Initiation Of Breakdown Research Articles

The formative time delay and electron avalanche number distributions for multielectron initiation of streamer breakdown

Electron avalanche statistics were mainly studied with a single electron initiation and there was no mathematical transition reported to the formative time delay distribution. This paper deals with the transition from the electron avalanche number distribution to the formative time delay distribution for multielectron initiation and streamer breakdown mechanism. The goal of the research is a detailed analysis of the formative time delay of electrical breakdown for different applications, such as spark counters, resistive plate chambers, gas-discharge switches, and high voltage systems with gas as an insulator. The electron avalanche number distribution for multielectron initiation is described by negative binomial distribution (NBD). Regarding the newly derived formative time delay distribution, when the number of initiating electrons k is small, the formative time delay distributions are asymmetric with the pronounced right tail. With increasing k, the formative time distributions shift to the shorter formative times and become narrower and higher, more symmetric and Gauss-like. As statistical tests show, for k≳20 the hypothesis that the formative time delay distributions are Gaussians cannot be rejected. As illustrations, the formative time delay distributions in neon and air at low reduced electric field are shown and briefly discussed. The distributions for methylal, ether and methane are presented in order to compare the estimated distribution parameters with available experimental data and to model the measured distributions.

Comparison of machine learning-based breakdown pressure prediction and initiation criteria in hydraulic fracturing testing

Comparison of machine learning-based breakdown pressure prediction and initiation criteria in hydraulic fracturing testing

Validation of prediction capability of operating space for plasma initiation in MAST-U

Abstract DYON is a plasma initiation modelling code that solves the differential equation system of the full circuit equations (plasma current, active coil currents and eddy currents in full passive structures) and 0D global energy and particle balance equations[1]. In order to test the capability of the full electromagnetic plasma initiation model to predict individual discharges in experiments and thus the operating space in the device, a dedicated experimental database was built in MAST-U by scanning the prefilled gas pressure p0 and the induced loop voltage Vloop . In the experimental operating space of p0 and Vloop the lower and the upper limits of p0 are determined by the plasma breakdown failure and the plasma burn-through failure, respectively. The lower limit of Vloop is determined by the plasma burn-through failure. By directly reading the control room data used in each discharge (i.e. currents in the solenoid, poloidal field coils, and toroidal field coils, p0 , and gas puffing rate), the full electromagnetic DYON consistently predicted the failed breakdown, failed burn-through, and successful plasma initiation discharges in the experimental database, demonstrating its capability to predict the operating space for inductive plasma initiation. The Paschen curve calculated with the effective connection length in MAST-U indicates a much higher p0 required for plasma breakdown than the experimental data, indicating that individual field line evaluation is necessary to calculate the quantitative requirements for Townsend breakdown. The demonstration in this paper shows that the full electromagnetic DYON could be a useful simulation tool to assess the feasibility of inductive plasma initiation and to optimise operating scenarios in future devices. [1] Hyun-Tae Kim 2022 Nuclear Fusion 62 126012

Overview of recent experimental results on the EAST Tokamak

Since the last IAEA-FEC in 2021, significant progress on the development of long pulse steady state scenario and its related key physics and technologies have been achieved, including the reproducible 403 s long-pulse steady-state H-mode plasma with pure radio frequency (RF) power heating. A thousand-second time scale (∼1056 s) fully non-inductive plasma with high injected energy up to 1.73 GJ has also been achieved. The EAST operational regime of high β P has been significantly extended (H 98y2 > 1.3, β P ∼ 4.0, β N ∼ 2.4 and n e/n GW ∼ 1.0) using RF and neutral beam injection (NBI). The full edge localized mode suppression using the n = 4 resonant magnetic perturbations has been achieved in ITER-like standard type-I ELMy H-mode plasmas with q 95 ≈ 3.1 on EAST, extrapolating favorably to the ITER baseline scenario. The sustained large ELM control and stable partial detachment have been achieved with Ne seeding. The underlying physics of plasma-beta effect for error field penetration, where toroidal effect dominates, is disclosed by comparing the results in cylindrical theory and MARS-Q simulation in EAST. Breakdown and plasma initiation at low toroidal electric fields (<0.3 V m−1) with EC pre-ionization is developed. A beneficial role on the lower hybrid wave injection to control the tungsten concentration in the NBI discharge is observed for the first time in EAST suggesting a potential way toward steady-state H-mode NBI operation.

