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Penicillenols belong to the family of tetramic acids with anticancer and antibacterial activities. Here, we report the discovery of the biosynthetic gene cluster (

The Diels-Alder cycloaddition is one of the most powerful approaches in organic synthesis and is often used in the synthesis of important pharmaceuticals. Yet, strictly controlling the stereoselectivity of the Diels-Alder reactions is challenging, and great efforts are needed to construct complex molecules with desired chirality via organocatalysis or transition metal strategies. Nature has evolved different types of enzymes to exquisitely control cyclization stereochemistry, however, most of the reported Diels-Alderases have been shown to only facilitate the energetically favorable diastereoselective cycloadditions. Here we report the discovery and characterization of CtdP, as a member of a new class of bifunctional oxidoreductase/Diels-Alderase, which was previously annotated as an NmrA-like transcriptional regulator. We demonstrate that CtdP catalyses the inherently unfavored cycloaddition to form the bicyclo[2,2,2]diazaoctane scaffold with a strict α-anti-selectivity. Guided by computational studies, we reveal a NADP(+)/NADPH-dependent redox mechanism for the CtdP-catalysed inverse electron demand Diels-Alder cycloaddition, which serves as the first example of a bifunctional Diels-Alderase that utilizes this mechanism.

Genome mining of cryptic natural products (NPs) remains challenging, especially in filamentous fungi, owing to their complex genetic regulation. Increasing evidence indicates that several epigenetic modifications often act cooperatively to control fungal gene transcription, yet the ability to predictably manipulate multiple genes simultaneously is still largely limited. Here, we developed a multiplex base-editing (MBE) platform that significantly improves the capability and throughput of fungal genome manipulation, leading to the simultaneous inactivation of up to eight genes using a single transformation. We then employed MBE to inactivate three negative epigenetic regulators combinatorially in Aspergillus nidulans, enabling the activation of eight cryptic gene clusters compared to the wild-type strains. A group of novel NPs harboring unique cichorine and polyamine hybrid chemical scaffolds were identified, which were not reported previously. We envision our scalable and efficient MBE platform can be readily applied in other filamentous fungi for genome mining of novel NPs, providing a powerful approach for the exploitation of fungal chemical diversity.

Atmospheric pressure plasma jet (APPJ) arrays have shown a potential in a wide range of applications ranging from material processing to biomedicine. In these applications, targets with complex three-dimensional structures often easily affect plasma uniformity. However, the uniformity is usually crucially important in application areas such as biomedicine, etc. In this work, the flow and electric field collaborative modulations are used to improve the uniformity of the plasma downstream. Taking a two-dimensional sloped metallic substrate with a 10° inclined angle as an example, the influences of both flow and electric field on the electron and typical active species distributions downstream are studied based on a multi-field coupling model. The electric and flow fields modulations are first separately applied to test the influence. Results show that the electric field modulation has an obvious improvement on the uniformity of plasma while the flow field modulation effect is limited. Based on such outputs, a collaborative modulation of both fields is then applied, and shows a much better effect on the uniformity. To make further advances, a basic strategy of uniformity improvement is thus acquired. To achieve the goal, an artificial neural network method with reasonable accuracy is then used to predict the correlation between plasma processing parameters and downstream uniformity properties for further improvement of the plasma uniformity. An optional scheme taking advantage of the flexibility of APPJ arrays is then developed for practical demands.

Atmospheric pressure plasma jets (APPJs) have attracted considerable interests due to their low-temperature and easily controlled features. In order to cover a larger area for broader applications, spatially extended atmospheric plasma arrays are designed and constructed by a number of packed plasma jets. Despite their outstanding stability and flexibility, it remains a challenge for the jets to form large-scale uniformity on downstream substrates due to complex electrical and hydrodynamic interactions. In this paper, we use a 2-D temporally evolving code coupling electric, flow, and temperature fields, as well as chemical reactions to study synergistic effects of the multi-fields coupling on discharge and propagation characteristics of the jet array based on a USim software framework. The influence of gas flow and a substrate downstream on the plasma jets dynamics and uniformity are investigated by the model. Also, essential strategies for uniformity modulation are acquired. Results show that the balanced evolution of the ionization process and the effective merging of discharge channels could be realized by modulating the gas flow field. Therefore, a homogeneous distribution of plasma downstream can be obtained by adjusting the gas flow field properly, which provides a new efficient way for the uniformity control in the field of plasma processing.

