Saturation Mechanism Research Articles

One-dimensional steady-state model for stimulated Raman and Brillouin backscatter processes in laser-irradiated plasmas

AbstractA one-dimensional steady-state model for stimulated Raman backscatter (SRS) and stimulated Brillouin backscatter (SBS) processes in laser-irradiated plasmas is presented. Based on a novel “predictor-corrector” method, the model is capable to deal with broadband scattered light and inhomogeneous plasmas, exhibiting robustness and high efficiency. Influences of the electron density and temperature on the linear gains of both SRS and SBS are investigated, which indicates that the SRS gain is more sensitive to the electron density and temperature than that of the SBS. For the low-density case, the SBS dominates the scattering process, while the SRS exhibits much higher reflectivity in the high-density case. The nonlinear saturation mechanisms and competition between SRS and SBS are included in our model by a phenomenological method. The typical anti-correlation between SRS and SBS versus electron density is reproduced in the model. Calculations of the reflectivities are qualitatively in agreement with the typical results of experiments and simulations.

Novel Saturation Mechanism of Feedback Instability due to Electron Landau Damping

The feedback instability occurs in a coupling system of the magnetosphere and the ionosphere, and is a theoretical model explaining spontaneous structure formation of the quiet aurora. In this stud...

Saturation of radiative heat transfer due to many-body thermalization

Radiative heat transfer between two bodies saturates at very short separation distances due to the nonlocal optical response of the materials. In this work, we show that the presence of radiative interactions with a third body or external bath can also induce a saturation of the heat transfer, even at separation distances for which the optical response of the materials is purely local. We demonstrate that this saturation mechanism is a direct consequence of a thermalization process resulting from many-body interactions in the system. This effect could have an important impact in the field of nanoscale thermal management of complex systems and in the interpretation of measured signals in thermal metrology at the nanoscale.

Saturation of stimulated Raman backscattering due to beam plasma instability induced by trapped electrons

A new saturation mechanism, beam plasma instability with trapped electrons resulting in saturation of stimulated Raman backscattering (SRBS), is presented. The beam plasma instability can be excited due to interaction between beam electrons and background electrons and it will produce a new mode. Here, the trapped electrons can be considered as beam electrons with velocity vp (phase velocity of the plasma wave) and density nt (density of the trapped electrons normalized by nc, nc is the critical density for the incident laser light). This new mode will induce a sinusoidal density modulation and then this modulation will produce harmonic wave with modulation wave-number period δk to enhance this new mode. As reported previously, the density modulation can increase Landau damping of the plasma wave and decrease the resonant region where SRBS occurs. Therefore, SRBS will saturate due to beam plasma instability induced by the trapped electrons.

Nonlinear excitation of parasitic modes in harmonic gyrotrons

The investigation of interaction between modes resonant with electrons at different cyclotron harmonics is presented. The study is focused on mode interaction in the region of currents lower than the start current of the parasitic mode at the fundamental. In this region, the excitation of such parasitic modes is possible due to nonlinear excitation of them by the harmonic modes only. It is shown that, when the beam current increases, various oscillation regimes replace each other: when the current exceeds the start current of the harmonic mode only slightly, the single-frequency oscillations of this mode are stable. At higher currents, due to nonlinear excitation, the oscillations of both modes with constant amplitudes are possible. Then, this regime is replaced by the auto-modulation of two modes with periodically varying mode amplitudes. Finally, at even higher currents, the parasitic mode at the fundamental suppresses the parental second harmonic mode. The role of two saturation mechanisms (the orbital phase bunching and the depletion of the free rotational energy in the process of beam-wave interaction) in the nonlinear excitation of parasitic modes at the fundamental is analyzed. The parallels between this set of events and events that take place in the prey–predator theory are discussed.

Measurement of Kinetic-Scale Current Filamentation Dynamics and Associated Magnetic Fields in Interpenetrating Plasmas.