On the modes of nanosecond pulsed plasmas for combustion ignition of quiescent CH4-air mixtures

The effect of transient plasma modes on ignition kernel development are discussed here for a quiescent CH4-air combustion model system. A 10 ns high-voltage pulse was applied to a pin-to-pin electrode in lean fuel-air mixtures at room temperature and atmospheric pressure. High-impedance streamer, transient spark and low-impedance spark discharges were identified based on pulse waveforms of voltage and current. A sustained ignition kernel expansion was observed when the plasma discharge transitioned into a transient spark or spark discharge. The minimum ignition energy was obtained at the transient spark mode, which has less than a third of the energy or Coulomb transfer compared to the low-impedance spark. Employing repetitive 10-pulse sequence at 10 kHz, the lean-fuel limit was extended from an equivalence ratio of 0.6 for the single pulse ignition to 0.5. The use of repetitive pulses also allowed streamer breakdown or spark initiation to occur at a lower voltage.

Early activation of hepatic stellate cells induces rapid initiation of retinyl ester breakdown while maintaining lecithin:retinol acyltransferase (LRAT) activity

Lecithin:retinol acyltransferase (LRAT) is the main enzyme producing retinyl esters (REs) in quiescent hepatic stellate cells (HSCs). When cultured on stiff plastic culture plates, quiescent HSCs activate and lose their RE stores in a process similar to that in the liver following tissue damage, leading to fibrosis. Here we validated HSC cultures in soft gels to study RE metabolism in stable quiescent HSCs and investigated RE synthesis and breakdown in activating HSCs.HSCs cultured in a soft gel maintained characteristics of quiescent HSCs, including the size, amount and composition of their characteristic large lipid droplets. Quiescent gel-cultured HSCs maintained high expression levels of Lrat and a RE storing phenotype with low levels of RE breakdown. Newly formed REs are highly enriched in retinyl palmitate (RP), similar to freshly isolated quiescent HSCs, which is associated with high LRAT activity. Comparison of these quiescent gel-cultured HSCs with activated plastic-cultured HSCs showed that although during early activation the total RE levels and RP-enrichment are reduced, levels of RE formation are maintained and mediated by LRAT. Loss of REs was caused by enhanced RE breakdown in activating HSCs. Upon prolonged culturing, activated HSCs have lost their LRAT activity and produce small amounts of REs by DGAT1. This study reveals unexpected dynamics in RE metabolism during early HSC activation, which might be important in liver disease as early stages are reversible. Soft gel cultures provide a promising model to study RE metabolism in quiescent HSCs, allowing detailed molecular investigations on the mechanisms for storage and release.

Interface charge accumulation in bi-layer dielectrics between cable and accessory insulation under temperature gradient

Bi-layer dielectrics are a feature of high-voltage direct-current cable and accessory systems, giving rise to the interfaces between the cable and accessory insulation. The interfaces act as preference sites for space charge build-up and are therefore regarded as potential breakdown initiation sites. A further complication is that during the operation of cable, a temperature gradient (TG) will be produced across the bi-layer insulation. In this paper, carbon black (CB) nanoparticles with 0, 0.5, 1, 1.5, 3.5 and 5.5 wt% are doped into silicon rubber (SR), space charge distributions in bi-layer dielectrics composed of cross-linked polyethylene (XLPE) and CB/SR nanocomposites under DC voltage and TG are measured using the pulsed electroacoustic method, and the effects of the doping proportion of CB nanoparticles and the TG on the bi-layer interface charge behavior are investigated. The experimental results show that under the TG, CB nanoparticles doped into SR can effectively suppress the interface charge accumulation, the amount of the interface charge decreases with the increase of the doping proportion from 0 to 1.5 wt% but increases with the increase of the doping proportion from 1.5 to 5.5 wt%, and the CB nanoparticle with 1.5 wt% shows relatively better suppression effect on the interface charge accumulation. With the increase of the TG, the amount of the interface charge increases initially and then decreases.

