Showing papers on "Harmonics published in 2015"

Linking high harmonics from gases and solids

Abstract: When intense light interacts with an atomic gas, recollision between an ionizing electron and its parent ion creates high-order harmonics of the fundamental laser frequency. This sub-cycle effect generates coherent soft X-rays and attosecond pulses, and provides a means to image molecular orbitals. Recently, high harmonics have been generated from bulk crystals, but what mechanism dominates the emission remains uncertain. To resolve this issue, we adapt measurement methods from gas-phase research to solid zinc oxide driven by mid-infrared laser fields of 0.25 volts per angstrom. We find that when we alter the generation process with a second-harmonic beam, the modified harmonic spectrum bears the signature of a generalized recollision between an electron and its associated hole. In addition, we find that solid-state high harmonics are perturbed by fields so weak that they are present in conventional electronic circuits, thus opening a route to integrate electronics with attosecond and high-harmonic technology. Future experiments will permit the band structure of a solid to be tomographically reconstructed.

A Review of Multilevel Selective Harmonic Elimination PWM: Formulations, Solving Algorithms, Implementation and Applications

Abstract: Selective harmonic elimination pulse width modulation (SHE-PWM) offers tight control of the harmonic spectrum of a given voltage and/or current waveform generated by a power electronics converter. Owing to its formulation and focus on elimination of low-order harmonics, it is highly beneficial for high-power converters operating with low switching frequencies. Over the last decade, the application of SHE-PWM has been extended to include multilevel converters. This paper provides a comprehensive review of the SHE-PWM modulation technique, aimed at its application to multilevel converters. This review focuses on various aspects of multilevel SHE-PWM, including different problem formulations, solving algorithms, and implementation in various multilevel converter topologies. An overview of current and future applications of multilevel SHE-PWM is also provided.

Generation of Bright Phase-matched Circularly-polarized Extreme Ultraviolet High Harmonics

Abstract: Circularly-polarized extreme ultraviolet and X-ray radiation is useful for analysing the structural, electronic and magnetic properties of materials. To date, such radiation has only been available at large-scale X-ray facilities such as synchrotrons. Here, we demonstrate the first bright, phase-matched, extreme ultraviolet circularly-polarized high harmonics source. The harmonics are emitted when bi-chromatic counter-rotating circularly-polarized laser pulses field-ionize a gas in a hollowcore waveguide. We use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase-matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to linearly polarized high harmonic sources. This work represents a critical advance towards the development of table-top systems for element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution.

Analysis of the Phase-Shifted Carrier Modulation for Modular Multilevel Converters

Abstract: The modular multilevel converter (MMC) is an emerging topology for high-power applications and is considered as the development trend of the high-voltage power converters. In this paper, general implementation of the phase-shifted carrier (PSC) modulation with a capacitor voltage balancing method for MMC is first introduced. Then, the mathematical analysis of PSC modulation for MMC is performed to identify the PWM harmonic characteristics of the output voltage and the circulating current. Moreover, influence of the carrier displacement angle between the upper and lower arms on these harmonics is also studied. Using this analysis, the optimum displacement angles are specified for the output voltage harmonics minimization and the circulating current harmonics cancellation, respectively. The harmonic features of the line-to-line voltage and the dc-link current are also investigated. Moreover, an extension of the PSC modulation for MMC with full-bridge submodules is also proposed which can increase the equivalent switching frequency of the output voltage and circulating current by two times compared with the conventional MMC. Finally, the findings are verified experimentally on a prototype of MMC.

Modulated Model Predictive Control for a Three-Phase Active Rectifier

Abstract: Model predictive control (MPC) has a number of desirable attributes which are difficult to achieve with classical converter control techniques. Unfortunately, the nature of power electronics imposes restriction to the method, as a result of the limited number of available converter states. This, combined with the spread spectrum nature of harmonics inherent with the strategy, complicates further design. This paper presents a method for removing this characteristic without compromising the desirable functionality of predictive control. The method, named modulated MPC, is applied to a two-level three-phase converter and compared with a number of similar approaches. Experimental results are used to support theoretical analysis and simulation studies.

