D MoS2 nanosheets. And the mechanism is investigated by quenching experiments. The results show that superoxide anion radical (•O2-), hydroxyl radical (•OH) and hole (h+) all have contributions to photocatalysis. This work widens the range of materials to synthesize the composites of 2D MoS2 nanosheets and provides a new and gentle method for preparing flexible large-scale heterostructures for environmental protection.

White light-emitting diode (LED) with effective energy conservation and long service life could be employed in numerous applications. In this study, the high-performance n-ZnO films were first prepared via pulsed laser deposition on p-GaN substrates and then the n-ZnO/p-GaN heterojunction LED was fabricated. This LED exhibits blue and yellow light emission, and their emission intensities can be tuned by adjustment of the fabrication parameters (e.g., oxygen pressure) and/or by introduction of a semi-insulating i-ZnO layer to form a p-GaN/i-ZnO/n-ZnO heterojunction. Thus, a facile approach has been proposed for the preparation of white LED.

In order to realistically collect Electroencephalogram (EEG) signals in complicated environments, low impedance and flexibility are considered as two crucial characteristics for EEG electrode. We designed and manufactured a type of dry EEG electrode based on graphene with matrix micro-pyramid structure on the surface. This EEG electrode could not only reduce the contact impedance, but also improve the stability of the electrode. Furthermore, based on these flexible graphene EEG electrodes, we assembled a system that could monitor human EEG signals in real time. When a tested person was answering the phone or driving while tired, the system could give him appropriate warnings, so as to reduce the occurrence of accidents. This brain-computer interface system was wearable and suitable for intelligent detection of EEG signals.

A nonlinear Schrodinger equation with varying dispersion, nonlinearity and gain (or absorption) is studied for ultrashort optical pulses propagating in inhomogeneous optical fibres in the case of normal dispersion. Using the modified Hirota method and symbolic computation, the bilinear form and analytic soliton solution are derived. Stable bright and dark solitons are observed in the normal dispersion regime. A periodically varying soliton and compressed soliton without any fluctuation are obtained. Combined and kink-shaped solitons are observed. Possibly applicable soliton control techniques, which are used to design dispersion-managed systems, are proposed. The proposed techniques may find applications in soliton management communication links, soliton compression and soliton control.

In this paper, two types of the (2+1)-dimensional breaking soliton equations are investigated, which describe the interactions of the Riemann waves with the long waves. With symbolic computation, the Hirota bilinear forms and Backlund transformations are derived for those two systems. Furthermore, multisoliton solutions in terms of the Wronskian determinant are constructed, which are verified through the direct substitution of the solutions into the bilinear equations. Via the Wronskian technique, it is proved that the Backlund transformations obtained are the ones between the (N − 1)- and N-soliton solutions. Propagations and interactions of the kink-/bell-shaped solitons are presented. It is shown that the Riemann waves possess the solitonic properties, and maintain the amplitudes and velocities in the collisions only with some phase shifts.

Two-dimensional materials have been widely used as optical modulator materials in mode-locked fiber lasers. In terms of the performance of the fiber laser, one with an ultrashort pulse and high stability has great commercial value. Herein, the MoSe2 grown by the chemical vapor deposition (CVD) method with high modulation depth, quality lattice structure and uniformity is successfully applied in a mode-locked erbium-doped fiber laser. The pulse duration and signal-to-noise ratio of the laser are 207 fs and 85 dB, respectively. The multifarious performance comparisons indicate that the CVD-based MoSe2 saturable absorber with the tapered fiber structure has unique advantages not only in the generation of ultrashort pulses, but also in the optimization of laser stability.

Shear banding as a ubiquitous phenomenon is mostly observed in planar slip metals under complex plastic deformation conditions, especially in cyclic deformation. The deformation substructures and local damages during fatigue vary from grain to grain, which depends greatly on the crystallographic orientation. Despite previous extensive studies, the orientation-dependent deformation substructure is still elusive due to the complexity of damage evolution and the limitation of spatial resolution in both reciprocal and real sample spaces by conventional characterization methods. In this study, we conducted synchrotron-based X-ray micro-diffraction (μXRD) experiments to explore the dependence of localized elastic strains on grain orientation. It is demonstrated that the typical intersection of slip shear bands (SSBs) with Lomer-Cottrell locks (LCLs) are the potential damage locations, which are most likely to form in the near [001]//LD grains. The advanced μXRD with ability of spatially quantify the localized strains/fine orientations opens new opportunities for understanding the microscopic mechanisms of damage and failure in bulk materials.

