Saturation Regime Research Articles

Non-premixed counterflow methane flames in DC/AC/NS electric fields

The response of counterflow diffusion flames to sub-breakdown DC and AC electric fields, as well as their superposition with ns pulse discharge waveforms, is studied in the plane-to-plane electrode geometry. Sub-breakdown DC and low-frequency AC electric fields cause the flame displacement toward the grounded electrode, in the direction of the applied field, indicating that the body force on the positive ions exceeds that on the electrons and negative ions. As the AC frequency increases, the flame response becomes less pronounced, due to the reduction of the electrohydrodynamic (EHD) body force impulse over the AC half-period. The electric field in the electrode gap is determined by ps Electric Field Induced Second Harmonic (E-FISH) generation, with absolute calibration using sub-breakdown ns pulses overlapped with the measured electric field waveform. The results show that the electric field distribution across the flame in the current saturation regime follows the Laplacian field. This indicates that the space charge density in the gap is too low to distort the applied DC or AC field, consistent with the kinetic modeling predictions. Combining a nanosecond pulse discharge with a sub-breakdown DC field generates a diffuse plasma across the entire gap. Time-resolved and spatially resolved measurements of the electric field in the discharge indicate the ionization wave propagation between the electrodes. The present results do not exhibit a detectable flame displacement enhancement by ns discharge pulses combined with a sub-breakdown field, observed previously. Kinetic modeling calculations show that the absence of this effect in the plane-to-plane geometry is due to the rapid plasma self-shielding. This indicates that alternative electrode geometries limiting the self-shielding would be more effective for the plasma / electric field enhanced flameholding and flame stabilization applications.

The strongly irradiated planets in Praesepe

ABSTRACT We present an analysis of XMM–Newton observations of four stars in the young (670 Myr) open cluster Praesepe. The planets hosted by these stars all lie close in radius–period space to the radius–period valley and/or the Neptunian desert, two features that photoevaporation by X-ray and extreme ultraviolet (EUV) photons could be driving. Although the stars are no longer in the saturated regime, strong X-ray and extreme ultraviolet irradiation is still ongoing. Based on EUV time evolution slopes, we derived in a previous paper, in all four cases, two-thirds of their EUV irradiation is still to come. We compare the XMM–Newton light curves to those simultaneously measured with K2 at optical wavelengths, allowing us to search for correlated variability between the X-ray and optical light curves. We find that the X-ray flux decreases and flattens off while the optical flux rises throughout for K2-100, something that could result from active regions disappearing from view as the star spins. Finally, we also investigate possible futures for the four planets in our sample with simulations of their atmosphere evolution still to come, finding that complete photoevaporative stripping of the envelope of three of the four planets is possible, depending on the current planet masses.

Negative differential resistance in photoassisted field emission from Si nanowires

Field emission (FE) from semiconducting nanowires (NWs) is studied for expanding electron gun performances and functionality in terms of stability, brightness, and pulsed emission. Here, we report on a pronounced and robust double negative differential resistance (NDR) in the FE IV characteristics measured during photoassisted field emission experiments on highly crystalline p-type silicon NWs. The main feature is a double NDR in the current saturation regime, which can be modulated by both temperature and light intensity. These results contrast with previous FE studies in which only a barely noticeable single NDR was reported. Several mechanisms for the physical explanation of the NDR are currently under consideration: photogenerated carrier instabilities in the depletion region, which give rise to a pulsed space-charge current in the nanowire or tunneling through a double quantum well formed by confinement at the NW apex. Because NDRs are signatures of pulsed currents, these results suggest new functionalities for which pulsed electron sources can potentially be achieved at high repetition rates.

