Parasitic Oscillations Research Articles

Terahertz multiple modes defined by fractal symmetry in complementary meta-atoms

Low quality (Q) factors of the intrinsic inductive–capacitive (LC) mode as well as the parasitic dipole oscillation mode restrict high-resolution sensing using split-ring resonators (SRR). Although the trapped Fano-mode of the high-Q factor is found in asymmetric SRR, the conventional design limits the scaling down of resonators. As such, excitation and manipulation of multiple trapped modes of SRR is significant for driving innovative designs of terahertz metamaterials and metasurfaces. In this work, we present a novel approach to manipulating multiple terahertz modes by increasing the fractal levels as well as the geometric symmetry of complementary SRR. It is found that the multiple trapped modes become achievable only in the case that the gap of adjacent fractal SRR opposes each other. By increasing the fractal level, the intrinsic resonance modes change slightly, and more trapped modes appear in between the frequency range of the two major intrinsic modes. The map of surface currents and magnetic field distribution reveal that intrinsic LC resonance in the first or second level SRR dominates the intrinsic modes. By contrast, the trapped mode arises from the hybridization of high-order localized dipole oscillation as well as the multiple localized LC resonances. These findings create new design opportunities for scalable metasurfaces across the terahertz spectrum and beyond, with ability to create high-resolution sensors.

Analysis and Modeling of SiC JFET Bi-Directional Switches Parasitic Oscillation

Silicon carbide (SiC) junction field-effect transistor (JFET) based bi-directional switches (BDSs) have great potential in the construction of several power electronic circuits, including matrix converters, multi-level converters, solid state breakers, and so on. Parasitic oscillation in SiC JFET-based BDSs has direct impact on the stability and reliability of these circuits. Proper handling of parasitic oscillation becomes highly critical. This paper focuses on the parasitic oscillation suppression in SiC JFET-based BDSs. A parallel snubber capacitor or a series ferrite ring was often used to damp parasitic oscillation in switching circuits in the literature. However, conventionally, the snubber capacitance was obtained by time-consuming and labour-intensive trial-and-error methods. The main contribution of this study is to derive the simplified equivalent transient circuit of the SiC JFET-based BDS considering all parasitic elements and quantitatively define the reasonable range of the snubber capacitance. And the combined effect of the selected snubber capacitance and the selected ferrite ring is investigated. In the end, simulation and experimental results validate the effectiveness of the proposed method.

Investigation of the Root Causes of Electromagnetic Noise of an Interleaved DC–DC Converter With GaN or Si Transistors and Corresponding Optimization Strategies

The proposed paper unveils various sources of conducted and radiated electromagnetic interferences (EMIs) of an interleaved dc-dc converter with gallium nitride (GaN) transistors and describes different optimization strategies. Instead of striving to achieve conformity to electromagnetic compatibility (EMC) standards, it rather focuses on deriving a deeper understanding of the source of EMI phenomena related to fast switching power converters generalizing critical aspects that need to be kept in mind during the design stage. The fast switching transients of GaN transistors are the main reason for increased EMI, which is why the characteristics of the applied transistors are investigated closely, paying special attention to the parasitic switching oscillations. Various observed interference effects are presented and corresponding conclusions are drawn. The effects are divided into intra-EMI issues of capacitive and inductive near-field coupling related to signal instability and inter-EMI issues of conducted and radiated noise. Based on these findings, different optimization strategies are presented addressing signal filtering, an optimized printed circuit board design, as well as proper high-frequency designs of passive components such as capacitors for the dc link and the power choke. All findings are put into context with the EMI characteristics of an equivalent dc-dc converter utilizing Si-insulated gate bipolar transistors (IGBTs).

Room-temperature lasing on Fe2+:ZnSe with meniscus inner doped layer fabricated by solid-state diffusion bonding

An Fe2+:ZnSe laser on a polycrystalline active element with an inner doped layer in the form of a meniscus operating at room temperature was investigated. The active element was manufactured using the solid-state diffusion bonding (SSDB) technique. The billets, in the form of plano-concave and plano-convex ZnSe lenses with a radius of curvature of 200 mm, were subjected to diffusion bonding. Before bonding, zinc selenide was doped with Fe2+ ions by diffusion through spherical lens surfaces by the process of hot isostatic pressing (HIP). After the SSDB procedure, the prepared sample was subjected to an additional HIP treatment. An Fe2+:ZnSe polycrystal was pumped by an electric-discharge hydrogen fluoride (HF) laser. The use of the active element with an inner doped layer in the form of the meniscus permitted an increase in the pump spot diameter, at which there was not yet any reduction in laser output energy associated with the development of transverse parasitic oscillation. This increase was ~19% (compared to samples with a single flat doped layer). The output energy of 480 mJ was achieved at a total efficiency (with respect to absorbed energy) of 38% on the 20 mm diameter Fe2+:ZnSe sample with the meniscus inner doped layer.

