Pulsed Regime Research Articles

Burst mode in a cooled packed-bed dielectric barrier discharge reactor for CO2 splitting * *(A manuscript offered to ‘ Plasma Research Express ’).

The so called ‘burst mode’, a rapidly pulsed power regime for dielectric barrier discharges (DBDs) has previously been reported to greatly improve the performance of an uncooled and unpacked DBD reactor for CO2 splitting. Here we explore the interaction between a DBD in burst mode and each of active cooling and a dielectric packing material (1.0–1.2 mm glass beads) in a coaxial, metal-dielectric DBD reactor for CO2 splitting. The effect of burst mode on CO2 conversion and energy efficiency under different combinations of cooling and packing were investigated and compared with those under continuous wave mode operation. The burst mode operation showed similar CO2 conversion and energy efficiency to continuous wave mode in the unpacked and uncooled configuration under the conditions investigated in this study. However, burst mode significantly outperformed continuous wave mode in the packed and uncooled configuration. When active cooling was employed, burst mode was found to provide only minor benefit or no benefit, depending on specific energy input (SEI), compared to continuous wave mode for each the packed and unpacked configurations. Maximum conversion and energy efficiency were achieved with both packing and active cooling across all SEI examined, and performance in this configuration was found to be relatively insensitive to the power delivery mode.

Influence of the Ar pressure on the structure of the NiTi foams produced by self-propagating high-temperature synthesis

The influence of the Ar pressure in the reactor on the structure and martensitic transformation of the NiTi foams produced by self-propagating high-temperature synthesis (SHS) was studied. It was shown that an increase in Ar pressure significantly affected the porous structure due to the process of combustion wave propagation changed from the stationary to the pulse regime. Regardless of the Ar pressure, the porous partitions included the NiTi phase as well as Ti2Ni, Ni3Ti2, Ni4Ti3 precipitates. An increase in Ar pressure decreased the transformation temperatures, which might be caused by an increase in the Ni concentration in the NiTi phase due to a decrease in the volume fraction of the Ni-rich precipitates.

Dynamics of Self-Pulsation in Gas-Centered Swirl Coaxial Injector: An Experimental Study

Experimental investigations on the self-pulsation dynamics in a gas-centered swirl coaxial atomizer are conducted using water and air. Experiments are performed in the pulsation regime by precisely varying the momentum flux ratio for different values of swirl number, recess length, and lip thickness. High-speed imaging is used to understand the underlying mechanism of the onset of pulsation. While the transition to the pulsation regime takes a route of intermittency in a nonrecessed injector, the transition in a recessed injector is seen to be spontaneous. The spray width fluctuates significantly in the pulsation regime, and hence the temporal variation in the spray width is used to characterize the pulsation frequencies. Three factors, the constrained shear layer instability between the liquid and the gas streams inside the recess region, the spread rate of the gas jet, and its interaction location with the liquid stream, are shown to dictate the presence of dominant pulsation frequencies and even their higher harmonics in many recessed configurations. It is further shown that the nondimensional pulsation frequency depends only on the lip thickness and is independent of the flow and other geometrical parameters of the injector considered in the present paper.

All-normal dispersion ytterbium-doped fiber laser mode locked by nonlinear multimode interference.

We report a mode-locking operation using a nonlinear multimode interference (MMI)-based saturable absorber in an ytterbium-doped fiber laser under an all-normal dispersion configuration. The MMI is observed in a step-index single mode-multimode-single mode (SMS) fiber structure. The intensity and wavelength-dependent transmission of the SMS structure has been studied numerically to understand the Kerr-induced saturable absorber properties of MMI in a step-index fiber. Numerical investigations show that presence of a narrow-band wavelength-tunable spectral filter is necessary for a nonlinear MMI-based saturable absorber to be effective for mode locking. A single-pulse mode-locking operation in the laser is realized at a very low pump power of ${\sim}{56.3}\;{\rm mW}$ at which the laser generates 180 ps duration pulses at ${\sim}{9}\;{\rm MHz}$ repetition rate with measured signal-to-noise ratio of ${\sim}{50}\;{\rm dB}$. This all-fiber integrated laser operates in multi-pulse and noise-like rectangular pulse regimes at increased pump power.