Effects of solutes on thermal stability, microstructure and mechanical properties in CrMnFeCoNi based alloys after high pressure torsion

The equiatomic Cantor alloy and Ni-enriched derivates of it ((CrMnFeCo)xNi1-x with x = 0.8, 0.4, 0.08 and 0) were deformed by high pressure torsion to the saturation regime and subsequently annealed in a wide temperature range. The microstructural stability of the alloys was investigated in terms of grain growth and decomposition tendencies with an emphasis on Ni92 and Ni60 that are marking a transition from dilute solid solutions to HEA alloys. Ni92 and Ni100 show a larger grain size after HPT than the HEA type alloys Ni20 and Ni60, yet, they do exhibit a lower resistance to grain growth. Both HEA type alloys exhibit decomposition tendencies in the form of secondary phases (Ni20) or nanosized Cr-rich precipitates and Mn segregations to grain boundaries (Ni60), that reduce the grain boundary mobility. A comparison of the activation energies for grain growth and diffusion coefficients of the alloys shows a good qualitative agreement. The differences in microstructural stability are also reflected by the mechanical properties. All alloys show a small amount of hardening after annealing, followed by a softening for higher temperatures in case of dilute solid solutions. In contrast, the hardness of the HEA type alloys remains constant up to 450 °C (Ni60) or even increases up to 500 °C (Ni20) followed by a softening at higher annealing temperatures. The second phase formation with limited volume fractions has no effect on the modulus.

Parameterization of mixing in stratified open channel flow

The dynamics and parameterization of mixing in temporally evolving turbulent open-channel flow is investigated through direct numerical simulations as the flow transitions from an initially neutral state to stable stratification. We observe three distinctly different mixing regimes separated by transitional values of turbulent Froude number $Fr$ : a weakly stratified regime for $Fr >1$ ; an intermediate regime for $0.3< Fr<1$ ; and a saturated regime for $Fr<0.3$ . The mixing coefficient $\varGamma =B/\epsilon _K$ , (where $B$ is the buoyancy flux and $\epsilon _K$ is the dissipation rate of kinetic energy), is well predicted by the parameterization schemes of Maffioli et al. (J. Fluid Mech., vol. 794, 2016) and Garanaik & Venayagamoorthy (J. Fluid Mech., vol. 867, 2019, pp. 323–333) across all three regimes through instantaneous measurements of $Fr$ and the ratio $L_E/L_O$ , where $L_E$ and $L_O$ are the Ellison and Ozmidov length scales, respectively. The flux Richardson number $R_f = B/(B+\epsilon _K)$ shows linear dependence on the gradient Richardson number $Ri_g$ up to a transitional value of $Ri_g =0.25$ , past which it saturates again to a constant value independent of $Fr$ or $Ri_g$ . By examining the flow as a balance of inertial, shear and buoyancy forces, we derive physically based scaling relationships to demonstrate that $Ri_g \sim Fr^{-2}$ and $Ri_g \sim Fr^{-1}$ in the weakly and moderately stratified regimes and that $Ri_g$ becomes independent of $Fr$ in the saturated regime. Our results suggest that the $L_E/L_O \sim Fr^{-1}$ scaling of Garanaik & Venayagamoorthy (J. Fluid Mech., vol. 867, 2019, pp. 323–333) in the intermediate regime manifests due to the influence of mean shear. The differences in the relationships between $Fr$ and $L_E/L_O$ for non-sheared flows within this regime are discussed.

Negative Differential Resistance Behavior in n‐Channel Organic Thin‐Film Transistors Based on C60and PTCDI‐C8: Electrical Characterization and Parameter Extraction

Organic thin‐film transistors (OTFTs), based on two n‐type structures of fullerene (C60) andN,N′‐dioctyl‐3,4,9,10‐perylenedicarboximide (PTCDI‐C8), are investigated. The two OTFT devices reveal a negative differential resistance (NDR) behavior in the saturation region of the output curves. The OTFT based on C60shows a more pronounced NDR than that based on PTCDI‐C8. The main origin of the NDR is ascribed to the trapping states. In addition, the contact resistance (Rc) of the two devices, extracted using the developed voltage transition method (Vtrmethod), as well as the obtained values ofRc, are in fair agreement with the experimental values extracted using the total resistance. The device based on C60exhibits a higherRcthan that based on PTCDI‐C8. OTFT parameters, such as transconductance (gm), mobility (μmax), threshold voltage (Vth,), turn‐on voltage (Von), ratio currentIon/Ioff, and subthreshold slope (SS), are obtained in the linear and saturation regimes. The extracted value of SS for the OTFT based on C60is very small compared with that of the device based on PTCDI‐C8, which indicates an increase in the density of trapping states in the C60film.

