Non-saturated Regime Research Articles

Insight in NO synthesis in a gliding arc plasma via gas temperature and density mapping by laser-induced fluorescence

A gliding arc (GA) plasma, operating at atmospheric pressure in a gas mixture of 50% N2 and 50% O2, is studied using laser-induced fluorescence spectroscopy. The main goal is to determine the two-dimensional distribution of both the gas temperature and the NO ground state density in the afterglow. As GA plasma discharges at atmospheric pressure normally produce rather high NO x densities, the high concentration of relevant absorbers, such as NO, may impose essential restrictions for the use of ‘classical’ laser-induced fluorescence methods (dealing with excitation in the bandhead vicinity), as the laser beam would be strongly absorbed along its propagation in the afterglow. Since this was indeed the case for the studied discharge, an approach dealing with laser-based excitation of separate rotational lines is proposed. In this case, due to a non-saturated absorption regime, simultaneous and reliable measurements of both the NO density and the gas temperature (using a reference fitting spectrum) are possible. The proposed method is applied to provide a two-dimensional map for both the NO density and the gas temperature at different plasma conditions. The results show that the input gas flow rate strongly alters the plasma shape, which appears as an elongated column at low input gas flow rate and spreads laterally as the flow rate increases. Finally, based on temperature map analysis, a clear correlation between the gas temperature and NO concentration is found. The proposed method may be interesting for the plasma-chemical analysis of discharges with high molecular production yields, where knowledge of both molecular concentration and gas temperature is required.

A generalization of the linear adsorption model to include electrosorption

The adsorption phenomenon on surfaces is investigated both with and without the presence of an external electric field. A simple interpretation of the adsorption coefficient and desorption time, as they appear in Langmuir’s isotherm model, is presented. A model for electrosorption is proposed, which explicitly takes into account the applied external voltage and the reaction field resulting from the adsorbed charges. Both saturation, stemming from the finite density of adsorption sites, and non-saturation regimes are considered. The case of a system in contact with a reservoir of particles is also analyzed for its experimental importance. The model’s validity is assessed by analyzing experimental data published by several research groups. A comparative study of the proposed model is conducted in relation to the commonly used pseudo-first and pseudo-second order models. Unlike the latter models, the proposed model allows for the assessment of the impact of the external voltage on the model parameters. The model takes into account ion generation resulting from electrolysis phenomena.

The Kinetics of Sorption-Desorption Phenomena: Local and Non-Local Kinetic Equations.

The kinetics of adsorption phenomena are investigated in terms of local and non-local kinetic equations of the Langmuir type. The sample is assumed in the shape of a slab, limited by two homogeneous planar-parallel surfaces, in such a manner that the problem can be considered one-dimensional. The local kinetic equations in time are analyzed when both saturation and non-saturation regimes are considered. These effects result from an extra dependence of the adsorption coefficient on the density of adsorbed particles, which implies the consideration of nonlinear balance equations. Non-local kinetic equations, arising from the existence of a time delay characterizing a type of reaction occurring between a bulk particle and the surface, are analyzed and show the existence of adsorption effects accompanied by temporal oscillations.

Planar laser induced fluorescence mapping of a carbon laser produced plasma.

We present measurements of ion velocity distribution profiles obtained by laser induced fluorescence (LIF) on an explosive laser produced plasma. The spatiotemporal evolution of the resulting carbon ion velocity distribution was mapped by scanning through the Doppler-shifted absorption wavelengths using a tunable, diode-pumped laser. The acquisition of these data was facilitated by the high repetition rate capability of the ablation laser (1Hz), which allowed for the accumulation of thousands of laser shots in short experimental times. By varying the intensity of the LIF beam, we were able to explore the effects of fluorescence power against the laser irradiance in the context of evaluating the saturation vs the non-saturation regime. The small size of the LIF beam led to high spatial resolution of the measurement compared to other ion velocity distribution measurement techniques, while the fast-gate operation mode of the camera detector enabled the measurement of the relevant electron transitions.