Large Mode Area Solid-Core Photonic Bandgap Yb-Doped Fiber With Hetero-Structured Cladding for Compact High-Power Laser Systems

We report on the design, realization and testing in pulsed amplification regime of a double-cladding, large mode area, polarization maintaining, solid core photonic bandgap fiber with Yb-doped core obtained from a Sol-Gel -made material. A specific hetero-structuration of the cladding has been applied to efficiently reject the higher-order modes, making the fiber single-mode in practice. When used as the last stage of a high power fiber amplifier, more than 90 W average power has been extracted at 1.03 μm wavelength with a slope efficiency of 75 % with respect to the absorbed pump power. These performances were obtained for a 6 m-long piece of fiber bent on a radius smaller than 12.5 cm. A mode field diameter of 35 μm is reported for the amplified beam with very good spatial quality and polarization extinction ratio.

1940 nm, 1966 nm and 2066 nm multi-wavelength CW and passively-Q-switched operation of L-shaped Tm3+:Lu2O3 ceramic laser in-band fiber-laser pumped at 1670 nm

A Tm3+:Lu2O3 ceramics laser with the L-shaped cavity in-band pumped by a fiber laser at 1670 nm was studied in CW and passively Q-switched multi-wavelength regimes. The CW laser operated at 2066 nm at the weak pump power and self-tuned to 1966 nm and further to 1940 nm at higher pump power. The maximum power in the CW regime at 1940 nm reached 9.5 W, while the average power of 3.6 W was registered in the Q-switched regime with the pulse repetition rate of 110–170 kHz and the pulse duration of 200–240 ns. The Q-switched operation was obtained using a Cr2+:ZnSe saturable absorber. Pulsed regime started at the shortest wavelength of 1940 nm and self-tuned to the double-wavelength of 1940 nm and 1966 nm at higher pump power.

A thermodynamic scaling law for electrically perturbed lipid membranes: Validation with steepest entropy ascent framework

Experimental evidence has demonstrated the ability of transient pulses of electric fields to alter mammalian cell behavior. Strategies with these pulsed electric fields (PEFs) have been developed for clinical applications in cancer therapeutics, in-vivo decellularization, and tissue regeneration. Successful implementation of these strategies involve understanding how PEFs impact the cellular structures and, hence, cell behavior. The caveat, however, is that the PEF parameter space (i.e., comprising different pulse widths, amplitudes, number of pulses) is large, and design of experiments to explore all possible combinations of pulse parameters is prohibitive from a cost and time standpoint. In this study, a scaling law based on the Ising model is introduced to understand the impact of PEFs on the outer cell lipid membrane so that an understanding developed in one PEF pulse regime may be extended to another. Combining non-Markovian Monte Carlo techniques to determine density-of-states with a novel non-equilibrium thermodynamic framework based on the principle of steepest entropy ascent, the applicability of this scaling model to predict the behavior of both thermally quenched and electrically perturbed lipid membranes is demonstrated. A comparison of the predictions made by the steepest-entropy-ascent quantum thermodynamic (SEAQT) framework to experimental data is performed to validate the robustness of this computational methodology and the resulting scaling law.

Wear performance and osteogenic differentiation behavior of plasma electrolytic oxidation coatings on Ti-6Al-4V alloys: Potential application for bone tissue repairs

Plasma electrolytic oxidation (PEO) can be used to obtain oxide coatings able to improve the biocompatibility, wear and corrosion properties of metal implants, due to complex morphology and the formation of oxides with the possibility of incorporating bioactive elements. This study aims at producing coatings on Ti-6Al-4V alloys with potential application for bone tissue repairs, evaluating cytocompatibility behavior and the potential for osteogenic differentiation, by using mesenchymal stem cells. Coatings have been produced on Ti6Al4V alloys by PEO with a calcium and phosphate-based electrolyte, by applying the potentials of 200 V (Ti-PEO200V) and 300 V (Ti-PEO300V), with a pulsed electrical regime. The SEM/FEG characterization shown that the samples obtained are typically porous, have different thicknesses and roughness, which were found to be dependent on the maximum applied voltage. The bioactive elements (Ca and P) were incorporated into the coatings and their concentrations grew with the maximum applied voltage, as indicated by RBS analysis. In addition, the same analysis indicated that the coatings were mainly composed of Ti oxides. XRD patterns confirmed that the coatings consist mainly of titanium oxides in the rutile and anatase phases. The tribological properties were evaluated by a ball-on-plate tribometer. Both samples show very similar tribological behavior. The wear resistance of Ti-PEO300V was more impaired by the microcracks present in its morphology and its great surface roughness than the Ti-PEO200V sample. Biological properties were assessed by studying the cytocompatibility with MSC cells. The coating obtained for the Ti-PEO200V sample showed a great cytocompatibility level and initial adhesion, as well as an innate ability for osteo-differentiation of MSCs, displaying considerable potential for usage as bone-repair applications.