Multi-Spectrally Constrained Transceiver Design Against Signal-Dependent Interference

This paper focuses on the joint synthesis of constant envelope transmit signal and receive filter aimed at optimizing radar performance in signal-dependent interference and spectrally contested-congested environments. To ensure the desired Quality of Service (QoS) at each communication system, a precise control of the interference energy injected by the radar in each licensed/shared bandwidth is imposed. Besides, along with an upper bound to the maximum transmitted energy, constant envelope (with either arbitrary or discrete phases) and similarity constraints are forced to ensure compatibility with amplifiers operating in saturation regime and bestow relevant waveform features, respectively. To handle the resulting NP-hard design problems, new iterative procedures (with ensured convergence properties) are devised to account for continuous and discrete phase constraints, capitalizing on the Coordinate Descent (CD) framework. Two heuristic procedures are also proposed to perform valuable initializations. Numerical results are provided to assess the effectiveness of the conceived algorithms in comparison with the existing methods.

Identification of the parameters of a simplified 2 degree of freedom model of a nonlinear vibroacoustic absorber coupled to an acoustic system in linear and nonlinear forced regimes

This article presents a method for identifying the parameters of a simplified 2 degree of freedom model representative of a linear primary system coupled to a non-linear absorber in a forced harmonic regime over a wide range of amplitudes and forcing frequencies covering different dynamical regimes. This is a priori a difficult operation because it is necessary to combine two apparently contradictory steps. The first step consists in establishing models representing the physics of the system which are analytically soluble, which imposes severe approximations. The second step consists in adjusting the parameters of the models to experimental data, which reveal some phenomena ignored by the models. To do so, two approximate analytic methods, Harmonic Balance and Complexification Averaging under 1:1 resonance, are used to describe the dynamics of the nonlinear system for its different operating regimes: linear behavior, nonlinear behavior without energy pumping, energy pumping, and saturation regime. Then, using a non-linear regression, the parameters of the simplified model are identified from experiments. The values obtained correspond to the expected physical quantities.

Dedicated to the memory of V.D. Aleksandrov Effect of annealing in an inert atmosphere on the electrical properties of crystalline pentacene films

The results of a study of the effect of annealing at 150^oC in an inert atmosphere (Ar + 5% H2) on the electrical properties of organic field-effect transistors based on pentacene are presented. Crystalline pentacene films with a thickness of 95±5 nm were obtained using thermal vacuum deposition. The transfer and output characteristics of field-effect transistors before and after annealing for 15 hours are investigated. It was found that as a result of heat treatment, the hole mobility in the saturation regime increased by an average of 30%, and the threshold voltage decreased approximately two times. According to the data of atomic force microscopy, annealing led to a more than twofold decrease in the surface roughness of pentacene films, as well as to a noticeable enlargement of grains, which led to a decrease in the concentration of traps for hole electric transport in the channel of the field-effect transistor. Keywords: pentacene, vacuum thermal deposition, crystalline films, organic field-effect transistors, hole mobility, annealing in an inert atmosphere.

Cooling flows around cold clouds in the circumgalactic medium: steady-state models and comparison with TNG50

