Sub-nanosecond UV filaments and their applications for remote spectroscopy and high-voltage discharges

Intensive investigations during the past two decades have focused on potential applications of near-infrared (NIR) femtosecond filament for remote spectroscopy. The short length (less than 1 m) and low energy (only several millijoule) of a single NIR filament limit these applications. Long-pulse UV filaments have therefore been proposed at UNM to overcome such limitations. This dissertation describes our investigation and optimization of the high-power UV source, focusing on details of the generation and characterization of the generated pulses, as well as the applications of UV filaments for remote sensing and high-voltage discharge. On the topic of pulse generation, a 266 nm UV system delivering laser pulses below 200 ps with up to 0.4 J per pulse at 1.25 Hz repetition rate is developed. Two aspects of the laser source are closely investigated. On the one hand, the spatio-temporal profile of the laser pulses that has been overlooked for decades is systematically studied. It is shown that a curved energy front, i.e., pulses away from the beam center delayed from the center pulse, originates from the Q-switched unstable cavity Nd:YAG oscillator and evolves during the processes of laser amplification and pulse compression. To eliminate the energy front curvature, a possible solution is proposed based on the numerical simulation. On the other hand, a long existing debate on the minimum pulse duration that can be achieved through stimulated Brillouin scattering (SBS) pulse compression is resolved by this work. It is demonstrated that the lower limit of the compressed pulse duration is not set by the phonon lifetime of the SBS medium. The energy exchange between the pump and compressed Stokes pulses is responsible for the pulse compression below phonon lifetime. Next, using the newly developed powerful and stable UV source, the generation of UV filaments in air is studied. It is shown that, when focused by a 3 m lens, a single filament is generated inside the laboratory, while multiple filaments are observed in an open environment with a 9 m focusing lens. Detailed characterization of the filament spatial profile and the conductivity of the plasma channel created by the filament are performed. Two applications of the UV filaments are investigated. The single UV filament is applied to spectroscopy studies, including both Raman and Laser Induced Breakdown Spectroscopy (LIBS). A UV filament is shown to be very efficient in exciting forward stimulated Raman scattering (SRS) in gases. Backward emission of SRS signal, which could be utilized for remote sensing, has not been observed. However, a side experiment carried out in water discloses a new mechanism of driving efficient backward SRS generation, which can possibly be employed in the case of gas medium. A second study with the single filament is carried out for LIBS. The dynamics of self-absorption dip in LIBS spectrum is investigated, which can be further applied for the high resolution spectroscopy. The last application is filament-induced high-voltage discharge. A fully guided 40 cm long discharge is demonstrated with the UV filament alone, at 1/2 the self-breakdown voltage in air. Two additional lasers are tested to improve the discharge triggering by photo-detaching oxygen negative ions and heating the plasma. The anticipated improvement in reducing the discharge delay or enhancing the discharge probability has not been observed

HV discharge acceleration by sequences of UV laser filaments with visible and near-infrared pulses

We investigate the triggering and guiding of DC high-voltage discharges over a distance of 37 cm by filaments produced by ultraviolet (266 nm) laser pulses of 200 ps duration. The latter reduce the breakdown electric field by half and allow up to 80% discharge probability in an electric field of 920 kV/m. This high efficiency is not further increased by adding nanosecond pulses in the Joule range at 532 nm and 1064 nm. However, the latter statistically increases the guiding length, thereby accelerating the discharge by a factor of 2. This effect is due both to photodetachment and to the heating of the plasma channel, that increases the efficiency of avalanche ionization and reduces electron attachment and recombination.Comment: 12 pages, 6 figure

Fast electromagnetic transient simulation methods and prospects of high-frequency isolated power electronics transformers

With the increasing uptake of distributed renewable energy and low-carbon technologies (e.g. energy storage and electric vehicle), the conventional AC distribution network is transferring to hybrid AC/DC or pure DC. Power electronics transformers (PETs), also known as solid-state transformers, are multifunctional of integrating distributed generation, regulating bidirectional power flow, and achieving grid interconnection, reactive power compensation, harmonic control, etc. Therefore, PETs can serve as key interfaces for energy conversion in future distribution networks. At present, a number of medium- and low-voltage distribution networks using PETs have commissioned in China, such as the Xiaoertai substation in Zhangbei and Tangjia Bay three-terminal DC distribution network in Zhuhai. The off-line and real-time electromagnetic transient (EMT) simulation studies are of great significance for system-level analysis and prototype development of PETs, which should be investigated timely

Fast electromagnetic transient simulation methods and prospects of high-frequency isolated power electronics transformers

With the increasing uptake of distributed renewable energy and low-carbon technologies (e.g. energy storage and electric vehicle), the conventional AC distribution network is transferring to hybrid AC/DC or pure DC. Power electronics transformers (PETs), also known as solid-state transformers, are multifunctional of integrating distributed generation, regulating bidirectional power flow, and achieving grid interconnection, reactive power compensation, harmonic control, etc. Therefore, PETs can serve as key interfaces for energy conversion in future distribution networks. At present, a number of medium- and low-voltage distribution networks using PETs have commissioned in China, such as the Xiaoertai substation in Zhangbei and Tangjia Bay three-terminal DC distribution network in Zhuhai. The off-line and real-time electromagnetic transient (EMT) simulation studies are of great significance for system-level analysis and prototype development of PETs, which should be investigated timely

The First Example of Tb-3-Containing Metallopolymer-Type Hybrid Materials with Efficient and High Color-Purity Green Luminescence