Evaluation of interfacial pH change during water splitting at pulsed regime using finite element method

Electrochemical water splitting is a well-known reaction, investigated since 1869 due to its wide applicability for different purposes. Despite occurring at the electrode/solution interface, this set of reactions can be governed not only by interfacial properties but also by reactant transport from the solution toward the electrode surface. For water splitting specifically, the reaction mechanism itself leads to a pH change at the interface although the bulk value remains constant. In this work, we simulated such pH variations using the finite element method. The results show an interfacial pH change of up to 9 pH units. This modification in the reaction condition affects many related parameters such as electric power consumption, which increases. Thence, we investigated the effect of using an AC perturbation. In the conditions we have studied, it is observed an important delay in the pH change, although it always occurs.

Generation of intense coherent electromagnetic radiation during the interaction of a multi-terawatt laser pulse with a nanowire target* *Reported at the 4th International Conference on Ultrafast Optical Science (Russia, Moscow, FIAN, 28 September – 2 October 2020).

Generation of coherent radiation in the IR and terahertz ranges during the propagation of a multi-terawatt laser pulse along a nanowire target is investigated. In the process of interaction, dense electron bunches are displaced from the target and accelerated in the laser field, generating intense electromagnetic radiation. Three regimes of interaction can be realised depending on the duration and shape of the laser pulse. In the first regime, when the laser pulse is long enough (tens and hundreds of femtoseconds), electrons are only partially forced out of the target. The characteristics of the low-frequency part of the spectrum of the generated radiation are determined in this case by the duration of the laser pulse, as well as by its amplitude and target parameters (geometric dimensions and concentration of electrons). In the second regime, the laser pulse has a large amplitude and a steep rising edge (the amplitude of the first half-wave is of the order of the maximum pulse amplitude); as a result, most of the electrons are displaced from the target already at the initial moment of interaction. In this regime, unipolar and bipolar pulses with a duration of tens of laser field periods can be formed. Changing the target length makes it possible to control the period of field oscillations and their number in the generated radiation. In the intermediate regime of short laser pulses with an insufficiently steep rising edge, oscillations of the formed electron bunches can occur in the macroscopic Coulomb attraction field of the charged target, which gives rise to radiation with a frequency several times lower than that of laser radiation. In this case, the pulses of the generated radiation contain a few cycles of the field with decreasing amplitude and increasing frequency. Using numerical simulation in three regimes of interaction, the characteristics of IR and terahertz radiation are found, in particular, the pulse shapes, ranges of generated frequencies, amplitudes and angular distributions of radiation are determined. It is shown that the amplitude of the generated pulse can reach subrelativistic and relativistic values (the field strength is more than 1 TV m−1 at a frequency ten times lower than the laser radiation frequency), and the energy conversion efficiency can be of the order of one percent.

A high efficiency 10W MMIC PA for K-b and satellite communications

AbstractThis paper discusses the design steps and experimental characterization of a monolithic microwave integrated circuit (MMIC) power amplifier developed for the next generation of K-band 17.3–20.2 GHz very high throughput satellites. The technology used is a commercially available 100-nm gate length gallium nitride on silicon process. The chip was developed taking into account the demanding constraints of the spacecraft and, in particular, carefully considering the thermal constraints of such technology, in order to keep the junction temperature in all devices below 160°C in the worst-case condition (i.e., maximum environmental temperature of 85°C). The realized MMIC, based on a three-stage architecture, was first characterized on-wafer in pulsed regime and, subsequently, mounted in a test-jig and characterized under continuous wave operating conditions. In 17.3–20.2 GHz operating bandwidth, the built amplifier provides an output power >40 dBm with a power added efficiency close to 30% (peak >40%) and 22 dB of power gain.

Comparison of Biocompatible Coatings Produced by Plasma Electrolytic Oxidation on cp-Ti and Ti-Zr-Nb Superelastic Alloy

The paper compares the coatings produced by plasma electrolytic oxidation (PEO) on commercially pure titanium and a novel superelastic alloy Ti-18Zr-15Nb (at. %) for implant applications. The PEO coatings were produced on both alloys in the identical pulsed bipolar regime. The properties of the coatings were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS). The PEO process kinetics was modeled based on the Avrami theorem and Cottrell equation using a relaxation method. The resultant coatings contain TiO2, for both alloys, and NbO2, Nb2O5, ZrO2 for Ti-18Zr-15Nb alloy. The coating on the Ti-18Zr-15Nb alloy has a higher thickness, porosity, and roughness compared to that on cp-Ti. The values of the kinetic coefficients of the PEO process—higher diffusion coefficient and lower time constant for the processing of Ti-18Zr-15Nb—explain this effect. According to the electrochemical studies, PEO coatings on Ti-18Zr-15Nb alloy provide better corrosion protection. Higher corrosion resistance, porosity, and roughness contribute to better biocompatibility of the PEO coating on Ti-18Zr-15Nb alloy compared to cp-Ti.