ABSTRACT Cold, non-self-gravitating clumps occur in various astrophysical systems, ranging from the interstellar and circumgalactic medium (CGM), to active galactic nucleus outflows and solar coronal loops. Cold gas has diverse origins such as turbulent mixing or precipitation from hotter phases. We obtain the analytical solution for a steady pressure-driven 1D cooling flow around cold, local overdensities, irrespective of their origin. Our solutions describe the slow and steady radiative cooling-driven gas inflow in the saturated regime of non-linear thermal instability in clouds, sheets, and filaments. Such a cooling flow develops when the gas around small clumps undergoes radiative cooling. These small-scale, cold ‘seeds’ are embedded in a large volume-filling hot CGM maintained by feedback. We use a simple two-fluid treatment to include magnetic fields as an additional polytropic fluid. To test the limits of applicability of these analytical solutions, we compare with the gas structure found in and around small-scale cold clouds in the CGM of massive haloes in the TNG50 cosmological magnetohydrodynamic simulation from the IllustrisTNG suite. Despite qualitative resemblance of the gas structure, we find deviations from steady-state profiles generated by our model. Complex geometries and turbulence all add complexity beyond our analytical solutions. We derive an exact relation between the mass cooling rate ($\dot{\rm M}_{\rm cool}$) and the radiative cooling rate ($\dot{\rm E}_{\rm cool}$) for a steady cooling flow. A comparison with the TNG50 clouds shows that this cooling flow relation only applies in a narrow temperature range around $\rm \sim 10^{4.5}$ K where the isobaric cooling time is the shortest. In general, turbulence and mixing, instead of radiative cooling, may dominate the transition of gas between different temperature phases.

Accurate electron beam phase-space theory for ionization-injection schemes driven by laser pulses

Abstract After the introduction of the ionization-injection scheme in laser wake field acceleration and of related high-quality electron beam generation methods, such as two-color and resonant multi-pulse ionization injection (ReMPI), the theory of thermal emittance has been used to predict the beam normalized emittance obtainable with those schemes. We recast and extend such a theory, including both higher order terms in the polynomial laser field expansion and non-polynomial corrections due to the onset of saturation effects on a single cycle. Also, a very accurate model for predicting the cycle-averaged distribution of the extracted electrons, including saturation and multi-process events, is proposed and tested. We show that our theory is very accurate for the selected processes of ${\mathrm{Kr}}^{8^{+}\to {10}^{+}}$ and ${\mathrm{Ar}}^{8^{+}\to {10}^{+}}$ , resulting in a maximum error below 1%, even in a deep-saturation regime. The accurate prediction of the beam phase-space can be implemented, for example, in laser-envelope or hybrid particle-in-cell (PIC)/fluid codes, to correctly mimic the cycle-averaged momentum distribution without the need for resolving the intra-cycle dynamics. We introduce further spatial averaging, obtaining expressions for the whole-beam emittance fitting with simulations in a saturated regime, too. Finally, a PIC simulation for a laser wakefield acceleration injector in the ReMPI configuration is discussed.

Flexible, solution-processed, indium oxide (In2O3) thin film transistors (TFT) and circuits for internet-of-things (IoT)

Over the last decade, novel approaches to explore low voltage flexible devices and low power flexible circuits are being widely researched by the scientific community. To realize the true potential of energy thrifty Internet-of-Things (IoT) objects, low power circuits and hence their low-voltage operating devices are a paramount prerequisite, especially when their power is constrained by autonomous energy scavenging. At present, through advanced manufacturing processes, silicon-based semiconductor devices are powering the modern electronics industry. However, processing temperatures are inhibiting them from flexible and printed electronics, as well as being too costly for scalability to the trillions of IoT objects anticipated. Therefore, development of solution-processed metal oxide semiconductors creates huge opportunities for IoT and wearables. Here, flexible solution-processed indium oxide (In2O3) thin film transistors (TFT) and inverter circuits with low operating voltage are reported. The operating voltage of the TFTs is ≤ 3 V with threshold voltage (Vth) 0.82 V, on/off ratio 105 and extracted mobility (μ) in saturation regime is 14.5 cm2/V·s. The gain of the inverter at VDD 1, 2 and 3 V was determined to be 10, 22 and 32 respectively. Furthermore, measured transconductance (gm) and subthreshold swing (S) are found to be 140 μS and 0.22 V/dec, respectively.