Stoichiometric dependence and laser heating effect on the luminescence thermometric performance of Er3+, Yb3+: YuGdwVO4 microparticles in the non-saturation regime

The performance of YuGdwVO4 microcrystals with different atomic concentrations of Y3+ and Gd3+ ions, co-doped Er3+-Yb3+, as thermometers, exploiting two different calculation methods based on the fluorescence intensity ratio is investigated in the non-saturation regime. At a low excitation power density of 20 W cm−2, the thermometric parameters are approximately the same for all the analyzed sensors. Whereas for an excitation power density of 155 W cm−2, they are widely different all along the 303–513 K temperature range of study. Such as an increase on the Gd3+ ion concentration in the microcrystals induces a lattice expansion that can result in the decreasing of the lattice field surrounding the Er3+ ions and then it could enhance greatly the thermometric performances. For both calculation methods, it has been demonstrated that the ∆EFIR (fitted from the FIR curves) and the ∆ETS (determined using the TeSen Calculator program) values are lower than the ∆E` experimental (fitted from the emission spectra measured at room temperature), whenever the sample is heated and then the thermometric parameters are less competitive.

Two-photon absorption laser induced fluorescence: rate and density-matrix regimes for plasma diagnostics

Two-photon absorption laser induced fluorescence (TALIF) technique employing nanosecond lasers is often used to measure space- and time-resolved distributions of key atomic and molecular radicals in reactive environments such as plasmas and combustions. Although the technique was applied for about four decades, particularly in high pressure non-equilibrium plasmas accurate measurements of species densities remain challenging. With atomic oxygen as an example, central aspects of the technique including the role of photon statistics and line profiles on the two-photon absorption rates, selection rules, spatial and temporal resolutions, photolytic and quenching effects, and absolute calibration methods are discussed. Simulations using rate equations which include non-depletion regime, three-level and six-level models are compared and criteria for non-saturation regimes are given for low- and high-pressure plasmas. Solutions of the density-matrix model, which include coherent excitation and Stark detuning phenomena, and the rate equation model are compared. The validity criteria for non-depletion and photolytic-free regimes and rate models are given. The nanosecond TALIF quench-free regime at high laser intensities is investigated using the density-matrix model. The two-photon cross-sections for O, H and N atomic radicals and their ratio with Kr and Xe rare gases used for calibrations are revisited and recommendations are proposed. For TALIF applying ultrafast lasers, the appropriate model for the fluorescence probability is discussed.

HADES RV Programme with HARPS-N at TNG

Aims. We extend the relationship between X-ray luminosity (Lx) and rotation period (Prot) found for main-sequence FGK stars, and test whether it also holds for early M dwarfs, especially in the non-saturated regime (Lx ∝ Prot−2) which corresponds to slow rotators. Methods. We use the luminosity coronal activity indicator (Lx) of a sample of 78 early M dwarfs with masses in the range from 0.3 to 0.75 M⊙ from the HArps-N red Dwarf Exoplanet Survey (HADES) radial velocity (RV) programme collected from ROSAT and XMM-Newton. The determination of the rotation periods (Prot) was done by analysing time series of high-resolution spectroscopy of the Ca II H & K and Hα activity indicators. Our sample principally covers the slow rotation regime with rotation periods from 15 to 60 days. Results. Our work extends to the low mass regime the observed trend for more massive stars showing a continuous shift of the Lx∕Lbol versus Prot power law towards longer rotation period values, and includes a more accurate way to determine the value of the rotation period at which the saturation occurs (Psat) for M dwarf stars. Conclusions. We conclude that the relations between coronal activity and stellar rotation for FGK stars also hold for early M dwarfs in the non-saturated regime, indicating that the rotation period is sufficient to determine the ratio Lx∕Lbol.