A Repetitive Control Scheme for Harmonic Suppression of Circulating Current in Modular Multilevel Converters

Abstract: In a modular multilevel converter (MMC), the interaction between switching actions and fluctuating capacitor voltages of the submodules results in second- and other even-order harmonics in the circulating currents. These harmonic currents will introduce extra power loss, increase current stress of power devices, and even cause instability during transients. Traditional methods for circulating current harmonic suppression have problems such as limited harmonic rejection capability, limited application area, and complex implementation. This paper presents a plug-in repetitive control scheme to solve the problem. It combines the high dynamics of PI controller and good steady-state harmonic suppression of the repetitive controller, and minimizes the interference between the two controllers. It is suitable for multiple harmonic suppression, easy to implement, and applicable for both single-phase and three-phase MMCs. Simulation and experimental results on a single-phase MMC inverter proved the validity of the proposed control method.

Non-collinear generation of angularly isolated circularly polarized high harmonics

Abstract: We generate angularly isolated beams of circularly polarized extreme ultraviolet light through the first implementation of non-collinear high harmonic generation with circularly polarized driving lasers. This non-collinear technique offers numerous advantages over previous methods, including the generation of higher photon energies, the separation of the harmonics from the pump beam, the production of both left and right circularly polarized harmonics at the same wavelength and the capability of separating the harmonics without using a spectrometer. To confirm the circular polarization of the beams and to demonstrate the practicality of this new light source, we measure the magnetic circular dichroism of a 20 nm iron film. Furthermore, we explain the mechanisms of non-collinear high harmonic generation using analytical descriptions in both the photon and wave models. Advanced numerical simulations indicate that this non-collinear mixing enables the generation of isolated attosecond pulses with circular polarization. Scientists have demonstrated non-collinear circularly polarized high-harmonic generation and showed that this method generates bright circularly polarized extreme-ultraviolet beams with both left and right helicity simultaneously.

Frequency Adaptive Selective Harmonic Control for Grid-Connected Inverters

Abstract: In this paper, a frequency adaptive selective harmonic control (FA-SHC) scheme is proposed. The FA-SHC method is developed from a hybrid SHC scheme based on the internal model principle, which can be designed for grid-connected inverters to optimally mitigate feed-in current harmonics. The hybrid SHC scheme consists of multiple parallel recursive (nk ± m)-order (k = 0,1,2,. . ., and m ≤ n/2) harmonic control modules with independent control gains, which can be optimally weighted in accordance with the harmonic distribution. The hybrid SHC, thus, offers an optimal tradeoff among cost, complexity, and also performance in terms of high accuracy, fast response, easy implementation, and compatible design. The analysis and synthesis of the hybrid SHC are addressed. More important, in order to deal with the harmonics in the presence of grid frequency variations, the hybrid SHC is transformed into the FA-SHC, being the proposed fractional-order controller, when it is implemented with a fixed sampling rate. The FA-SHC is implemented by substituting the fractional-order elements with the Lagrange-polynomial-based interpolation filters. The proposed FA-SHC scheme provides fast on-line computation and frequency adaptability to compensate harmonics in grid-connected applications, where the grid frequency is usually varying within a certain range (e.g., 50 ± 0.5 Hz). Experimental tests have demonstrated the effectiveness of the proposed FA-SHC scheme in terms of accurate frequency adaptability and also fast transient response.

Direct Power Control of a Pulse Width Modulation Rectifier Using Space Vector Modulation Under Unbalanced Grid Voltages

Abstract: Compared with conventional table-based direct power control (DPC), DPC using space vector modulation (DPC-SVM) exhibits several specific features, such as a constant switching frequency and small ripples in both active power and reactive power. However, conventional DPC-SVM exhibits highly distorted grid currents when the grid voltages are unbalanced. In this study, we propose a novel and simple DPC-SVM that is effective under both ideal and unbalanced grid voltage conditions by using an extension of original instantaneous power theory. After deducing the power slopes of both active power and reactive power, the suitable converter voltage reference to nullify the errors of active power and reactive power is analytically derived, which is subsequently synthesized by SVM. The proposed DPC-SVM does not require the extraction of complex positive/negative sequence from the grid voltage/current or power compensation. Compared to prior DPC-SVM using original imaginary power, the proposed method exhibits much better performance by obtaining highly sinusoidal line currents and eliminating twice grid-frequency ripples in both active power and the reactive power under unbalanced conditions. Simulations and experimental results supported the theoretical study and confirmed the effectiveness of the proposed method.