Design of Low Voltage Quasi-floating Self Cascode Current Mirror

In this paper, a modified structure of self-cascode structure is proposed. In the proposed structure, the MOSFET working in saturation mode is replaced by a Quasi-floating gate MOSFET by which the threshold voltage can be scaled, resulting in an increase in the drain-to-source voltage of other MOSFET operating in the linear region. The increased drain-to-source voltage results in a change in the operating region, which here is from linear to saturation regime. To exploit the performance of the proposed structure, the design of the current mirror circuit is shown in this paper. The proposed architecture when compared with its conventional design showed improvement in performance without affecting the other parameters. The complete design is done using MOSFET models of 180nm technology using Spice at supply dual supply of 0.5V.

Surface-potential-based drain current model for two-dimensional organic TFTs using the multiple trapping and release conduction theory

A new analytical surface-potential-based drain current model is presented for the organic thin-film transistors (TFTs) when carriers are confined in two dimensions. Following the carrier multiple trapping and release (MTR) conduction theory, i.e., the assumption that the trapped carrier concentration is much higher than the free carrier concentration, the model is developed. The presented model can account for the linear regime and saturation regime by a single formulation. The calculated results of the presented model are verified by the available experimental drain current considering the temperature characteristics. Comparing with the previous model using the variable range hopping and percolation (VRH) conduction theory, although the presented model and the previous model are similar in mathematics, the presented model is more efficient to estimate the density of trap states for the organic TFTs.

Effect of irrigation regime and varietal selection on the yield, water productivity, energy indices and economics of rice production in the lower Gangetic Plains of Eastern India

Water is the major environmental constraint for sustainable rice cultivation. A field experiment was conducted during 2016 and 2017 to evaluate the effects of three irrigation methods viz., continuous ponding (CP), alternate wetting and drying (AWD) and saturation (SAT)) and three popular local rice varieties (IET-4786, IR-36 and GB-1) laid out in a factorial randomized block design with three replications on growth, yield, water and energy productivity and economics of winter rice grown at farmers’ field. Results of the study showed that all the growth parameters, yield attributes and yield were significantly higher in CP followed by AWD method. Among the varieties, GB-1 was superior in both CP and AWD conditions to produce higher yields and yield variables than IET-4786 and IR-36. Crop water productivity (CWP) was found maximum in SAT regime followed by AWD while GB-1 as well as followed by IET-4786 displayed highest CWP among rice varieties. Energy use efficiency (EUE) and economics were higher in AWD with GB-1 followed by CP with GB-1. AWD irrigation coupled with GB-1 variety was the best treatment combination for deriving higher grain yield with maximum water saving, CWP, EUE and benefit-cost ratio.

Maximizing precision in saturation-limited absorption measurements

Quantum fluctuations in the intensity of an optical probe is noise which limits measurement precision in absorption spectroscopy. Increased probe power can offer greater precision, however, this strategy is often constrained by sample saturation. Here, we analyse measurement precision for a generalised absorption model in which we account for saturation and explore its effect on both classical and quantum probe performance. We present a classical probe-sample optimisation strategy to maximise precision and find that optimal probe powers always fall within the saturation regime. We apply our optimisation strategy to two examples, high-precision Doppler broadened thermometry and an absorption spectroscopy measurement of Chlorophyll A. We derive a limit on the maximum precision gained from using a non-classical probe and find a strategy capable of saturating this bound. We evaluate amplitude-squeezed light as a viable experimental probe state and find it capable of providing precision that reaches to within > 85% of the ultimate quantum limit with currently available technology.

Electrohydrodynamic conduction pumping of a viscoelastic dielectric fluid with the Onsager–Wien effect

Due to the Onsager–Wien effect (enhanced electric field dissociation), electrohydrodynamic (EHD) conduction pumping problems in the Oldroyd-B viscoelastic dielectric liquid are solved using the finite volume method. Fully coupled nonlinear equations, including the Naiver–Stokes (N–S) equation, the Nernst–Plank equation of the ion conduction mechanism, and the constitutive equation of viscoelastic fluid, are thoroughly implemented. Our numerical model is fully verified with two benchmark cases, and simulations are performed under different conduction numbers C0 (0.01–103), Weissenberg numbers Wi (0.01–50), and viscosity ratios (0.3–0.9). Two limiting operating states of EHD conduction pumping, the Ohmic and the saturation regimes, are discussed in detail. Elastic instability is more likely to occur in the Ohmic regime, and the thickness of the heterocharge layer (called the dissociation layer) remains sensitive to C0 and the elasticity values. Current density and pressure generation show crucial degrees of dependence on the fluid elasticity and viscosity ratio.