Transition metal dichalcogenides (TMDs) have been successfully used as broadband optical modulator materials for pulsed fiber laser systems. However, the nonlinear optical absorptions of exfoliated TMDs are strongly limited by their nanoflakes morphology with uncontrollable lateral size and thickness. In this work, we provide an effective method to fully explore the nonlinear optical properties of MoSe2. Large-area and high quality lattice MoSe2 grown by chemical vapor deposition method was adopted as an optical modulator for the first time. The large-area MoSe2 shows excellent nonlinear optical absorption with a large modulation depth of 21.7% and small saturable intensity of 9.4 MW cm-2. After incorporating the MoSe2 optical modulator into fiber laser cavity as a saturable absorber, a highly stable Q-switching operation with single pulse energy of 224 nJ is achieved. The large-area MoSe2 possessing superior nonlinear optical properties compared to exfoliated nanoflakes affords possibility for the larger-area two-dimensional materials family as high performance optical devices.

The development of high-performance photocatalysts is central to efforts focused on taking advantage of solar energy to overcome environmental and energy crises. Integrating different functional materials artfully into nanostructures can deliver more efficient photocatalytic activity. Here, sandwiched ZnO@Au@CdS nanorod films were synthesized via successive ZnO nanorod electrodeposition, Au sputtering and CdS electrodeposition. The as-synthesized composites were characterized by UV-vis spectrophotometer, x-ray diffractometer, scanning and transmission electron microscopy. Their photocatalytic activity was assessed by degrading Rhodamine B solution under visible light irradiation. ZnO@Au@CdS exhibited better photocatalytic performance than ZnO@CdS throughout the visible light region, and the corresponding enhancement factor of Au nanoparticles was measured as a function of CdS loading amount, and it could reach 190% with CdS deposition for 1 min. The normalized rate constant could reach 0.387 h-1 for ZnO@Au@CdS-1min, which was equivalent to or better than results in reference photocatalysts. The enhancement mechanism of Au nanoparticles was estimated by comparing the monochromatic photocatalytic action spectra with the absorption spectrum of ZnO@Au@CdS, and it was mainly determined by incident photon energy. With selective excitation of Au nanoparticles by incident photons, the excited hot electrons in Au NPs are transferred to the conduction band of ZnO to boost photocatalytic reaction. With selective excitation of CdS, the enhanced interband absorption of CdS and relay station effect of Au nanoparticles should be responsible for the enhanced photocatalytic performance. Our work not only opens the door to the design of efficient supported photocatalysts, but also helps to understand the enhancement mechanism of LSPR effect on the photoelectric conversion of semiconductors.

All-optical switches have become one of the research focuses of nonlinear optics due to their fast switching speed. They have been applied in such fields as ultrafast optics, all-optical communication and all-optical networks. In this paper, based on symbolic computation, bidirectional all-optical switches are presented using analytic two-soliton solutions. Various types of soliton interactions are analyzed through choosing the different parameters of high-order dispersion and nonlinearity. Results indicate that bidirectional all-optical switches can be effectively achieved using highly nonlinear optical fibers.

Hollow-core photonic crystal fibers (HC-PCFs) can be used for the supercontinuum generation. Because the design of HC-PCFs is flexible, the dispersion and nonlinear effects is variable, and the pulses in HC-PCFs can show different transmission characteristics. In this paper, the transmission of high-order solitons in HC-PCFs is studied. Through adjusting the group-velocity dispersion and nonlinear effects of HC-PCFs, we present the different transmission of high-order solitons, and analyze their characteristics. Results are conducive to the applications of HC-PCFs in nonlinear optics and ultrafast optics.

Nanostructured heterojunctions play key role for transfer and separation of interfacial photo-carriers. At visible light illumination, the effects of Au nanoparticles (NPs) for the photocatalysis and gas-sensing performance of Au@ZnO nanstructures on porous silicon layer are reported. At optimized loading amount of Au NPs, the local surface plasmon resonance (LSPR) effect of Au NPs is studied. Generated by visible light irradiation, the LSPR effect of Au NPs promotes desorption and activation of surface adsorption oxygen species which is initiated by hot electrons transfer through Au–ZnO nanojunctions. This mechanism is responsible for the enhanced photocatalysis of methyl orange molecular, as well as enhancing the detecting performance for ammonia (NH3) gas at room temperature.

Dynamic solitons for a perturbed derivative nonlinear Schrodinger equation in nonlinear optics are presented for the first time in this paper. The analytic one-soliton solution for the perturbed derivative nonlinear Schrodinger equation is obtained with the Hirota method. The stable transmission soliton is observed and the influences of third-order dispersion and nonlinear coefficients are discussed. The characteristics and properties of solitons are analyzed and the stability analysis for the solitons is made. The salient features of the solitons reveal the possibility for the stable transmission of pulses in nonlinear optics.