Perturbative measurement of magnetoresistance

In this paper, we report the development of a novel technique in which the magnetoresistance of nanostructures is perturbatively measured by transversally modulating the DC magnetic field. It measures the electrical transport counterpart of the transverse magnetic AC-susceptibility. The technique was developed in a conventional four-probe configuration in which a small DC current flows through the sample and a small transverse AC-field perturbates the equilibrium position of the sample magnetization. Lock-in detection, in-phase with the AC-perturbation, is used to measure the voltage signal between the two inner electrodes of the four-probe station. We successfully demonstrated that this signal is proportional to the product of the first derivative of sample resistance with respect to the equilibrium position of the magnetization times the second derivative of the energy with respect to the equilibrium position of the magnetization. These dependences add new features to the technique investigated here that were not captured by the investigations previously reported on the same subject. To show the effective use of the technique, we discuss its application in measuring magnetic properties of thin magnetic films in the non-saturated regime.

Catastrophic rotational braking among Sun-like stars

Context. Observations of young open clusters show a bimodal distribution of stellar rotation. In those clusters, Sun-like stars group into two main populations of fast and slow rotators. Beyond an age of approximately 600 Myr, the two populations converge towards a single sequence of slow rotators.Aims. The present study addresses the origin of this bimodal distribution and the cause of its observed evolution.Methods. New prescriptions of mass-loss rate and Alfven radius dependences on Rossby number suggested by observations are implemented in a phenomenological model of angular-momentum loss and redistribution. The obtained model is used to calculate the time evolution of a rotation-period distribution of solar-mass stars similar to that observed in the 5 Myr-old NGC 2362 open cluster. The simulated distributions at subsequent ages are compared with those of h Per, the Pleiades, M 50, M 35, and M 37.Results. The model is able to reproduce the appearance and disappearance of a bimodal rotation-period distribution in open clusters providing that a brief episode of large-angular-momentum loss is included in the early evolution of Sun-like stars.Conclusions. I argue that a transitory episode of large-angular-momentum loss occurs on Sun-like stars with Rossby numbers between 0.13 and 0.3. This phenomenon of enhanced magnetic braking by stellar wind would be mainly driven by a rapid increase of mass loss at a critical rotation rate. This scenario accounts for the bimodal distribution of stellar rotation in open clusters with ages between 20−30 Myr and approximately 600 Myr. The mass-loss rate increase could account for a significant fraction of the X-ray luminosity decay of Sun-like stars in the 0.13−0.3 Rossby number range where a transition from the saturated to the non-saturated regime of X-ray emission is observed. Observed correlations between Li abundance and rotation sequences in the Pleiades and M 34 clusters support this scenario.

Analysis of CSMA/CA Mechanism of IEEE 802.15.6 under Non-Saturation Regime

We have developed an analytical model for a non-saturated IEEE 802.15.6 wireless body area network (WBAN) operating under an error-prone channel. The most suitable vehicle for improving network performance was found to be the choice of access phase lengths based on traffic loads for different user priorities (UPs). It was also found that the deployment of exclusive access phase (EAP) is not necessary in a typical WBAN; in fact, short exclusive and random access phases (EAP and RAP, respectively) lead to inefficient use of available bandwidth. We have also found that four user priorities (out of the eight available) typically suffice to achieve even the most stringent requirements for WBAN performance.

Supersaturation and activity-rotation relation in PMS stars: the young cluster h Persei