Effect of multiple conduction bands on high-harmonic emission from dielectrics

Abstract: We find that, for sufficiently strong mid-IR fields, transitions between different conduction bands play an important role in the generation of high-order harmonics in a dielectric. The transitions make a significant contribution to the harmonic signal, and they can create a single effective band for the motion of an electron wave packet. We show how high harmonic spectra produced during the interaction of ultrashort laser pulses with periodic solids provide a spectroscopic tool for understanding the effective band structure that controls electron dynamics in these media.

Super high power mid-infrared femtosecond light bullet

Abstract: Mid-infrared ultrashort high energy laser sources are opening up new opportunities in science, including keV-class high harmonic generation and monoenergetic MeV-class proton acceleration. As new higher energy sources become available, potential applications for atmospheric propagation can dramatically grow to include stand-off detection, laser communications, shock-driven remote terahertz enhancement and extended long-lived thermal waveguides to transport high power microwave and radiofrequency waves. We reveal a new paradigm for long-range, low-loss, ultrahigh power ultrashort pulse propagation at mid-infrared wavelengths in the atmosphere. Before the onset of critical self-focusing, energy in the fundamental wave continually leaks into shock-driven spectrally broadened higher harmonics. A persistent near-invariant solitonic leading edge on the multi-terawatt pulse waveform transports most of the power over hundred-metre-long distances. Such light bullets are resistant to uncontrolled multiple filamentation and are expected to spark extensive research in optics, where the use of mid-infrared lasers is currently much under-utilized. A mechanism for the propagation of mid-infrared femtosecond laser pulses in air is theoretically investigated. A numerical simulation predicts that the propagation of multiple-terawatt pulses is possible over hundreds of metres.

Model-Predictive Flux Control of Induction Motor Drives With Switching Instant Optimization

Abstract: Conventional model-predictive torque control (MPTC) requires tedious and time-consuming tuning work for stator flux weighting factor, and presents relatively high torque ripples. To solve these problems, this paper proposes a model-predictive flux control (MPFC) for two-level inverter-fed induction motor (IM) drives. The references of stator flux magnitude and torque in conventional MPTC are converted into an equivalent reference of stator flux vector in the proposed MPFC. As only the tracking error of stator flux vector is required in the cost function, the use of weighting factor is eliminated. The optimal voltage vector is selected based on the principle of stator flux error minimization and its switching instant is optimized rather than being in the beginning of each control period. The proposed MPFC with and without switching instant optimization are both implemented in a 32-bit floating digital signal processor, and they are compared in detail in terms of torque ripple, current harmonics, and average switching frequency. Both digital simulations and experimental tests were carried out on a two-level inverter-fed IM drive, and the obtained results validate the effectiveness of the proposed method.

Potential Harmonic Resonance Impacts of PV Inverter Filters on Distribution Systems

Abstract: This paper presents a clarification study to identify the potential resonance phenomenon between photovoltaic (PV) inverters and the distribution system. LCL and LC filters are widely applied in PV inverters to mitigate high-order harmonic components generated by PV inverters. There is a possibility that these filters will excite harmonic resonance by interacting with the system impedance. The mechanism of this phenomenon is investigated here by mathematical analysis and measurement. The results indicate that the resonance can be attenuated if the damping resistance, such as damping resistor and residential linear loads, is large enough. Alternatively, three sets of field tests are conducted in the laboratory to verify and clarify the potential harmonic resonance and its factors. Furthermore, a full case of an actual North American distribution system with PV installations is also studied. The results indicate that the harmonic resonance caused by the PV filter is almost attenuated and cannot cause serious problems. At the same time, the filter may have some advantages in mitigating harmonics. Finally, to complete this paper, other sustainable energy resources with voltage-source converters (VSCs) are compared.