Tunability is an important feature for the filter. As a special electromagnetic medium, the plasma has its permittivity being altered in a wideband range. In this work, based on the surface plasmon polaritons of plasma–dielectric–plasma waveguides, we propose a double-stubs structure submerged in a gaseous discharge plasma medium to realize tunable filtering properties in the giga-hertz (GHz) regime. The finite element method is applied to numerically compute filtering properties. The coupled mode theory and orthogonal design method are introduced to verify simulation results and estimate the effect of simulation parameters on the filtering properties. It is shown that the height of two stubs has the most important influence on filtering performance. Although once the filter is fabricated, its size cannot be modified, one can, nevertheless, vary the plasma frequency to effectively adjust the plasma frequency for the best filtering. Thus, such a plasma-modified filter provides a feasible scheme to dynamically adjust the filtering frequency.

Tailored voltage waveforms, formed by a fundamental frequency waveform superimposed with higher harmonics, show promise in realizing independent control and optimization of plasma parameters in conventional atmospheric dielectric barrier discharge systems (DBDs). In this paper, a self-consistent fluid model developed by a semi-kinetic treatment of electrons is applied to study the dependency of the electron energy and density distributions on the number of applied higher harmonics, applied fundamental frequency, and contributions from secondary electron emission—in a DBD system driven by tailored voltage waveforms. The mechanisms for achieving selective control over the modulated parameters are proposed, which allow for optimal selection of applied parameters for various downstream applications. This work exhibits dual-advantages for its novelty in presenting practical methods to modulate atmospheric plasma parameters, while in-depth analysis and discussions reveal underlying theoretical principles for the modulation of plasma parameters in atmospheric pressure discharges driven by tailored voltage waveforms. Published version

An innovative, hybrid, inductively coupled, dual-antenna 13.56-MHz radio frequency source of dense low-temperature plasma was developed and experimentally investigated. The electrical and plasma properties of the conventional single-coil inductively coupled plasma source and the upgraded dual-antenna configuration were comparatively studied. As compared to the conventional setup, the dual antenna hybrid scheme maintains the low-temperature plasma with significantly higher electron densities reaching to $1.85\times 10^{12}$ cm−3 at a low electron temperature of 1.7 eV over a large area, and much faster transits to a steady-state plasma. It was also found that the dual-coil system heats electrons more effectively and produces a synergistic effect on the enhancement of power transfer efficiency to the discharge. Significant advantages warrant the efficient employment of this system in many techniques requiring cost effective, convenient, and efficient systems for low electron temperature high-density plasma-based applications.

The performance of atmospheric pressure dielectric barrier discharge (DBD) driven by dual-frequency sources of 200 kHz/13.56 MHz has been investigated numerically here in a one-dimensional fluid model. By varying the ratio of dual frequency excitation amplitudes, strong nonlinear coupling in the discharge is observed, with the electron density rising significantly due to the synergistic effect governing the ionization process. Moreover, it is shown that the phase shift between the dual frequencies also has an influence on the electron temperature and density. As to different high frequency components, the modulation of the electron temperature and density induced by the phase-shift also varies significantly. Those results then demonstrated the possibility of applying amplitude and/or phase modulations in a wide operation window to optimize the plasma parameters in dual-frequency DBDs with fixed frequencies.The performance of atmospheric pressure dielectric barrier discharge (DBD) driven by dual-frequency sources of 200 kHz/13.56 MHz has been investigated numerically here in a one-dimensional fluid model. By varying the ratio of dual frequency excitation amplitudes, strong nonlinear coupling in the discharge is observed, with the electron density rising significantly due to the synergistic effect governing the ionization process. Moreover, it is shown that the phase shift between the dual frequencies also has an influence on the electron temperature and density. As to different high frequency components, the modulation of the electron temperature and density induced by the phase-shift also varies significantly. Those results then demonstrated the possibility of applying amplitude and/or phase modulations in a wide operation window to optimize the plasma parameters in dual-frequency DBDs with fixed frequencies.