In the series of homo-leptic trinuclear complexes {[Ln(3)(L)(4)Cl-4(MeOH)(H2O)]center dot Cl} (Ln = La, 1; Ln = Eu, 2; Ln = Tb, 3 or Ln = Gd, 4) self-assembled from the allyl-modified benzimidazole-type ligand HL (4-allyl-2-(1H-benzo[d]imidazol-2-yl)-6-methoxyphenol) and LnCl(3)center dot 6H(2)O, a suitable energy level match endows efficient green luminescence (Phi(overall) = 72%) of Tb-3-arrayed complex 3. The copolymerization between each of these complex monomers 1-4 and C=C-containing MMA (methyl methacrylate) or NBE (norbornene) shows that degradative chain transfer of the terminal four flexible allyl groups within restrains their radical polymerization with MMA while it does not hinder their effective ring-opening metathesis polymerization (ROMP) with NBE. Thus, two kinds of PMMA-supported doping hybrid materials 1@PMMA, 2@PMMA, 3@PMMA and 4@PMMA and PNBE-supported metallopolymer-type hybrid materials Poly( NBE-1), Poly(NBE-2), Poly(NBE-3) and Poly(NBE-4) are obtained, respectively. Especially for both 3@PMMA and Poly(NBE-3) with high color-purity characteristic green emission of Tb3+ ions, improved physical properties including significantly enhanced luminescence (Phi(overall) = 76% or 83%) are observed, and covalent-bonding endows a higher-concentration self-quenching as compared to physical doping.National Natural Science Foundation 21373160, 91222201, 21173165Program for New Century Excellent Talents in University from the Ministry of Education of China NCET-10-0936Doctoral Program of Higher Education 20116101110003Science and Technology and Innovation Project of Shaanxi Province 2012KTCQ01-37Graduate Innovation and Creativity Fund (Visiting Learner) of Northwest University in P. R. ChinaChemistr

Dual harmonic injection for reducing the sub-module capacitor voltage ripples of hybrid MMC

Reducing the capacitor voltage ripples of the half-bridge sub-modules (HBSM) and full-bridge sub-modules (FBSM) in a hybrid modular multilevel converter (MMC) is expected to reduce the capacitance, volume and costs. To address this issue, this paper proposes a dual harmonic injection method which injects the second harmonic circulating current and third order harmonic voltage into the conventional MMC control. Firstly, the mathematical model of the proposed control is established and analyzed. Then, the general strategy of determining the amplitude and phase angle of each injection component is proposed to suppress the fluctuations of the fundamental and double frequency instantaneous power. The proposed strategy can achieve the optimal power fluctuation suppression under various operating conditions, which also has the advantage of reducing the voltage fluctuation difference between HB and FB SMs. The correctness and effectiveness of the proposed strategy are verified in simulations in PSCAD/EMTDC

Modeling for complex modular power electronic transformers using parallel computing

The modular power electronic transformer (PET) faces difficulty carrying out microsecond-level electromagnetic transient (EMT) simulations. This paper provides a high-speed and high-precision simulation method capable of eliminating the internal nodes and reducing the order of the nodal admittance matrix. Meanwhile, the parallel computing is integrated into the whole solution process, which achieves a significant simulation speedup. A physical prototype is established to prove that the detailed model is sufficient to reflect the dynamics of physical devices. Moreover, simulations in PSCAD/EMTDC are carried out to compare the proposed method with the detailed model in terms of accuracy and time efficiency. Simulation results show that the proposed method is accurate to simulate the external and internal dynamics of PET with hundreds of times simulation speed acceleration

Accelerated electromagnetic transient (EMT) equivalent model of solid-state transformer

Accurate and efficient electromagnetic transient (EMT) simulation of various types of solid-state transformers (SST) is extremely time-consuming due to the complex module structure, flexible topology connections, large number of electrical nodes and simulation time-steps limited in the range of micro-seconds. Therefore, it is urgent to develop the EMT equivalent modelling and fast simulation of SSTs for system level studies. Taking the modular multilevel converter (MMC) based SST as an example, this paper proposes an accelerated EMT model which focuses on the equivalence of the dual active bridge (DAB) based high-frequency link (HFL) in the SST. Compared with the existing algorithms, two critical factors of the proposed method that contribute the most to the efficiency improvement are the preprocessing of the nodal admittance equation and the conversion of the short-circuit admittance parameters. The proposed model is verified in PSCAD/EMTDC by comparing it with the detailed EMT model. The results show that the accelerated model is one to two orders of magnitude faster than the detailed model without sacrificing the accuracy. The experiment validation also confirms the validity of the proposed model

BacHBerry: BACterial Hosts for production of Bioactive phenolics from bERRY fruits

BACterial Hosts for production of Bioactive phenolics from bERRY fruits (BacHBerry) was a 3-year project funded by the Seventh Framework Programme (FP7) of the European Union that ran between November 2013 and October 2016. The overall aim of the project was to establish a sustainable and economically-feasible strategy for the production of novel high-value phenolic compounds isolated from berry fruits using bacterial platforms. The project aimed at covering all stages of the discovery and pre-commercialization process, including berry collection, screening and characterization of their bioactive components, identification and functional characterization of the corresponding biosynthetic pathways, and construction of Gram-positive bacterial cell factories producing phenolic compounds. Further activities included optimization of polyphenol extraction methods from bacterial cultures, scale-up of production by fermentation up to pilot scale, as well as societal and economic analyses of the processes. This review article summarizes some of the key findings obtained throughout the duration of the project