Design and Implementation of a Single-Stage Driver for Supplying an LED Street-Lighting Module With Power Factor Corrections

Abstract: This paper proposes a novel single-stage light-emitting diode (LED) driver for street-lighting applications with power factor corrections (PFC). The presented driver integrates a modified bridgeless PFC ac-dc converter with a half-bridge-type LLC dc-dc resonant converter into a single-stage conversion circuit topology. The proposed ac-dc resonant driver provides input current shaping, and it offers attributes of lowered switching losses to the soft-switching functions obtained on two power switches and two output-rectifier diodes. The proposed driver features cost-effectiveness, high circuit efficiency (>92%), high power factor (>0.99) and low input current total harmonics distortion (<;8%). A prototype driver is developed for supplying a 144-W-rated LED street-lighting module with utility-line input voltages ranging from 100 to 120 V, and experimental results demonstrate the functionalities of the proposed LED driver.

Power System Harmonics and Passive Filter Designs

Abstract: Description: Power System Harmonics provides comprehensive coverage of generation, effects, and control of harmonics, and presents its state–of–the–art technology and advancements This book is the first to cover Power System Harmonics in–depth, including real world, illustrative case studies. Written by a well–known author with extensive experience designing harmonic filters, this book is written in a thorough and easy to read way, making it an invaluable resource for practicing engineers, consultants, and academia. Coverage includes new harmonic mitigation technologies, detailed step–by–step design of passive filters, interharmonics and flicker, and more. Readers who are new to the field will be able to grasp the subject through the many examples, case studies and illustrations. More advanced readers who are already familiar with Power System Harmonics will be able to learn about new insights and the latest developments. Covers new harmonic mitigation technologies, such as advances in converter technologies for harmonic mitigation and simultaneous improvement of power factor, and detailed step–by–step design of passive filters not covered in other publications Covers comprehensive models of power system components and power systems for harmonic analysis Includes simulations of wind and solar power plants In depth coverage of the effects of harmonics Includes EMTP simulations (Electromagnetic Transients Program) for real world situations Richly illustrated; covers IEEE and IEC standards, and provides extensive references for further reading Power system harmonics, and mitigation of their deleterious effects, has become a fast–developing topic and this book will perform as a useful reference now, and in the future.

Towards enabling femtosecond helicity-dependent spectroscopy with high-harmonic sources

Abstract: Recent advances in high-harmonic generation gave rise to soft X-ray pulses with higher intensity, shorter duration and higher photon energy One of the remaining shortages of this source is its restriction to linear polarization, since the yield of generation of elliptically polarized high harmonics has been low so far We here show how this limitation is overcome by using a cross-polarized two-colour laser field With this simple technique, we reach high degrees of ellipticity (up to 75%) with efficiencies similar to classically generated linearly polarized harmonics To demonstrate these features and to prove the capacity of our source for applications, we measure the X-ray magnetic circular dichroism (XMCD) effect of nickel at the M2,3 absorption edge around 67 eV There results open up the way towards femtosecond time-resolved experiments using high harmonics exploiting the powerful element-sensitive XMCD effect and resolving the ultrafast magnetization dynamics of individual components in complex materials

Analysis of the Torque Production Mechanism for Flux-Switching Permanent-Magnet Machines

Abstract: This paper investigates the principles underlying torque production in a flux-switching permanent-magnet (FSPM) machine. Because the phase windings and permanent magnets (PMs) in FSPM machines are both located on the stator, the torque production mechanism is not the same as for a conventional PM synchronous machine. Spatial harmonic analysis is applied to examine the frequency components present in the electric and magnetic loading of the machine. Since torque is proportional to the product of the electric and magnetic loading, understanding the source of the principal harmonics in these waveforms yields powerful insights into the components that result in torque production. The analysis is first presented for a specific FSPM machine (12-slot/10-pole) and then extended to a general FSPM machine. The primary torque-producing harmonics in the air-gap flux density waveform are found to be the heterodyned harmonics of the magnetomotive force produced by the stator magnets and the air-gap permeance seen by the stator looking into the rotor. Analytical results are compared to results from finite-element analysis and exhibit good agreement.