Experimental research of ECW pre-ionization and assisted startup in EAST

Experimental research on the electron cyclotron wave (ECW) pre-ionization and assisted start-up was carried out systematically for the first time in EAST tokamak, which is a superconducting device with ITER-like full metal wall. Breakdown and plasma initiation at low toroidal electric fields (<0.3 V m−1) with ECW pre-ionization and startup assistance has been demonstrated. Also, the parameter domain of breakdown is significantly extended towards higher prefill gas pressure. The effect of ECW injection timing, power, toroidal injection angle on breakdown were also investigated. Injecting ECW earlier leads to an earlier breakdown and a higher plasma current ramp rate. The electron cyclotron heating (ECH) power threshold for breakdown in EAST is approximately 0.4 MW. In the range of ECH power tested in this work, higher ECH power is advantageous for achieving earlier and faster breakdown. Furthermore, the breakdown with radial ECW injection occurs earlier compared with oblique injections (co-current and counter-current). During the ECW-assisted startup, the process of burn-through is prolonged by the higher pre-filled gas pressure even though it enhances the ease of breakdown. In addition, compared to the low hybrid wave assistance, the ECW assistance has an effect in averting the generation of runaway electrons and improving the safety of device during startup. Moreover, the ECW assistance exhibits a high tolerance to the impurity and thus ensures a high ramp rate of plasma current even with a high impurity level.

Combined effects of bedding anisotropy and matrix heterogeneity on hydraulic fracturing of shales from Changning and Lushan, South China: An experimental investigation

Shales are intrinsically anisotropic and heterogeneous due to the existence of bedding planes and natural microfractures. Although the roles of shale bedding anisotropy and inherent heterogeneity have been well studied, their combined effects remain unclear. To fill this knowledge gap, we performed six hydraulic fracturing tests on shales with varying bedding inclinations (0°, 45° and 90°) collected from Changing (Sichuan) and Lushan (Jiangxi), South China. The mineral components and microstructure of shales collected from these two places were compared. The results indicate that the shale from Lushan is more heterogeneous due to its higher content of microfractures and pores than the shale from Changning. Through hydraulic fracturing analysis, we show that the fracture initiation pressure and breakdown pressure first increase and then decrease with increasing bedding inclination, and a strong linear correlation between the two pressures is found to be independent of shale heterogeneity. Second, we find inconsistency between the high breakdown pressure and large hoop deformation, which can be attributed to the anisotropy effect of shale’s bedding strength relative to the maximum principal stress. In addition, the analysis of the amplitudes and frequency of acoustic emission signals suggests that tensile cracks preferentially occur in a short-transverse mode, while deviating the bedding plane from the axial direction suppresses the initiation of tensile cracks. Shale heterogeneity is mainly reflected in reducing the magnitude of critical pressures and deflecting local fracture branch behaviors, which are less significant than the impacts of bedding anisotropy during the hydraulic fracturing process.

Mesoscopic interpretation of hydraulic fracture initiation and breakdown pressure using discrete element method

The relationship between initiation pressure and breakdown pressure, two vital parameters in hydraulic fracturing operations, requires further investigation, with the explicit mechanism underlying breakdown pressure yet to be revealed. This study presents an enhanced particle discrete element model with a regular-shaped borehole, wherein the pressure-updating equation of the pipe network flow model is improved to incorporate comprehensive fluid–solid coupling. Through simulation of the mesoscopic process of hydraulic fracturing initiation and propagation, the characteristic curve of borehole fluid pressure is obtained. By comparing the curves of borehole pressure and fluid flow into the fracture, the physical meaning of breakdown pressure is explained at the mesoscopic level. The results show that the breakdown pressure fundamentally arises from the unstable propagation of the fracture, with its numerical value corresponding to the borehole fluid pressure when the fluid flow into the fracture reaches the injection rate for the first time. Additionally, employing fluid–solid coupling modeling, the impacts of fluid viscosity, injection rate, and borehole diameter on breakdown pressure are investigated. Finally, the evolution of fracture morphology under different injection rates and elastic moduli is analyzed.

Low-Grade Inflammatory Mediators and Metalloproteinases Yield Synchronous and Delayed Responses to Mechanical Joint Loading.