We present the first local, quantitative measurements of ion current filamentation and magnetic field amplification in interpenetrating plasmas, characterizing the dynamics of the ion Weibel instability. The interaction of a pair of laser-generated, counterpropagating, collisionless, supersonic plasma flows is probed using optical Thomson scattering (TS). Analysis of the TS ion-feature revealed anticorrelated modulations in the density of the two ion streams at the spatial scale of the ion skin depth c/ω_{pi}=120 μm, and a correlated modulation in the plasma current. The inferred current profile implies a magnetic field amplitude ∼30±6 T, corresponding to ∼1% of the flow kinetic energy, indicating that magnetic trapping is the dominant saturation mechanism.

Hydrogeology of Reclaimed Floodplain in A Permafrost Area, Yakutsk, Russia

A study was performed to evaluate the current permafrost and groundwater conditions in the reclaimed floodplain of the Lena, one of the largest rivers in the permafrost zone. Data from ongoing hydrogeological monitoring were compared with earlier observations conducted during the reclamation process. The results demonstrate that the placement of dredged fill led to the development of suprapermafrost thaw zones (taliks). The anthropogenic taliks vary in thickness from 10 to 15 m in areas of buried bars to 20 m or more in the former locations of oxbow lakes. There is similarity in seasonal groundwater fluctuation patterns and response to river stage variations across the study area suggesting that a continuous aquifer connected to surface water. The connection with the river is most evident during the spring flood period. Two mechanisms of ground saturation are identified during this time. One is lateral seepage flow from the Lena River into the fill mass. The zone of its influence is limited to 150–170 m from the stream. The second is hydraulic pressure transmission from the river through the subchannel flow connected with the anthropogenic suprapermafrost aquifer. Its influence extends across the entire fill area. Continuous water movement at the base of the fill prevents permafrost aggradation from below. The study results should be taken into account when developing and implementing design and construction standards for engineering structures in the reclaimed floodplains of the permafrost zone.

Low-Frequency Noise Parameter Extraction Method for Single-Layer Graphene FETs

In this paper, a detailed parameter extraction methodology is proposed for low-frequency noise (LFN) in single layer (SL) graphene transistors (GFETs) based on a recently established compact LFN model. Drain current and LFN of two short channel back-gated GFETs (L=300, 100 nm) were measured at lower and higher drain voltages, for a wide range of gate voltages covering the region away from charge neutrality point (CNP) up to CNP at p-type operation region. Current-voltage (IV) and LFN data were also available from a long channel SL top solution-gated (SG) GFET (L=5 um), for both p- and n-type regions near and away CNP. At each of these regimes, the appropriate IV and LFN parameters can be accurately extracted. Regarding LFN, mobility fluctuation effect is dominant at CNP and from there the Hooge parameter aH can be extracted while the carrier number fluctuation contribution which is responsible for the well-known M-shape bias dependence of output noise divided by squared drain current, also observed in our data, makes possible the extraction of the NT parameter related to the number of traps. In the less possible case of a Lambda-shape trend, NT and aH can be extracted simultaneously from the region near CNP. Away from CNP, contact resistance can have a significant contribution to LFN and from there the relevant parameter SDR^2 is defined. The LFN parameters described above can be estimated from the low drain voltage region of operation where the effect of Velocity Saturation (VS) mechanism is negligible. VS effect results in the reduction of LFN at higher drain voltages and from there the IV parameter hOmega which represents the phonon energy and is related to VS effect can be derived both from drain current and LFN data.

COVID-19 Response in the Global Epicenter: Converting a New York City Level 1 Orthopedic Trauma Service into a Hybrid Orthopedic and Medicine COVID-19 Management Team.