Detailed Oscillation Analysis and Parameter Selection Principle for Boost PFC Converter With <italic>RC</italic> Snubber Operated in DCM

The parasitic parameters of a boost power factor correction converter lead to input current distortion when the converter operates in discontinuous conduction mode, and using an RC snubber is a simple and effective way to suppress the parasitic oscillation. The effect of the RC snubber on the parasitic oscillation is analyzed in detail, and three different forms of the characteristic roots are obtained according to different RC values. The optimal RC values and their relationship between the inductance and parasitic capacitance are figured out, which can achieve a lowest total harmonic distortion. A 1-kW prototype has been built and tested at low power and the experimental waveforms and data are presented to validate the theoretical analysis.

High efficiency frequency doubling with a passive enhancement cavity

Highly efficient frequency doubling of a continuous-wave Yb-doped fiber laser with a passively-locked enhancement cavity is achieved. The increase in efficiency is a result of improved transverse mode matching between the fiber and doubling cavities, which is enabled by an alignment method presented here. An optical conversion efficiency as high as 36.8% from a launched 976 nm diode pump to a 532 nm green laser is obtained. The maximum 532 nm output is 24.5 W, which is limited by the onset of parasitic oscillations in the fiber laser.

Research on a new converter topology for energy transmission between spacecrafts

A novel power converter based on dual-transistor forward converter topology is proposed to meet the requirements of high power transmission between spacecrafts. Compared with the traditional two-transistor forward converter, this converter eliminates the secondary filter inductance and the diode of the rectifier. Theoretical analysis and experimental results show that the new topology reduces the use of magnetic elements, improves the power density of the converter, and improves the parasitic oscillation and reverse recovery of the rectifier diode. The new topology can better meet the requirements of power supply system for high reliability, reducing device stress, increasing the availability of key devices, optimizing system structure and so on.

Silicone Oil Damping for Quasi-static Micro Scanners with Electrostatic Staggered Vertical Comb Drives

Many applications like image projection, distance sensors or spectroscopy require high speed quasi-static operating scanners with the size of several millimeters for the laser beam guidance. Quasi-static micro scanners with electrostatic staggered vertical comb drives of sizes up to 5 mm achieve remarkable mechanical deflections of ±8 degree and eigenfrequencies of several hundred Hertz. Closed-loop control of the scanner position highly reduces parasitic eigenfrequency oscillation when driving an arbitrary trajectory like triangle or saw tooth, but requires position feedback and high performant drive electronics for real-time control. In this contribution we investigate silicone oil to attenuate undesired oscillations by passive mechanic damping. We present experimental results with different viscosity and determine the resulting system parameters such as damping coefficient, eigenfrequency and permittivity, supported by transient simulations. While the permittivity, refractive index and damping increase significantly, the deceasing eigenfrequency results in slower admissible trajectories. The overall performance is assessed and compared to the active damping approach using closed-loop control of the micro scanner operating in air. Finally, a potential packaging technology is presented.

Parasitic Memcapacitor Effects on HP TiO2Memristor Dynamics

In this paper, we study an HP TiO 2 memristor model with a parasitic memcapacitor, and it is shown that the parasitic element has significant effects on the volt-ampere characteristics and the dynamics of the memristor circuits. Further, the study shows that the pinched point on the hysteresis loop of the HP TiO 2 memristor may deviate from the origin. We also show that the memristor with a parasitic memcapacitor will eventually lose its stored flux when its power is switched off. Furthermore, in order to study the parasitic effects in circuits, we derive a simple three-element series circuit that contains an inductor, a negative resistor, and an HP TiO 2 memristor. If the parasitic memcapacitor of the memristor is considered, we observe a parasitic periodic oscillation occurred in the circuit.

Modified equations for variational integrators applied to Lagrangians linear in velocities

Variational integrators applied to degenerate Lagrangians that are linear in the velocities are two-step methods. The system of modified equations for a two-step method consists of the principal modified equation and one additional equation describing parasitic oscillations. We observe that a Lagrangian for the principal modified equation can be constructed using the same technique as in the case of non-degenerate Lagrangians. Furthermore, we construct the full system of modified equations by doubling the dimension of the discrete system in such a way that the principal modified equation of the extended system coincides with the full system of modified equations of the original system. We show that the extended discrete system is Lagrangian, which leads to a construction of a Lagrangian for the full system of modified equations.