We demonstrate a nonlinear polarization evolution mode-locked Yb-doped fiber oscillator with the broadband spectrum output operating in a dispersion-managed regime. Pumped by a 976 nm single-mode laser diode, stable mode-locked ultrashort pulses are emitted with an average power of 198 mW at a repetition rate of 124 MHz, corresponding to a pulse energy of 1.6 nJ. The output spectrum spans from 950 nm to 1150 nm so that the transform-limited pulse duration is as short as 23 fs. Due to the imperfect dispersion compensation, we compress the pulses to 32 fs in this experiment.

Periodic solitons are studied in dispersion decreasing fibers with a cosine profile. The variable-coefficient nonlinear Schrodinger equation, which can be used to describe the propagation of solitons, is investigated analytically. Analytic soliton solutions for this equation are derived with the Hirota's bilinear method. Using the soliton solutions, we obtain periodic solitons, and analyze the soliton characteristics. Influences of physical parameters on periodic solitons are discussed. The presented results can be used in optical communication systems and fiber lasers.

We demonstrate an all-normal-dispersion ytterbium (Yb)-doped fiber laser mode-locked by a higher modulation depth carbon nanotube saturable absorber (CNT-SA) based on an evanescent field interaction scheme. The laser cavity consists of pure normal dispersion fibers without dispersion compensation and an additional spectral filter. It is exhibited that the higher modulation depth CNT-SA could contribute to stabilize the mode-locking operation within a limited range of pump power and generate the highly chirped pulses with a high-energy level in the cavity with large normal dispersion and strong nonlinearity. Stable mode-locked pulses with a maximal energy of 29 nJ with a 5.59 MHz repetition rate at the operating wavelength around 1085 nm have been obtained. The maximal time-bandwidth product is 262.4. The temporal and spectral characteristics of pulses versus pump power are demonstrated. The experimental results suggest that the CNT-SA provides a sufficient nonlinear loss to compensate high nonlinearity and catch up the gain at a different pump power and thus leads to the stable mode locking.

In this letter, long-distance interactions between optical solitons with an oscillating structure are investigated. Analytic two-soliton solutions for the variable-coefficient nonlinear Schrodinger equation are obtained. Different from the elastic interactions reported previously, the interaction solitons are accompanied with an oscillating structure during their interactions. Reasons for long-distance interactions are discussed, and influences of the corresponding parameters are analyzed. Those studies may provide a new insight into the soliton interactions.

In this work breathers are obtained in a hollow-core photonic crystal fiber (HC-PCF) for the first time. The nonlinear Schr?dinger equation describing the propagation of pulses in a HC-PCF is investigated using the Hirota bilinear method and the auxiliary function method. Analytic breather solutions are derived by an appropriate choice of parameters. Dynamical behavior of breathers is exhibited, and the influences of different parameters on the characteristics of breathers are discussed. The presented results could be used in fiber lasers, nonlinear optics and Bose?Einstein condensates.

Elastic and inelastic interactions between optical spatial solitons in nonlinear optics, which to our knowledge have not been reported before, are investigated in this paper. Analytic soliton solutions for the coupled nonlinear Schrodinger equation are obtained using the Hirota method. The coefficient constraints, which can be used to determine whether the soliton interactions are elastic or inelastic, are presented. With different coefficient constraints, the characteristics of soliton interactions are exhibited. The results of this paper may have applications in the design of directional couplers and all-optical logical gates.

Butterfly-shaped pulse in optical fibers with variable group-velocity dispersion is reported for the first time. The nonlinear Schrodinger equation, which describes the pulse propagation in optical fibers with variable group-velocity dispersion, is investigated, and the analytic solution for this equation is derived. The cosine dispersion profile of optical fibers is suggested to obtain the butterfly-shaped pulse. The physical effects affecting the butterfly-shaped pulse properties is discussed.

A D-shaped fiber was fabricated by femtosecond laser-induced water breakdown for a fiber laser mode-locked by the evanescent field interaction of carbon nanotubes. Then a mode-locked double-clad fiber laser was constructed using the D-shaped fiber with carbon nanotubes as a saturable absorber and an ytterbium-doped double-clad fiber pumped by a 915 nm fiber coupled multimode laser diode as an amplifying medium. Stable mode-locked operation at 1.07 μm was achieved with a full width at half maximum of 18.1 nm. The output pulses had a repetition rate of 5.76 MHz with a pulse duration of 136.2 fs. The energy per pulse was 52.5 nJ and the slope efficiency was about 11%.