Dielectric barrier discharges (DBD) as a common method to generate plasmas with high density, low gas temperature, and abundant active particle under atmospheric pressure have been widely investigated in the past decades. Among these researches, the investigation on plasma parameters modulation represents a novel and budding focus. According to our previous works of DBD driven by dual-frequency, it has been demonstrated to provide a possible approach of controlling both averaged electron density and gas temperature independently based on the nonlinear frequency coupling effect. In this work, we used one-dimensional fluid model with semi-kinetic method, systemically studied the nonlinear behavior of the dual radio-frequency driven DBD. In term of the results of effective electron energy distribution function (EEDF) and electron impact ionization rate, it is found that the variations of power density and phase relationship provide separate control over the electron density, mean electron energy and gas temperature. Moreover, mode transitions are discussed in this paper. It is shown that there exist three kinds of discharge modes, which are governed by the nonlinear dynamics in the plasma sheath. Among which, the ionization in α mode is determined by the nonlinear coupling electron heating and local momentary charge density. While in γ mode, the ionization is caused mainly by the electron avalanches. In summary, the dual radio-frequency applied in DBD system is found to generate a nonlinear synergistic effect between two frequencies, which can provide a possible approach to enhance control over the plasma parameters.

Resonant absorption is a process that electromagnetic (EM) energy is converted to plasma energy with a mode conversion on the resonant layer where the incident EM wave frequency equals to the local frequency of a plasma normal modes. With a finite collision between charged and background neutral particles in a plasma, the plasma oscillation is dissipated to widen the resonance layer and heat the plasma. In this work a modified scattering matrix methods (SMM) are applied to study the effects of the collision frequency, incident angle, and plasma thickness on resonant absorption. We analyze the energy absorption caused by resonance in comparison with collisional absorption for different parameters. It is found that the resonant absorption dissipates about nearly half of the incident EM energy in an overdense inhomogeneous plasma when the collision is weak, and the rest half portion is reflected. If the collision is strong, however, the collisional absorption is then more significant than the resonant and affects the entire wave propagation process.

A novel technique based on sub-wavelength plasma structure effects on enhancement of RF communication signals on a receiving antenna is carried out in this paper in laboratory experiments and analyzed by corresponding numerical simulations. Considerable intensification on receiving signal gain up to ~10 dB in comparison with that without the plasma modulation is observed experimentally in ~1 GHz RF band, with an effective enhancement bandwidth of ~340 MHz and the fractional bandwidth of ~34%. Then, the optimal modulation parameters of plasma are further studied by a numerical simulation. It is shown that the number density, the layer thickness, and the collision frequency of the plasma, as well as the relative distance between the plasma layer and antenna synergistically affect the modulation. Compared to the metallic antenna with the same overall dimension, the modulated antenna covered by the sub-wavelength plasma structure features higher receiving efficiency and lower radar cross section in the studied RF band. The mechanism of the reception enhancement is further revealed by analyzing characteristics of electromagnetic scattering and electric field distribution in the sub-wavelength plasma layer. The results then exhibit scientific significance and application potential of sub-wavelength plasma modulation on compact receiving antennas with higher performance and better feature of radar stealth.