The SARS-COV-2 (COVID-19) pandemic has placed unprecedented challenges on the health care system in the United States with New York City at its epicenter. By the end of the 8 week (4/23/2020) since the virus's emergence in New York City, there have been 142,432 confirmed COVID-19 cases and 10,977 deaths attributed to complications from COVID-19-related illnesses. Secondary to policies enacted by the New York State government to limit spread of the virus, Orthopedic Surgery departments at hospitals around the area have witnessed an abrupt change in clinical demands. At a local level one trauma hospital in Queens, New York, Orthopedic Surgery elective cases have been cancelled, trauma consult volume has experienced a sharp decline, and both residents and attendings have been repurposed to meet the new clinical demands of this medical crisis. Our own orthopedic surgery service has adopted care for patients normally admitted to an internal medicine service in a novel Ortho-Medical COVID-19 management team. We prepared this primer to make our experience with caring for COVID-19 patents available as a reference for other surgical subspecialty services preparing to adjust the clinical focus of their hospital teams during this or future pandemics. LEVEL OF EVIDENCE:: Level V.

Diagnostic Accuracy of Physical Examination and History Taking in Traumatic Rib Fracture; A Single Center Experience.

Objective:To evaluate the diagnostic accuracy of history taking and physical examination in the patients with traumatic rib fractures.Methods:In a cross-sectional study, all patients with multiple traumas who referred to the emergency department were evaluated for the mechanism of injury, chief complaints, vital signs and oxygen saturation. History taking and physical examination were performed according to Barbara Bates reference. Fracture was diagnosed based on chest x-ray results and CT scan, if needed. The results were analyzed by receiver operating characteristic (ROC) curves and area under the curve (AUC) analysis.Results:Isolated rib fractures of thoracic bones were found in 8 out of 99 subjects with mean age of 33.4±19.43 years. In the sensitivity analysis of history taking and physical exam tests, the highest sensitivity was chest tenderness and deformity with 100% sensitivity for each one and the lowest was for the dyspnea with 28.10%; however, the highest sensitivity was for dyspnea with 62.50% sensitivity; and pulmonary hearing aid and chest deformity were not specific (0%). For heart rate, AUC analysis was significant. Heart rate above 80/min was associated with 87.5% sensitivity and 62.5% specificity for rib fractures.Conclusion:Proper and physical examination and history taking can help to detect rib fractures with high sensitivity and specificity denoting to the importance of the issue; while, radiographic or surgical approval is required to diagnose rib fractures.

Impact of fast ions on a trapped-electron-mode dominated plasma in a JT-60U hybrid scenario

The impact of fast ions on a trapped electron mode (TEM) is extensively analysed by linear and nonlinear gyrokinetic simulations for a JT-60U plasma at high and low magnetic shear using the Gene code in local approximation. For the first time, it is shown that TEM-induced turbulent transport may remain unaffected by the steep fast ion pressure profile generated by the neutral beam injection. Unlike recent observations of ion temperature gradient (ITG)-induced turbulent transport reductions due to fast ions, TEM-dominated systems could act differently in the presence of a significant fast ion population. The possible role of zonal flows as a saturation mechanism is analyzed, showing that their weak impact in the reported JT-60U scenario might lead to different behaviour of fast ions with respect to the ITG-dominated discharges. It is also shown that Alfvénic shear modes are destabilized at low wave numbers (). They are identified as drift Alfvén waves destabilized by ITG, which is a form of Alfvénic ITG instabilities. Deep numerical analysis provides the physical parameter range in which ITG-driven BAEs are stabilized. These results open the way to new possibilities of tailoring future experimental scenarios in order to benefit from transport reduction by fast ions.

The effects of intersystem and reverse intersystem crossing on the optical limiting properties of C60

By presenting a theoretical steady-state rate equation analysis, we demonstrate an efficient optical power limiting in a fullerene system, which can be attributed to a reverse saturable absorption mechanism. The limiting behavior, characterized by means of the sample’s transmittance, is explained via the population redistribution among singlet and triplet levels. In addition, the effects of intersystem crossing and reverse intersystem crossing rates on the limiting behavior are investigated. Our numerical results show how these rates alter the optical limiting performance; indeed, the intersystem crossing has a constructive role in decreasing the optical limiting threshold. In addition, a theoretical investigation of the z-scan technique is employed to further characterize the power limiting properties.