Mechanical loading is an essential factor for the maintenance of joint inflammatory homeostasis and the sensitive catabolic-anabolic signaling cascade involved in maintaining cartilage tissue health. However, abnormal mechanical loading of the joint structural tissues can propagate joint metabolic dysfunction in the form of low-grade inflammation. To date, few studies have attempted to delineate the early cascade responsible for the initiation and perpetuation of stress-mediated inflammation and cartilage breakdown in human joints. Fifteen healthy human male participants performed a walking paradigm on a cross-tilting treadmill platform. Blood samples were collected before exercise, after 30 minutes of flat walking, after 30 minutes of tilted walking, and after an hour of rest. Serum concentrations of the following biomarkers were measured: interleukin (IL)-1β, IL-6, IL-10, tumor necrosis factor alpha (TNF)-α, matrix metalloproteinase (MMP)-1, MMP-3, MMP-9, MMP-13, transforming growth factor beta (TGF)-β, tissue inhibitor of matrix metalloproteinase 1 (TIMP)-1, and cartilage oligomeric protein (COMP). Luminex Multiplex analysis of serum showed increased concentrations of COMP, IL-1β, TNF-α, IL-10, and TGF-β from samples collected after flat and cross-tilted treadmill walking compared to baseline. Serum concentrations of MMP-1 and MMP-13 also increased, but primarily in samples collected after tilted walking. Pearson's correlation analysis showed positive correlations between the expression of COMP, TNF-α, IL-10, and MMP-13 at each study timepoint. Stress-mediated increases in serum COMP during exercise are associated with acute changes in pro and anti-inflammatory molecular activity and subsequent changes in molecules linked to joint tissue remodeling and repair.

Impacts of wellbore orientation with respect to bedding inclination and injection rate on laboratory hydraulic fracturing characteristics of Lushan shale

A series of hydraulic fracturing experiments were conducted using Lushan shale cores (50 mm in diameter and 100 mm in length) to unravel the formation mechanism of fracture networks under the combined effects of wellbore type (vertical or horizontal well), wellbore orientations relative to the bedding plane (0°, 30°,45°, 60°, and 90°), bedding inclinations (0°, 30°,45°, 60°, and 90°), and injection rates (0.03, 0.3, 3, and 30 mL/min). We monitored acoustic emission and surface displacements during the test and conducted post failure inspection via a fluorescent tracer and a 3D laser scanner. We show that adjusting the maximum principal stress perpendicular to the wellbore is beneficial for reducing the breakdown pressure. Inclined bedding tends to initiate along the bedding plane with relatively low fracture initiation pressure and breakdown pressure. Deploying the wellbore inclined to bedding inclination and/or adopting a low injection rate is beneficial to reduce the shale breakdown strength and create a complex fracture network. However, this will correspondingly increase the fracture surface roughness, which may impede the effective transportation of proppant. These findings can provide an important reference for field hydraulic fracturing.

Mathematical Model of Fuse Effect Initiation in Fiber Core

This work focuses on the methods of creating in-fiber devices, such as sensors, filters, and scatterers, using the fiber fuse effect. The effect allows for the creation of structures in a fiber core. However, it is necessary to know exactly how this process works, when the plasma spark occurs, what size it reaches, and how it depends on external parameters such as power and wavelength of radiation. Thus, this present study aims to create the possibility of predicting the consequences of optical breakdown. This paper describes a mathematical model of the optical breakdown initiation in a fiber core based on the thermal conductivity equation. The breakdown generates a plasma spark, which subsequently moves along the fiber. The problem is solved in the axisymmetric formulation. The computational domain consists of four elements with different thermophysical properties at the boundaries of which conjugation conditions are fulfilled. The term describing the heat source in the model is determined by the wavelength of radiation and the refractive indices of the core and the shell and also includes the radiation absorption on the released electrons during the thermal ionization of the quartz glass. The temperature field distributions in the optical fiber are obtained. Based on the calculations, it is possible to estimate the occurrence times of various phase states inside the fiber, in particular, the plasma spark occurrence time.