The nonlinear optical loop mirror: soliton and noise-like pulse emission in a figure-eight fiber laser

In this article, a symmetrical nonlinear optical loop mirror (NOLM) exhibiting a polarization-dependent transmission is evaluated to generate optical pulse emission in a figure-eight fiber laser in the soliton and noise-like pulse (NLP) regimes. The NOLM structure relies on a 50:50 fiber coupler, a loop with highly twisted single-mode optical fiber and a quarter-wave retarder (QWR) to break the polarization asymmetry. The pulse operation regime is determined by properly adjusting the NOLM low-power transmission, which is easily realized by the rotation of the QWR angle. Soliton pulses of 1.48 ps pulse duration and peak power of 18 W were observed with a peak to peak separation of 1.25 µs, corresponding to a fundamental cavity repetition rate of 0.8 MHz. Moreover, by incrementing the NOLM low-power transmission, NLP emission is generated exhibiting a wide and smooth spectrum of 8.5 nm bandwidth.

Multiple mode-locked regimes of an Er/Yb double-clad fiber laser based on NPR

In this paper, we experimentally demonstrate generation of diverse structures of robust optical pulses obtained with a ring cavity passively mode-locked fiber laser based on a double-clad erbium-ytterbium doped fiber, and operating at net-anomalous cavity dispersion. A half-wave and a quarter-wave retarder plates allow adjustable saturable absorbing action by the nonlinear polarization rotation technique. Pulsed laser operation in quasi- continuous wave, molecules of solitons, and single-wavelength noise-like pulses (NLPs) regimes is obtained by properly adjusting the wave retarders. In the soliton and NLPs operating regimes, the central laser wavelength lies in the 1540 nm region. The obtained NLPs exhibit narrow peak coherence of ∼231 fs.

Numerical simulation of arc, metal transfer and its impingement on weld pool in variable polarity gas metal arc welding

• A unified numerical model involving arc, droplet and molten weld pool for VP-GMAW process is presented. • The heat transfer behavior and driving forces of metal formation and falling on the weld pool is quantitatively discussed. • At the same welding current during pulse cycle, the arc temperature in EN current phase is greater than EP current phase. • The arc plasma generated at EN current phase shows a constricted arc shape at the electrode tip. • The phenomenon of finger-like shape penetration observed during experiment is simulated and explained. A unified numerical model of the variable polarity gas metal arc welding process is developed based on the solution of magnetohydrodynamic equations within the framework of the phase-field algorithm. The droplet formation, necking, detachment, transfer through the arc plasma, and impingement onto the weld pool are investigated at different electrode positive (EP) and electrode negative (EN) regions of the current pulse regime under the condition of one droplet per pulse. The current density, electromagnetic force, surface tension, transient melt-flow velocity, and Joule heating effects are calculated. The numerical results indicate that the arc plasma generated in the EN current phase has a constricted arc shape at the electrode tip compared with the typical bell shape arc in the EP current phase. At the same welding current during the pulse cycle, the arc plasma temperature in the EN current phase is greater than that in the EP current phase. The model shows that before the droplet detaches, the surface tension forces are dominant, whereas during the EP current phase, the electromagnetic forces strongly intensify along the liquid bridge, thereby causing droplet detachment. In addition, the phenomenon of finger-like shape penetration due to the momentum transferred by a falling droplet to the bottom of the weld pool is simulated. The predicted results well agree with the high-speed images and weld pool parameters, accordingly validating the formulated model.

Design Studies, Magnetic Calculations and Structural Assessment For the DTT Central Solenoid

The “Divertor Tokamak Test facility” (DTT) is an experimental fusion reactor being built in Frascati (IT) in the framework of the European Fusion Roadmap. The DTT Central Solenoid operates in a pulsed regime to generate the magnetic flux needed to induce the plasma current and is therefore subjected to burdensome load conditions. In DTT, this magnet will consist of a stack of six layer-wound independently energized modules, wound with Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Cable-in-Conduit Conductors. Different layout solutions have been studied to improve the performance of the CS in terms of magnetic flux density and structural integrity. The design presented is the result of the “Design Explorer” algorithm that has been specifically developed for this purpose. This code investigates all possible configurations to find the optimum conductor geometry and winding solution by considering measurable physical quantities to maximize the magnetic flux density while maintaining mechanical stresses at an acceptable level. This manuscript includes the electromagnetic calculation related to the heterogeneous plasma scenarios of the machine, the static structural and the fatigue life assessment and the full set of in-depth analyses performed for the qualification of the main components of the CS structure.