Parasitic Oscillations in Smooth-Wall Circular Symmetric Gyrotron Beam Ducts

In order to study parasitic oscillation that may occur in a realistic beam duct upstream to the gyrotron cavity, the self-consistent linear and spectral code TWANGlinspec has been modified. The large inhomogeneities in the smooth-wall beam duct geometry or in the magnetic field profile required the implementation of a numerical approach using a hybrid finite element method. The new model permits to characterize a large number of potentially spurious TE modes. Compared to previous studies on gyrotron beam duct instabilities, an extended interaction space including also the gyrotron cavity has been considered. The role of the connecting part between the beam duct and the cavity, called spacer, is highlighted and it is shown that the gyro backward-wave TE modes excited in this region generally have their minimum starting current. The sensitivity of the minimum starting current on electron beam velocity spread is also evaluated.

Off-axis eight-pass neodymium glass laser amplifier with high efficiency and excellent energy stability.

A new relay-imaged off-axial eight-pass laser amplifier with several joules energy and 1Hz repetition rate was demonstrated. The extraction efficiency and pulse-to-pulse energy stability were greatly improved. Under the single-pass small-signal gain of 3.6, a net gain of 900 and an effective extraction efficiency of 42.4% in the beam aperture were realized. Pulse-to-pulse energy stability of 0.83% (peak-valley) and 0.17% (root-mean-square) was achieved by the significant saturation of eight-pass amplification, which, to the best of our knowledge, is the best energy stability in several-joules-class amplifiers. The far-field quality was 2.52 times the diffraction limit, and the near-field modulation of the 90% beam aperture was 1.28. No parasitic oscillations or pencil beams were observed. Moreover, another key feature of the proposed amplifier was the ability to remarkably improve the pulse contrast with a unique design.

Computational Studies on the Suppression of Parasitic Oscillations in Gyrotron Beam Tunnels

Parasitic oscillations frequently appear in several modern gyrotron beam tunnels. In this paper, the CST Studio Suite is used to study the performance of dielectric-loaded beam tunnel designs in attenuating the spurious modes. Two common designs are considered and the effect of the ceramic material's dielectric constant on the beam tunnel's performance is studied. For this purpose, the ohmic quality factor Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ohm</sub> and the normalized shunt impedance R/Q are calculated to measure this performance. These studies focus on the backward propagating axisymmetric parasitic modes.

Modelling of a tuned liquid multi-column damper. Application to floating wind turbine for improved robustness against wave incidence

In this paper, the coupling of a float with a tuned liquid multi-column damper (TLMCD), a novel structural damping device inspired by the classical tuned liquid column damper (TLCD), is modelled using Lagrangian mechanics. We detail the tuning of the design parameters for each considered variant of the TLMCD, and compare each of them against a layout of multiple TLCDs. The results show that the proposed TLMCD is superior to multiple TLCDs for this application as it is more robust against wave incidence and it creates significantly less parasitic oscillations.

Analysis and Suppression of Unwanted Turn-On and Parasitic Oscillation in SiC JFET-Based Bi-Directional Switches

Silicon Carbide (SiC)-based Bi-Directional Switches (BDS) have great potential in the construction of several power electronic circuits including multi-level converters, solid-state breakers, matrix converters, HERIC (high efficient and reliable inverter concept) photovoltaic grid-connected inverters and so on. In this paper, two issues with the application of SiC-based BDSs, namely, unwanted turn-on and parasitic oscillation, are deeply investigated. To eliminate unwanted turn-on, it is proposed to add a capacitor (CX) paralleled at the signal input port of the driver IC (integrated circuit) and the capacitance range of CX is also analytically derived to guide the selection of CX. To mitigate parasitic oscillation, a combinational method, which combines a snubber capacitor (CJ) paralleled with the JFET (Junction Field Effect Transistor) and a ferrite ring connected in series with the power line, is proposed. It is verified that the use of CJ mainly improves the turn-off transient and the use of a ferrite ring damps the current oscillation during the turn-on transient significantly. The effects of the proposed methods have been demonstrated by theoretical analysis and verified by experimental results.

Study on the power scalability of all-fiber superfluorescent source operating near 980 nm.