The magnetic activity of late-type MS stars is characterized by different regimes, and their activity levels are well described by Ro, the ratio between P_rot and the convective turnover time. Very young PMS stars show, similarly to MS stars, intense magnetic activity. However they do not show clear activity-rotation trends, and it still debated which stellar parameters determine their magnetic activity levels. To bridge the gap between MS and PMS stars, we studied the activity-rotation relation in the young cluster h Per, a ~13 Myr old cluster, that contains both fast and slow rotators, whose members have ended their accretion phase and have already developed a radiative core. It offers us the opportunity to study the activity level of intermediate-age PMS stars with different rotational velocities, excluding any interactions with the circumstellar environment. We constrained the magnetic activity levels of h Per members measuring their X-ray emission from a Chandra observation, while P_rot were obtained by Moraux et al. (2013). We collected a final catalog of 414 h Per members with known P_rot, T_eff, M_star, with 169 of them having also detected X-ray emission. We found that h Per members, with 1.0 M_sun < M_star < 1.4 M_sun, display different activity regimes: fast rotators show supersaturation, while slower rotators have activity levels compatible to the non-saturated regime. At 13 Myr h Per is therefore the youngest cluster showing activity-rotation regimes analogous to that of MS stars, indicating that, at this age, magnetic field production is likely regulated by the alpha-Omega type dynamo. Moreover we observed that supersaturation is better described by P_rot than Ro, and that the observed patterns are compatible with the hypothesis of centrifugal stripping. In this scenario we inferred that coronae can produce structures as large as ~2 R_star above the stellar surface.

Low-field microwave absorption and magnetoresistance in iron nanostructures grown by electrodeposition on n-type lightly doped silicon substrates

In this study we investigate magnetic properties, surface morphology and crystal structure in iron nanoclusters electrodeposited on lightly doped (100) n-type silicon substrates. Our goal is to investigate the spin injection and detection in the Fe/Si lateral structures.The samples obtained under electric percolation were characterized by magnetoresistive and magnetic resonance measurements with cycling the sweeping applied field in order to understand the spin dynamics in the as-produced samples.The observed hysteresis in the magnetic resonance spectra, plus the presence of a broad peak in the non-saturated regime confirming the low field microwave absorption (LFMA), were correlated to the peaks and slopes found in the magnetoresistance curves.The results suggest long range spin injection and detection in low resistive silicon and the magnetic resonance technique is herein introduced as a promising tool for analysis of electric contactless magnetoresistive samples.

NATURAL SCALING FOR DARK ENERGY

We propose a dark energy density based on the Gauss–Bonnet four-dimensional invariant and its modification. This model avoids the necessity of introducing the black hole limit to define the holographic density, since it can be considered as a nonsaturated regime. This allows to describe the dark energy with an equation of state (EoS) and Hubble parameter behaving in a way that can be adjusted very well to recent observations. The model presents quintom behavior without any future fine-time singularities.

Analysis of Impact of TXOP Allocation on IEEE 802.11e EDCA under Variable Network Load

In this paper, we investigate the impact of transmission opportunity (TXOP), arbitration interframe space (AIFS), and contention window on the performance of an IEEE 802.11e cluster with four traffic classes under Poisson frame arrivals. We derive an analytical model of the cluster using queuing model of individual nodes, discrete time Markov chain, and probabilistic modeling of the backoff process. The analytical model demonstrates the complex interaction between TXOP, on one side, and AIFS and contention window, on the other. We derive saturation and stability points for all traffic classes and discuss their dependency on TXOP allocations. Our results indicate that use of nonzero TXOP parameter under Poisson frame arrivals improves performance slightly by separating points of saturation and instability. More substantial performance improvements should be expected by deploying TXOP differentiation under bursty traffic. Since all traffic classes need to operate in stable, nonsaturated regime, this work has important implications for the design of congestion control and admission control schemes in IEEE 802.11e clusters.

Reaching spatial or networking saturation in VANET

In this article, we investigate the network transition between non-saturation and saturation regimes for a Vehicular Ad hoc Network (VANET) which is composed of mobile nodes. We combine vehicular traffic theory, queuing model, and Markov chain to evaluate the performance of the network under spatial or networking saturation for multiple data classes over control channel and service channel. Our results indicate that the vehicle density growth can result in saturation of wireless medium around the roadside unit (RSU), further resulting in buffer overflows at on board units (OBUs). We also investigate the network saturation points for different transmission ranges of a RSU. Our results show that RSU's transmission coverage has to be chosen with respect to data patterns of OBUs, minimal distance between vehicles, and number of lanes in order to avoid network saturation condition.