Hybrid Cascaded Modular Multilevel Converter With DC Fault Ride-Through Capability for the HVDC Transmission System

Abstract: A new hybrid cascaded modular multilevel converter for the high-voltage dc transmission system is presented. The half-bridge cells are used on the main power stage and the cascade full-bridge (FB) cells are connected to its ac terminals. The main power stage generates the fundamental voltages with quite low switching frequency, resulting in relatively low losses. The cascaded FB cells only attenuate the harmonics generated by the main power stage, without contribution to the power transfer. Thus, the energy-storage requirement of the cascaded FB cells is low and the capacitance of FB cells is reduced significantly. Due to the dc fault reverse blocking capability of the cascaded FB cells, the proposed topology can ride-through the pole-to-pole dc fault. In addition, the soft restart is achieved after the fault is eliminated, without exposing the system to significant inrush current. Besides, the average-value model of the proposed topology is derived, based on which the control strategy is presented. The results show the feasibility of the proposed converter.

Passive Filter Design for China High-Speed Railway With Considering Harmonic Resonance and Characteristic Harmonics

Abstract: In order to make high-speed trains (HSTs) lighter and more reliable, LC or LCL high-pass filters, which are widely adopted to mitigate high-order harmonics, are not installed in most of China HSTs. Therefore, the harmonic problem is a concern, because of the significant adverse impacts it has on the tractive drive system of the train and power quality of the utility system. The harmful harmonic distortions in high-speed railways (HSRs) are mainly caused by harmonic resonance and massive characteristic harmonics emission. This paper presents the results of harmonic assessment and harmonic filter design for a typical HSR line in China. Harmonic penetration analysis (HPA) is implemented and carried out to determine the harmonic distorted types for a wide range of possible train-operating conditions in a timetable. Both statistical field test and numerical calculation are used in passive filter design for HSRs. A C-type filter is designed here to address these typical harmonic distortions. The studies will be validated by detailed simulations based on the train timetable by counting the 95% index of the 24-h profile of harmonic results.

Modeling and Elimination of Zero-Sequence Circulating Currents in Parallel Three-Level T-Type Grid-Connected Inverters

Abstract: Unique pitfalls in parallel three-level T-type inverters (3LT 2 Is) are potential zero-sequence circulating currents (ZSCCs) which are more complex than parallel two-level inverters and can cause current discrepancy, current waveform distortion, power losses, etc. In this paper, the ZSCC paths in the parallel 3LT 2 Is are first presented, and an equivalent model of the ZSCCs is developed. It is seen from this model that the ZSCCs consist of conduction, switching, and hybrid components. Based on the aforementioned analysis, an original sharing neutral bus structure is proposed to eliminate the conduction ZSCCs. With regard to the switching ZSCCs composed of high-frequency and low-frequency harmonics, modified LCL filters are proposed to eliminate the former, and zero-sequence control loops are put forward to suppress the latter. Furthermore, the proposed schemes are also proven to be effective to elimination of the hybrid ZSCCs. Experimental results validate the developed models and the proposed ZSCC elimination schemes.

Generation of intense high-order vortex harmonics.

Abstract: This Letter presents for the first time a scheme to generate intense high-order optical vortices that carry orbital angular momentum in the extreme ultraviolet region based on relativistic harmonics from the surface of a solid target. In the three-dimensional particle-in-cell simulation, the high-order harmonics of the high-order vortex mode is generated in both reflected and transmitted light beams when a linearly polarized Laguerre-Gaussian laser pulse impinges on a solid foil. The azimuthal mode of the harmonics scales with its order. The intensity of the high-order vortex harmonics is close to the relativistic region, with the pulse duration down to attosecond scale. The obtained intense vortex beam possesses the combined properties of fine transversal structure due to the high-order mode and the fine longitudinal structure due to the short wavelength of the high-order harmonics. In addition to the application in high-resolution detection in both spatial and temporal scales, it also presents new opportunities in the intense vortex required fields, such as the inner shell ionization process and high energy twisted photons generation by Thomson scattering of such an intense vortex beam off relativistic electrons.