The high-power all-fiber superfluorescent source operating near 980 nm is studied experimentally and numerically. In experiment, an all-fiber superfluorescent source operating near 980 nm is fabricated with the distributed side-coupled cladding-pumping (DSCCP) Yb-doped fiber (YDF). By optimizing the active fiber and angle-cleaving of the output port, a recorded 17.1-W output power and 14.6% slope efficiency of 980-nm ASE are obtained. No parasitic laser oscillation is observed at the maximum output power. The power scalability of the source is also numerically investigated. A simple but effective method is present to numerically determine the threshold of parasitic laser oscillation. It is found that the output power can be scaled up to 50 W and 100 W with the optical feedback of each output port suppressed to 1.2 × 10-6 and 7 × 10-7, respectively. It is also revealed that coupling coefficient should be larger than 6 to realize more than 50% slope efficiency. These results provide significant guidance for understanding and designing the high-power superfluorescent Yb-doped source (SYFS) operating near 980 nm and other sorts of three-level fiber sources.

Improved Suppression of Parasitic Oscillations in Gyrotron Beam Tunnels by Proper Selection of the Lossy Ceramic Material

Lossy dielectric materials are used in gyrotron beam tunnels to suppress possible parasitic oscillations by raising their starting currents. The dependence of the overall losses of the structure on the dielectric properties of the material is theoretically and numerically investigated and it is shown that materials with high values of the loss tangent are not suitable for the suppression of the parasitics. The beam tunnel is studied parametrically with a full-wave code, which calculates the diffraction and dielectric losses of the cold-cavity modes. A simplified geometry involving analytic relations is used to verify the results and investigate the underlying physics. It is shown that the proper selection of the material properties and its radial thickness can improve the absorption of the fields significantly.

Improvement of Mode Selectivity of High-Harmonic Gyrotrons by Using Operating Cavities with Short Output Reflectors

An important problem in realization of gyrotrons operating at high cyclotron harmonics is discrimination of parasitic oscillations excited at lower harmonics. In this paper, we propose to use an operating cavity with a short output irregularity providing a partial reflection of the operating wave. This allows decreasing the length of the cavity with no considerable change in both the operating electron current and the efficiency of the gyrotron operation. At the same time, since the output irregularity weakly reflects parasitic low-frequency waves, the reduction in the length of the cavity leads to a significant increase in the starting currents of the parasitic near-cutoff waves.

Generalization of one algorithm for constructing recurrent splines

We have analyzed two algorithms, close in composition, for constructing a smoothing spline, which imply a change only in the last link of the spline when new experimental data arrive. The main feature of the N. D. Dicoussar algorithm is the form of a polynomial representation in order to describe a link of the spline. It is shown that a given polynomial is one of the hierarchical form of the Hermitian polynomial. We have proposed a modification to the D. A. Silaev algorithm for constructing a smoothing spline with different orders of smoothness: from zero to the second, aimed at enhancing the stability of this algorithm. To this end, we substantiated recommendations related to the form of polynomials representation, which describe the links of splines of the specified form. For this purpose, we estimated conditionality of matrices used in the algorithm. For the spline of zero-order smoothness, the most advisable is to apply a polynomial in the N. D. Dicoussar form, and for splines with higher orders of smoothness of joining the links, it is appropriate to use different forms of the Hermitian polynomials. Based on computational examples, a possibility was demonstrated to generalize the D. A. Silaev algorithm to construct a spline with links of various lengths, which is determined by the rate of change in the examined parameter. That makes it possible to reduce the volume of information that contains a description of the spline itself, and to prevent such a widespread shortcoming of approximation when using polynomials as parasitic oscillations. It was shown as well that in the presence of significant measurement errors in experimental data there may occur a need to decrease the length of the spline's link (compared to that derived by the D. A. Silaev rule) in order to provide the spline with a property of robustness

Lasing characteristics of refractive-index-matched composite Y3Al5O12 rods employing transparent ceramics for solar-pumped lasers

We have proposed a new configuration of solar-pumped lasers employing transparent ceramic rods. The laser rod has a composite structure consisting of a Nd/Cr:YAG gain domain surrounded by Gd:YAG with the same refractive index as that of Nd/Cr:YAG. The lasing mode is well controlled by the output coupler, and the parasitic oscillation is suppressed, owing to the refractive index matching. A high laser slope efficiency and a low laser oscillation threshold were achieved owing to the suppressed absorption outside the lasing mode, which was previously a serious issue for the end-pumping configuration using a high-NA focusing optics. The laser oscillation threshold of 136 mW and the slope efficiency of 25.3% were derived. Thus, we have resolved the issue of useless absorption associated with the high-NA end-pumping, and achieved significant improvements compared with the conventional structure of uniform Nd/Cr:YAG.