Quantum dots as single-photon sources: Antibunching via two-photon excitation

Two-photon excitation induced photoluminescence (2PE-PL) microscopy of CdSe colloidal quantum dots at the single-entity level is demonstrated. We provide evidence for single nanoparticle microscopy in the two-photon excitation regime by varying the laser excitation average power, as well as by measuring the confocal point spread function in three dimensions with a single quantum dot. Model calculations of the point spread function are in good agreement with our experimental findings in the 2PE-PL nonsaturation regime. Ultimately, we observe photon antibunching and triggered single-photon emission at room temperature from those quantum nanostructures under two-photon excitation in a well-defined three-dimensional photonic volume.

IEEE 802.11e EDCA Under Bursty Traffic—How Much TXOP Can Improve Performance

In this paper, we analyze an IEEE 802.11e enhanced distributed channel access (EDCA) based single-hop ad hoc network under bursty traffic addressing all four differentiation parameters. We investigate the performance of the network considering both nonsaturation and saturation regimes under an error-prone channel. We develop an accurate analytical model for the network and validate it with a simulation model. Our results indicate that adjusting the transmission opportunity (TXOP) limit in accordance with the burst size distribution can considerably improve the performance of the network and the burst delay, which is an important parameter for the application. Although large TXOP allocations improve the medium utilization and extend the stability region of the network, serious unfairness and security concerns due to the possibility of denial of service attacks may arise. The results also show that under bursty traffic, the TXOP limit can play an important role in helping the lower priority classes avoid starvation.

Optimization of electromagnet for high-field polar magneto-optical microscopy

In this paper we propose a new method for simulation and optimization of the field of electromagnets. The method is applied to optimize the polar electromagnet with cylindrical symmetry for magnetooptic microscopy. The direct simulation is based on a discretization of Maxwell's equations by means of finite elements in the interior ferromagnetic parts that are coupled with boundary elements to model the exterior field. The ferromagnetic nonlinearity is treated by the Newton method. Further, we optimize shape of the ferromagnetics in order to achieve magnetic field above a pole head as high and as homogeneous as possible. The example of optimized electromagnet fits to a compact Zeiss polarization microscope adapted for polar magneto-optic domain visualization. Within the optimization we achieved the magnetic field of 0.23 T with inhomogeneity of 6% in a nonsaturated regime when applying a lower electric current, while in a saturated regime with a higher current applied the magnetic field was 0.47 T with inhomogeneity lower than 6%.

Fragmentation of N2 in 410 nm Intense Femtosecond Laser Field

Fragmentations of N2 in linearly polarized femtosecond 410 and 820 nm intense laser fields were studied by using the velocity mapping technique. Different behaviors of N2 at 410 and 820 nm were observed. Both the kinetic energy distributions and angular distributions of fragment ions in 410 nm field show weak dependency on laser intensities in the non-saturation regime, in contrast to the case in 820 nm. Different excited electronic states, i.e., non-Coulombic potentials populated via vertical excitation, are suggested to play crucial roles in fragmentations at short wavelength.

Consistency of homogenization schemes in linear poroelasticity

In view of extending classical micromechanics of poroelasticity to the non-saturated regime, one has to deal with different pore stresses which may be affected by the size and the shape of the pores. Introducing the macrostrain and these pore stresses as loading parameters, the macrostress of a representative volume element of a porous material can be derived by means of Levin's theorem or by means of the direct formulation of the stress average rule, respectively. A consistency requirement for a given homogenization scheme is obtained from the condition that the two approaches should yield identical results. Classical approaches (Mori–Tanaka scheme, self-consistent scheme) are shown to be only conditionally consistent. In contrast, the Ponte Castañeda–Willis scheme proves to provide consistent descriptions both of porous matrix-inclusion composites and of porous polycrystals. To cite this article: B. Pichler, L. Dormieux, C. R. Mecanique 336 (2008).