Analysis and Calculation of DC-Link Current and Voltage Ripples for Three-Phase Inverter With Unbalanced Load

Abstract: In this paper, an analysis and calculation of the dc-link current and voltage ripples are presented for a three-phase inverter with unbalanced load. A comparison of the dc-link average and root-mean-square (rms) currents between considering and ignoring high frequency harmonics of the output current is drawn. It is shown that high frequency harmonic currents have little effect on the dc-link current, and therefore, they can be ignored. Based on the symmetrical components method, the dc-link average and harmonic rms currents are derived, and the dc-link voltage ripple is compared between the balanced and unbalanced loads. It can be found that the dc-link current and voltage ripples consist of not only high frequency harmonics but also the double fundamental frequency harmonic, and the voltage ripple is independent of the positive-sequence component and determined by the negative-sequence component, under the unbalanced load. Experimental results are shown to verify the accuracy of the theoretical analysis.

Low Space Harmonics Cancelation in Double-Layer Fractional Slot Winding Using Dual Multiphase Winding

Abstract: One of the main drawbacks of nonoverlapped coils in fractional slot concentrated winding permanent magnet (PM) machines are the high eddy current losses in both rotor core and permanent magnets induced by the asynchronous harmonics of the armature reaction field. It has been shown in the literature that the reduction of low space harmonics can effectively reduce the rotor eddy current losses. This paper shows that employing a combined star-delta winding to a three-phase PM machine with fractional slot windings and with a number of slots equal to 12, or its multiples, yields a complete cancellation to the fundamental magneto-motive force (MMF) component, which significantly reduces the induced rotor eddy current. Besides, it offers a slight increase in machine torque density. A case study on the well-known 12-slot/10-pole PM machine is conducted to explore the proposed approach. With the same concept, the general n-phase PM machine occupying 4n slots and with a dual n-phase winding is then proposed. This configuration offers a complete cancelation of all harmonics below the torque producing MMF component. Hence, the induced eddy currents in both rotor core and magnets are significantly reduced. The winding connection and the required number of turns for both winding groups are also given. The concept is applied to a 20-slot/18-pole stator with a dual five-phase winding, where the stator winding is connected as a combined star/pentagon connection. The proposed concept is assessed through a simulation study based on 2-D finite element analysis.

Performance Comparison of Variational Mode Decomposition over Empirical Wavelet Transform for the Classification of Power Quality Disturbances Using Support Vector Machine

Abstract: This work considers the classification of power quality disturbances based on VMD (Variational Mode Decomposition) and EWT (Empirical Wavelet Transform) using SVM (Support Vector Machine). Performance comparison of VMD over EWT is done for producing feature vectors that can extract salient and unique nature of these disturbances. In this paper, these two adaptive signal processing methods are used to produce three Intrinsic Mode Function (IMF) components of power quality signals. Feature vectors produced by finding sines and cosines of statistical parameter vector of three different IMF candidates are used for training SVM. Validation for six different classes of power qualities including normal sinusoidal signal, sag, swell, harmonics, sag with harmonics, swell with harmonics is performed using synthetic data in MATLAB. Classification results using SVM shows that VMD outperforms over EWT for feature extraction process and the classification accuracy is tabled.