Thyroid Dysfunction in Patient with Cirrhotic Liver Disease and its Relation to the Severity of Cirrhosis

Background: Liver carries its vital significance due to ability to control over excretion, metabolism and transportation of the thyroid hormone. Peripheral change of T4 to T3 occurs in the live leading to initiation of action and breakdown of the hormones. Any morbidity in the liver can result in the decrease in TSH level and leading to thyroid dysfunction. So it is necessary to determine this impact in cases with liver cirrhosis. Methodology: 115 cases of chronic liver disease who were not on the any therapy were involved in this research. The age of &gt;18 years and both genders were selected. Data was collected from Department of Medicine, Lahore General Hospital, and Lahore in a duration of one year. All the demographics and baseline information mandatory for the selection of the appropriate sample was recorded. The participants were informed about the purpose of the research and after their consent they were selected for this study. Finally, the collected data was analyzed using SPSS 23.inc. Practical implication: Thyroid dysfunction in patients with cirrhotic liver disease has practical implications for their management and outcomes. Regular screening and early intervention are important to detect and treat thyroid dysfunction promptly. Managing both cirrhosis and thyroid dysfunction requires a multidisciplinary approach involving hepatologists and endocrinologists. Patient education and support are crucial for raising awareness and ensuring adherence to treatment plans. Further research is needed to understand the relationship between thyroid dysfunction and cirrhosis severity and to explore potential therapeutic targets. Integrating thyroid evaluation into the overall management of cirrhosis can improve patient outcomes and quality of life. Results: Mean age of 51(9.81) years were involved in this research. The male cases were higher in number 81(70.4%). The body mass index value was 27.5(3.65). Child Pugh A class was noted in 37(32.2%), B in 32(27.8%) &amp; C in 46(40%). Serum T3 level was 2.05, serum T4 level was 8.69, serum FT3 level was 3.06, serum FT4 was 1.24 and TSH level was having a mean value 3.15. A significant inverse relationship noted in the severity of cirrhosis and thyroid profile levels. Conclusion: Thyroid function depends on the smooth functioning of the liver through its hormones. As the liver compromise some disease, this normal function disturbs and thyroid dysfunction occurs which increase with the harshness of liver disease. Keywords: Liver Disease, Thyroid Dysfunction, HCV, TSH

The partial electrical breakdown mechanism by high voltage electric pulses in multi-fractured granite

High-voltage electric pulse (HVEP) technology has currently become a new efficient rock breaking method. In order to reveal the influence of internal fracture characteristics of heterogeneous granite on electric pulse rock breaking, the two-dimensional numerical model of multi-physical field coupling electrical breakdown of fractured rock is established, which the high-voltage electric pulse plasma generation in multi-fractured heterogeneous granite is realized from the coupling of circuit field, current field, breakdown field, heat transfer field and solid mechanics. This paper analyzes the influence of fracture porosity, fracture types and granite heterogeneity on the electrical breakdown of rock (i.e. the formation of plasma channel inside the rock). At the same time, the two-dimensional numerical model of electrical breakdown parameters, which considers the circuit structure parameters of the pulsed tool, the initiation and development of the electrical breakdown, the dependence between electrical breakdown intensity and time, and rock heterogeneity, are analyzed. The simulation results indicate that with the heterogeneity H of granite changing, the plasma channel inside the rock samples are different, while the time required for breakdown t and rock breaking volume Vrock of rock samples with different heterogeneous H will show obvious fluctuations; among the rock samples with different heterogeneity H, existing a single crack requires a longer breakdown time and the extreme value of the internal conductivity is also significantly larger. And then, the formation of plasma channel is directional selective, extending towards fractures and the weak electrical breakdown intensity. With the dip angle of the fracture increasing, the required breakdown time increases accordingly, and conversely, the broken volume decreases accordingly. The larger porosity of fracture is, the larger broken volume and the shorter required break down time of the rock sample. Furthermore, with the continuous action of electric pulses, multi-fractured rock form interpenetrating plasma channels to improve the rock breaking efficiency. The research results can provide help and theoretical support for optimization of electric pulse rock breaking parameters and the development of electric pulse drilling technology.

A temporally-resolved investigation on energy deposition from laser-induced plasmas in combustion environments: The pre-breakdown region and breakdown initiation

A temporally-resolved investigation on energy deposition from laser-induced plasmas in combustion environments: The pre-breakdown region and breakdown initiation

Rock breaking mechanism of electrode bit in heterogeneous granite formation and its optimization