Enhanced non-linear optical properties of Eu3+ activated glasses by embedding silver nanoparticles

Tri-positive lanthanide ion (Eu3+) activated glasses doped with different concentrations of silver (Ag0) nanoparticles obtained using thermal reducing agent were fabricated by applying the method of melt quench. The formation of Ag0 nanoparticles in glasses was revealed by the surface plasmon resonance (SPR) peak in the absorption spectra. Transmission electron microscopic measurements confirmed the presence of spherically shaped Ag0 nanoparticles of different size distribution. The absorption spectra showed a red–shift of the SPR peak with an increase in AgNO3 concentration occurring through Ostwald's ripening process because of the growth of particle size (as evidenced from microscope images). The non-linear optical (NLO) and optical limiting measurements were performed in the near infrared spectral region and femtosecond pulse excitation. The non-linear parameters were found to increase as the AgNO3 concentration increased to 0.6 mol %, however, the parameters subsequently decreased at higher doping level. The optical limiting threshold values demonstrated a reverse trend. The increase in non-linear optical properties regarding Ag nanoparticles concentration attributed to the enhancement of polarizabilities of glasses that occurred through local field stimulated by SPR of Ag nanoparticles when exposed to laser radiation of high energy. The increase in NLO coefficients (particularly the non-linear absorption coefficient) and the decrease in optical limiting threshold values with AgNO3 concentration (up to 0.6 mol %) indicated that these glasses containing 0.6 mol % AgNO3 are useful for the construction of the power optical limiters that function at the infrared region in the femtosecond pulse regime.

High-power amplified spontaneous emission pulses with tunable coherence for efficient non-linear processes

We report on a detailed study of an amplified spontaneous emission source operated in a pulsed regime with particular attention paid to the influence of high-intensity chaotic temporal events on the generation of nonlinear processes. To this aim, we have developed a monolithic high-power fiber system delivering partially coherent pulses of adjustable coherence. We also have demonstrated a non-linear method to characterize the stochastic properties of the source mitigating the bandwidth limitation of linear techniques. Measured parameters of the source for various configurations are presented. An enhanced classical model has been established to reproduce the statistical properties of the source and predict the behaviour when exciting non-linear processes. Finally, a non-linear process (second harmonic generation) is investigated comparing the efficiency when the process is pumped by a pulsed beam with maximal and low coherence.

Field-free and sub-ns magnetization switching of magnetic tunnel junctions by combining spin-transfer torque and spin–orbit torque

We investigate the magnetization switching via a combination of spin-transfer torque (STT) and spin–orbit torque (SOT). STT and SOT are simultaneously induced by a pulsed current flowing through an in-plane easy-axis magnetic tunnel junction and an underneath Ta/W channel. SOT allows the magnetization to be switched with the sub-ns pulse down to 200 ps and STT eliminates the necessity of an external field. The switching current is much smaller than the case driven solely by STT in the short pulse regime. We also compare the threshold current between two structures having orthogonal (Type Y) and collinear (Type X) magnetic easy axes to the longitudinal direction of the channel and find that the Type X achieves smaller switching current by a factor of 1/4 at 200 ps.

A parametric study of sub-picosecond laser ablation of thin metal foils

With the properties of ultrashort pulse width and ultrahigh peak power, femtosecond lasers excel at processing materials whose thickness is less than 500 μm. Numerous experiments and theoretical analyses have testified to the fact that there are solid grounds for the future applications of femtosecond laser micromachining [1, 2]. However, with the high costs and complexity of these devices, it is the sub-picosecond laser that might be an alternative when it comes to micromachining of thin metal foils. Furthermore, investigating the sub-picosecond laser interactions with matter could provide a better understanding of the ablation mechanisms and experimental verification of existing models concerning the ultrashort pulse regime. We present research on sub-picosecond laser interaction with metal foils with a thickness of less than 250 μm under various laser pulse parameters. The research was conducted by two types of ultrafast lasers: a lab-designed sub-picosecond Yb:KYW laser (650 fs) and a commercial femtosecond Ti:S laser (35 fs). The results show how variables, such as pulse duration, energy, repetition rate, wavelength and irradiation time, affect the micromachining process.