Control strategies of grid interfaced wind energy conversion system: An overview

Abstract: Wind energy conversion system (WECS) is interfaced with the utility system through power electronic converters which plays an important role in the integration of wind power into the electric grid. The main power quality disturbances due to integration of WECS to grid are variation in power and harmonics. To maintain grid synchronization and to keep total harmonic distortion (THD) within operational limits, appropriate control schemes are required for the grid side converter. The main objective of grid side controller is to control the power delivered to the grid, grid synchronization, to supply high quality power to grid and to meet grid code compliance. In this paper control schemes used in grid interfaced wind energy conversion system for generator side and grid side converter control, are reviewed thoroughly. The paper presents a comparative study of rotor flux oriented control and direct torque control (DTC) techniques applied in generator side converter of permanent magnet synchronous generator (PMSG) drives for wind turbine application. For grid side converter, various control schemes are developed mainly based on voltage oriented control (VOC) or on direct power control (DPC). The performance of VOC based control system basically depends on method applied for current control. A comparative study is done among them and findings are tabulated. Integration requirements of wind turbine to grid, grid synchronization and requirement of monitoring unit are also discussed.

Molecular harmonic extension and enhancement from H 2 + ions in the presence of spatially inhomogeneous fields

Abstract: Molecular high-order harmonic generation from the ${{\mathrm{H}}_{2}}^{+}$ ion driven by spatial inhomogeneous fields consisting of the chirped pulse and a terahertz pulse has been theoretically investigated by numerically solving the non--Born-Oppenheimer time-dependent Schr\"odinger equation. It shows that with the introduction of the chirp as well as the spatial inhomogeneity of the pulse, not only the harmonic cutoff is remarkably extended, but also the single short quantum path is selected to contribute to the harmonic spectra. Moreover, through investigation the effects of the laser and the molecular parameters on the inhomogeneous harmonic generation, we found 1.92- and 3.3-dB enhanced fields for the chirp-free and chirped inhomogeneous pulses, respectively. Isotopic effect shows that intense harmonics can be generated from the lighter molecule. Furthermore, with the enhancement of the initial vibrational state and by properly adding a terahertz controlling pulse, the harmonic yield is enhanced by almost five orders of magnitude compared with the initial single chirped case. As a result, a 362-eV supercontinuum (which corresponds to a 4.0-dB laser field enhancement) with five orders of magnitude improvement is obtained. Finally, by properly superposing the harmonics, a series of intense extreme ultraviolet pulses with durations from 22 to 52 as can be produced.

Interband Bloch oscillation mechanism for high-harmonic generation in semiconductor crystals

Abstract: High harmonic generation in semiconductors is analyzed for high mid-infrared laser intensities for which the electron-hole pair is driven beyond the first Brillouin zone and exhibits Bloch oscillations. We find that even a two-band analysis exhibits second and higher plateaus. Whereas the first plateau is shown to be consistent with high harmonic generation through electron-hole recollision, the higher plateaus arise from dynamic Bloch oscillations; however, the driving process is interband in nature, in contrast to the generally accepted intraband Bloch oscillation mechanism. Energy conservation is fulfilled, as harmonics beyond the first plateau come from a cascaded nonlinearity.

Current Harmonics Compensation Based on Multiresonant Control in Synchronous Frames for Symmetrical $n$ -Phase Machines

Abstract: Low-order odd current harmonics arise in practical multiphase drives due to machine and converter nonlinear behavior (e.g., deadtime and flux saturation). If the windings are distributed, some harmonics cause torque ripple, whereas others produce losses. The latter is aggravated by the small impedance in the no-torque subspaces. Current harmonics can be compensated without steady-state error by proportional–integral controllers in multiple synchronous frames (SFs); however, a heavy computational load is required. In three-phase systems, the computational burden of this multiple SF (MSF) scheme is often avoided by implementing instead resonant controllers (RCs) tuned at the harmonics that are multiples of six in an SF rotating with the fundamental frequency. A similar structure has been proposed for nonlinearities compensation in asymmetrical six-phase machines. This paper extends this multiple RC (MRC) strategy to symmetrical machines of any phase number. The optimum frequencies for the RCs and for the SF in each plane, so that the number of RCs is minimized, are established. Then, the computational load of the resulting generic MRC scheme is assessed and compared with that of the MSF structure. The conditions in which the former is particularly preferable over the latter are identified. Experimental results are provided.