Stimulated Brillouin scattering of backward stimulated Raman scattering

The rescattering of backward stimulated Raman scattering (BSRS) by stimulated Brillouin scattering (SBS) is found in the high electron density region by relativistic Vlasov-Maxwell simulation and particle-in-cell (PIC) simulation, where the BSRS is in the regime of absolute instability and dominates in all the scatterings. Both one dimension (1D) Vlasov simulation and two dimension (2D) PIC simulation have been given to verify that there exists SBS of BSRS in the regime of absolute instability for BSRS. The SBS of BSRS will be even stronger than forward stimulated Raman scattering (FSRS) and SBS in regime of absolute instability for BSRS. Thus, besides Langmuir decay instability and laser energy absorption, the SBS of BSRS is also an important saturation mechanism of BSRS in high electron density region.

Voltage Saturation Mechanism in V-I Characteristics for Natural Graphite Brush and Steel Slip-ring

Voltage Saturation Mechanism in V-I Characteristics for Natural Graphite Brush and Steel Slip-ring

Magnetic Inflation and Stellar Mass. V. Intensification and Saturation of M-dwarf Absorption Lines with Rossby Number

Abstract In young Sun-like stars and field M-dwarf stars, chromospheric and coronal magnetic activity indicators such as Hα, X-ray, and radio emission are known to saturate with low Rossby number (Ro ≲ 0.1), defined as the ratio of rotation period to convective turnover time. The mechanism for the saturation is unclear. In this paper, we use photospheric Ti i and Ca i absorption lines in the Y band to investigate magnetic field strength in M dwarfs for Rossby numbers between 0.01 and 1.0. The equivalent widths of the lines are magnetically enhanced by photospheric spots, a global field, or a combination of the two. The equivalent widths behave qualitatively similar to the chromospheric and coronal indicators: we see increasing equivalent widths (increasing absorption) with decreasing Ro and saturation of the equivalent widths for Ro ≲ 0.1. The majority of M dwarfs in this study are fully convective. The results add to mounting evidence that the magnetic saturation mechanism occurs at or beneath the stellar photosphere.

The Farley‐Buneman Spectrum in 2‐D and 3‐D Particle‐in‐Cell Simulations

AbstractSince the 1960s, the Farley‐Buneman instability has played an important role in probing the E‐region ionosphere. The intervening years have seen significant progress in the linear theory of this instability, its relation to other instabilities, and some of its observational signatures. However, the saturation mechanism and nonlinear behavior remain open topics because of their role in controlling energy flow in the E‐region plasma. This paper explores the saturated state of the Farley‐Buneman instability in 2‐D and 3‐D kinetic simulations of the high‐latitude ionosphere, at three different simulated altitudes: 107, 110, and 113 km. These simulations show irregularity amplitude growth and saturation in all runs, but irregularity growth takes much longer in 2‐D than in 3‐D. Once the simulations reach saturation, wave power in the meter‐scale regime falls off as a power law below the wavelength of peak growth, but the power law index is larger in 3‐D than in 2‐D. At longer wavelengths, the 3‐D spectrum is much flatter than the 2‐D spectrum. This implies that purely 2‐D simulations of the Farley‐Buneman instability may overestimate irregularity amplitudes at decameter scales and may also underestimate the efficiency of ion Landau damping at the ion mean‐free‐path scale. From a physical perspective, the relatively flat spectra above the wavelength of peak growth in 3‐D simulations imply a wavelength‐independent saturation mechanism across a range of altitudes. Finally, both 2‐D and 3‐D simulations demonstrate the importance of accounting for zeroth‐order ion drift when estimating the flow angle of density irregularities.