The dielectric barrier discharge (DBD) is a promising technology to generate high density and uniform cold plasmas in atmospheric pressure gases. The effective independent tuning of key plasma parameters is quite important for both application-focused and fundamental studies. In this paper, based on a one-dimensional fluid model with semi-kinetics treatment, numerical studies of ionization asymmetry effects on the properties modulation of atmospheric DBD sustained by tailored voltage waveforms are reported. The driving voltage waveform is characterized by an asymmetric-slope fundamental sinusoidal radio frequency signal superimposing one or more harmonics, and the effects of the number of harmonics, phase shift, as well as the fluctuation of harmonics on the sheath dynamics, impact ionization of electrons and key plasma parameters are investigated. The results have shown that the electron density can exhibit a substantial increase due to the effective electron heating by a spatially asymmetric sheath structure. The strategic modulation of harmonics number and phase shift is capable of raising the electron density significantly (e.g., nearly three times in this case), but without a significant increase in the gas temperature. Moreover, by tailoring the fluctuation of harmonics with a steeper slope, a more profound efficiency in electron impact ionization can be achieved, and thus enhancing the electron density effectively. This method then enables a novel alternative approach to realize the independent control of the key plasma parameters under atmospheric pressure.

The modulation and enhancement effect of sub-wavelength plasma structures on compact antennas exhibits obvious technological advantage and considerable progress. In order to extend the availability of this technology under complex and actual environment with inhomogeneous plasma structure, a numerical simulation analysis based on finite element method has been conducted in this paper. The modulation function of the antenna radiation with sub-wavelength plasma layer located at different positions was investigated, and the inhomogeneous plasma layer with multiple electron density distribution profiles were employed to explore the effect of plasma density distribution on the antenna radiation. It has been revealed that the optical near-field modulated distance and reduced plasma distribution are more beneficial to enhance the radiation. On the basis above, an application-focused research about communication through the plasma sheath surrounding a hypersonic vehicle has been carried out aiming at exploring an effective communication window. The relevant results devote guiding significance in the field of antenna radiation modulation and enhancement, as well as the development of communication technology in hypersonic flight.

Methane coupling under low temperature plasmas at atmospheric pressure is a green process by use of renewable sources of energy. In this study, CH4+H2 discharge plasma was on-line diagnosed by optical emission spectra so as to characterize the discharge system and to do spade work for the optimization of the technical parameters for future commercial production of methane coupling under plasmas. The study was focused on a calculation method for the online diagnosis of the electron excitation temperature in CH4+H2 discharge plasma at atmospheric pressure. The diagnostic method is easy, efficient and fairly precise. A serious error in a literature was corrected during the reasoning of its series of equations formerly used to calculate electron temperatures in plasmas.

The technology of radio frequency (RF) radiation intensification for radio compact antennas based on modulation and enhancement effects of sub-wavelength plasma structures represents an innovative developing strategy. It exhibits important scientific significance and promising potential of broad applications in various areas of national strategic demands, such as electrical information network and microwave communication, detection and control technology. In this paper, laboratory experiments and corresponding analyses have been carried out to investigate the modulation and enhancement technology of sub-wavelength plasma structure on the RF electromagnetic radiation. An application focused sub-wavelength plasma-added intensification up to ~7 dB higher than the free-space radiation is observed experimentally in giga-hertz (GHz) RF band. The effective radiation enhancement bandwidth covers from 0.85 to 1.17 GHz, while the enhanced electromagnetic signals transmitted by sub-wavelength plasma structures maintain good communication quality. Particularly, differing from the traditional RF electromagnetic radiation enhancement method characterized by focusing the radiation field of antenna in a specific direction, the sub-wavelength plasma-added intensification of the antenna radiation presents an omnidirectional enhancement, which is reported experimentally for the first time. Corresponding performance characteristics and enhancement mechanism analyses are also conducted in this paper. The results have demonstrated the feasibility and promising potential of sub-wavelength plasma modulation in application focused RF communication, and provided the scientific basis for further research and development of sub-wavelength plasma enhanced compact antennas with wide-range requests and good quality for communication.