Electro-pulse-boring (EPB) technology is one of the most potential and nearly industrialized new rock breaking methods in geothermal drilling. However, the mechanism of EPB is not clear, and the structural parameters of EPB electrode bit are not well matched with the corresponding strata parameters and voltage parameters, which seriously hindered the commercialization process of this technology. Based on the dynamic electrical breakdown model (DEBM), a single electric breakdown of rock is realized by coupling the circuit field, the current field and the breakdown field. At the same time, considering the heterogeneity of rock, the three-dimensional numerical simulation of full-bit electrical breakdown using five kinds of electrode bits with different structures is conducted. From these five kinds of electrode bits, an electrode bit suitable for EPB is selected. Finally, the adaptability between the structural parameters and voltage parameters of the selected electrode bit is studied. The results show that the initiation and development of electrical breakdown are affected by the mineral composition, mineral quantity and mineral position of the rock. The petal-structural electrode bits with the same clearance between the high voltage electrode and all ground electrodes are more suitable for EPB drilling due to their higher chip removal function. Moreover, the minimum initial output pulse peak voltage, i.e., the adaptive voltage, of petal-structural electrode bit under different structural parameters is calculated by fitting, which is the best rock breaking voltage parameter. This research can provide reliable help for the design of EPB electrode bit, the optimization of structural parameters and voltage parameters of electrode bit to increase geothermal resources.

Unraveling the passive film formation and degradation mechanism of NiTi shape memory alloy: An experimental investigation

TiO 2 passive film plays an important role in defining biocompatibility of NiTi shape memory alloy. The corrosion behaviour of Ni 50.8 Ti 49.2 was investigated in 0.9 wt.% aqueous NaCl solution. Grazing incident X-ray diffraction (GIXRD) reveals the formation of a compressive passive film having 2.12% d-spacing variation, noticeably in B19’ structure. The surface topography (undulation) and passive film thickness was recorded in a unique way using atomic force microscopy (AFM). Raman spectrometer and X-ray photoelectron spectroscopy (XPS) investigation confirm the formation of TiO 2 -dominated passive film. Film rupture and breakdown initiation sites were precisely captured by field emission scanning electron microscope (FESEM).

The jumping dielectric breakdown behavior induced by crack propagation in ferroelectric materials: A phase field study

Ferroelectric materials often experience fracture and dielectric breakdown under large mechanical and/or electrical loads. These two failure behaviors can affect each other and may lead to new phenomena. This study predicts a new type of jumping dielectric breakdown during crack propagation in ferroelectric materials using a phase field model involving multiple order parameters. The driving forces for the evolution of polarization, fracture, and dielectric breakdown were analysed based on the concept of configurational forces. The jumping dielectric breakdown was attributed to the competition between crack propagation and breakdown initiation at the moving crack tip. The breakdown pattern is related to the polarization domain structure and is significantly influenced by the applied electric field and crack boundary conditions. Furthermore, the jumping breakdown associated with possible mechanical degradation of the material can change the driving and resistance forces for crack propagation. This study introduces a novel dielectric breakdown behavior during crack propagation and provides new insights into the coupling failure of ferroelectric materials.

Numerical electric breakdown model of heterogeneous granite for electro-pulse-boring

The problems of cost, environmental protection and efficiency of hard rock fragmentation in petroleum exploration, drilling and other projects need to be solved urgently. Electro-pulse-boring (EPB) technology has attracted attention due to its advantages of low rock breaking energy consumption, low wear and low time cost. However, there is still no more accurate description model for EPB and rock fragmentation, and the existing models have limited guidance on the parameter optimization of EPB tools. This paper introduces an electric breakdown model, which realizes the whole electric breakdown process under a single high-voltage pulse by coupling the five fields: circuit field, current field, breakdown field, heat transfer field and solid mechanical field. The multi physics model comprehensively considers the circuit structure parameters of the electric impulse generator (EIG), the initiation and development of electric breakdown, the dependence of electric breakdown strength and time, and the heterogeneity of rock. The model results found that the breakdown channel originates from the weak electric strength in the rock, and the development of the channel is highly dependent on the heterogeneity of the rock medium. The rock fragmentation process with a single pulse is accompanied by local electric fragmentation and hydroelectric fragmentation, When the pulse voltage rise time (PVRT) is greater than 500 ns, the hydroelectric fragmentation mode is dominant. The temperature and thermal stress values of the discharge channel are consistent with the previous studies results, and the temperature distribution at the electrode tip is the highest, which shows that the ablation problem of the electrode in EPB process can not be ignored. This model has the potential to establish the relationship between rock-breakingparameters and rock-breaking efficiency of EPB, and can provide a practical method for the development and parameter optimization of EPB and rock breaking tools.