Theoretical analysis of the saturation phase of the 1/1 energetic-ion-driven resistive interchange mode

The aim of the present study is to analyze the saturation regime of the energetic-ion-driven resistive interchange mode (EIC) in the LHD plasma. A set of nonlinear simulations are performed by the FAR3d code that uses a reduced MHD model for the thermal plasma coupled with a gyrofluid model for the energetic particle (EP) species. The hellically trapped EP component is introduced through a modification of the averaged drift velocity operator to include their precessional drift. The nonlinear simulation results show similar 1/1 EIC saturation phases with respect to the experimental observations, reproducing the enhancement of the n/m = 1/1 resistive interchange modes (RIC) amplitude and width as the EP β increases, the EP β threshold for the 1/1 EIC excitation, the further destabilization of the 1/1 EIC as the population of the helically trapped EP increases and the triggering of burst events. The frequency of the 1/1 EIC calculated during the burst event is 9.4 kHz and the 2/2 and 3/3 overtones are destabilized, consistent with the frequency range and the complex mode structure measured in the experiment. In addition, the simulation shows the inward propagation of the 1/1 EIC due to the nonlinear destabilization of the 3/4 and 2/3 energetic particle modes, leading to the partial overlapping between resonances during the burst event. Finally, the analysis of the 1/1 EIC stabilization phase shows the excitation of the 1/1 RIC as soon as the flattening induced by the 1/1 EIC in the pressure profile vanishes, leading to the retrieval of the pressure gradient at the plasma periphery and the overcoming of the RIC stability limit.

Resistivity saturation in Kondo insulators

Resistivities of heavy-fermion insulators typically saturate below a characteristic temperature T*. For some, metallic surface states, potentially from a non-trivial bulk topology, are a likely source of residual conduction. Here, we establish an alternative mechanism: at low temperature, in addition to the charge gap, the scattering rate turns into a relevant energy scale, invalidating the semi-classical Boltzmann picture. Then, finite lifetimes of intrinsic carriers drive residual conduction, impose the existence of a crossover T*, and control—now on par with the gap—the quantum regime emerging below it. Assisted by realistic many-body simulations, we showcase the mechanism for the Kondo insulator Ce3Bi4Pt3, for which residual conduction is a bulk property, and elucidate how its saturation regime evolves under external pressure and varying disorder. Deriving a phenomenological formula for the quantum regime, we also unriddle the ill-understood bulk conductivity of SmB6—demonstrating a wide applicability of our mechanism in correlated narrow-gap semiconductors.

Quantum criticality in twisted transition metal dichalcogenides.

Near the boundary between ordered and disordered quantum phases, several experiments have demonstrated metallic behaviour that defies the Landau Fermi paradigm1-5. In moiré heterostructures, gate-tuneable insulating phases driven by electronic correlations have been recently discovered6-23. Here, we use transport measurements to characterize metal-insulator transitions (MITs) in twisted WSe2 near halffilling of the first moiré subband. We find that the MIT as a function of both density and displacement field is continuous. At the metal-insulator boundary, the resistivity displays strange metal behaviour at low temperatures, with dissipation comparable to that at the Planckian limit. Further into the metallic phase, Fermiliquid behaviour is recovered at low temperature, and this evolves into a quantum critical fan at intermediate temperatures, before eventually reaching an anomalous saturated regime near room temperature. An analysis of the residual resistivity indicates the presence of strong quantum fluctuations in the insulating phase. These results establish twisted WSe2 as a new platform to study doping and bandwidth-controlled metal-insulator quantum phase transitions on the triangular lattice.