Plasma ionization, excitation, mode transitions and associated electron heating mechanisms in atmospheric pressure dielectric barrier discharges (DBD) driven by dual radio frequency sources are investigated in this paper. The electrons are found to be heated mainly by the high frequency component in the plasma bulk when discharged in α mode. On the contrary, the low frequency component is primarily responsible for heating in the sheath which is caused by intense motion in the sheath. It was also found that variation of the lower frequency component ratio could effectively modulate the electron energy distribution as determined from time averaged EEDF. The results above have demonstrated that the independent control of plasma parameters via non-linear synergistic effect between the dual frequency sources can be achieved through reasonable selection of processing parameters.

Sub-atmospheric pressure plasma slabs exhibit the feature of relatively high plasma number density and high collisional frequency between electrons and neutral gases, as well as similar thickness to the electromagnetic (EM) wavelength in communication bands. The propagation characteristics of EM waves in sub-atmospheric pressure plasma slabs are attracting much attention of the researchers due to their applications in the plasma antenna, the blackout effect during reentry, wave energy injection in the plasma, etc. In this paper, a numerical model with a one-dimensional assumption has been established and therefore, it is used for the investigations of the propagation characteristics of the EM waves in plasma slabs. In this model, the EM waves propagating in both sub-wavelength plasma slabs and plasmas with thicker slabs can be studied simultaneously, which is superior to the model with geometrical optics approximation. The influence of EM wave frequencies and collisional frequencies on the amplitude of th...

Dielectric barrier discharges (DBDs) provide a promising technology of generating non-equilibrium cold plasmas in atmospheric pressure gases. For both application-focused and fundamental studies, it is important to explore the strategy and the mechanism for enabling effective independent tuning of key plasma parameters in a DBD system. In this paper, we report numerical studies of effects of dual-frequency excitation on atmospheric DBDs, and modulation as well as separate tuning mechanism, with emphasis on dual-frequency coupling to the key plasma parameters and discharge evolution. With an appropriately applied low frequency to the original high frequency, the numerical calculation demonstrates that a strong nonlinear coupling between two frequencies governs the process of ionization and energy deposition into plasma, and thus raises the electron density significantly (e.g., three times in this case) in comparisons with a single frequency driven DBD system. Nevertheless, the gas temperature, which is mainly determined by the high frequency discharge, barely changes. This method then enables a possible approach of controlling both averaged electron density and gas temperature independently.

Propagation characteristics of electromagnetic (EM) waves in partially ionized plasma slabs are studied in this paper. Such features are significant to applications in plasma antennas, blackout of re-entry flying vehicles, wave energy injection to plasmas, and etc. We in this paper developed a theoretical model of EM wave propagation perpendicular to a plasma slab with a one-dimensional density inhomogeneity along propagation direction to investigate essential characteristics of EM wave propagation in nonuniform plasmas. Particularly, the EM wave propagation in sub-wavelength plasma slabs, where the geometric optics approximation fails, is studied and in comparison with thicker slabs where the geometric optics approximation applies. The influences of both plasma and collisional frequencies, as well as the width of the plasma slab, on the EM wave propagation characteristics are discussed. The results can help the further understanding of propagation behaviours of EM waves in nonuniform plasma, and applications of the interactions between EM waves and plasmas.

The two-dimensional spatially extended atmospheric plasma arrays by many parallel radio-frequency glow discharge plasma jets packed densely, represent a feature option of large-scale low-temperature atmospheric plasma technologies with distinct capability of directed delivery of reactive species and good insusceptibility to sample variations. However, it is still a challenge to form plasma jet with large area of uniform active species on a downstream substrate due to the complex interactions between individual jets. This paper proposes to numerically study the strategy and mechanism of control/modulation for the array discharge to produce two-dimensional plasma uniformity in the downstream working area. In this work, a two dimensional fluid model is employed to investigate the characteristics in the jet region of helium radio-frequency atmospheric-pressure glow discharge (RF APGD) with array generators. The influences of upstream discharge characteristics, gas flow and their cooperative effects on the distribution of species densities, gas temperatures and the uniformity of active species in the material treating area is studied, and the essential strategy for the modulation method is acquired. The results will be significant for deep understanding of coupling behaviors of multiple plasma plumes in the RF APGD array and applications of the technology.