Large-scale and high-similarity experimental study of the effect of drilling fluid penetration on physical properties of gas hydrate-bearing sediments in the Gulf of Mexico

In this study, the process of drilling fluid penetrating into gas hydrate-bearing sediments in the Gulf of Mexico Hydrate Joint Industry Project was simulated with conditions similar to in situ drilling process parameters and reservoir temperatures and pressures. Natural sediments were replaced by artificial sediments columns with approximately similar physical properties. The temperature, pressure, and resistivity were measured in real time, and the effects of drilling fluid penetration on the physical properties were analyzed. The hydrate saturation, penetration depth, and influence mechanism of temperature and pressure difference were obtained. By orthogonal analysis, the porosity and resistivity of the artificial sediments columns were found to be optimized closely to natural conditions, with a difference of 1.29% and 4.0%, respectively. During penetration, a positive pressure difference was useful for maintaining hydrate stability, while the temperature difference had an opposite influence. High density, low temperature, and low filtration loss drilling fluid were found to be beneficial for field drilling. Hydrate decomposition gradually occurred with increasing depth, accompanied by reformation. The front edge of resistivity lagged behind the pressure and temperature, with a depth of ~0.65 m. Therefore, for resistivity logging after drilling, logging methods with a coordinated detection depth should be considered; thus, a dual laterolog with deep focused resistivity measurements would be a better choice.

Simulated response of an intermittent stream to rainfall frequency patterns

The response of intermittent catchments to rainfall is complex and difficult to model. This study uses the spatially distributed CATchment HYdrology (CATHY) model to explore how the frequency of daily rainfall (λ) can affect the hydrologic regime of intermittent catchments. After a multi‐objective calibration and validation of CATHY against experimental measurements of streamflow and groundwater levels in a catchment used as a pasture, the role of λ in affecting streamflow characteristics was explored using different scenarios. With different values of λ for the dry and wet periods of the year, CATHY showed that a series of frequent rainfall events was often associated with incipient streamflow, independent of the season. Activation of streamflow during the wet season was related to multiple factors and was not often associated with the shallow groundwater levels near the outlet of the catchment. The interplay between rainfall depth and intensity acted as the most important factor for the generation of streamflow. Using the difference between accumulated rainfall and evapotranspiration as a measure of wetness, saturated subsurface flow mechanism generated streamflow in simulations with wetness at least three times larger than mean wetness of other simulations. Although groundwater uprise near the outlet did not effectively contribute to streamflow in the initial days of flow, it strongly correlated with the magnitude of the runoff coefficient. Values of λ close or equal to the maximum value in the wet season can sustain the connectivity between groundwater and streamflow in the riparian zone. This connectivity increases the catchment wetness, which consequently results in an increase of the generated streamflow. Our study showed that rainfall regimes characterized by different λ were able to identify distinct flow regimes typical of either intermittent, ephemeral, or nonflowing catchments. Decrease of λ in the wet season is likely associated with a reduction of streamflow, with a shift of flow regime from intermittent to ephemeral or no‐flow.

Towards a high efficiency amplifier based on Raman amplification

Ultra high power laser amplifier systems based on plasma may provide a pathway to reach petawatt to exawatt powers, vastly exceeding the limitations imposed by the currently low damage threshold of solid state optical elements. In theory unfocused intensities of 1017 W cm−2 could be reached. The Raman amplification scheme has been demonstrated as a promising candidate through the observation of 10% energy transfer efficiency due to amplification of noise, which implies potentially much larger efficiencies. However, controlled amplification of a seed pulse has not hitherto exceeded an efficiency of 7%. Here we discuss several saturation mechanisms that can limit the gain, such as early pump scattering and thermal effects. We show that chirped pulse Raman amplification can mitigate these deleterious effects.

Role of Microtearing Turbulence in DIII-D High Bootstrap Current Fraction Plasmas.

We report on the first direct comparisons of microtearing turbulence simulations to experimental measurements in a representative high bootstrap current fraction (f_{BS}) plasma. Previous studies of high f_{BS} plasmas carried out in DIII-D with large radius internal transport barriers (ITBs) have found that, while the ion energy transport is accurately reproduced by neoclassical theory, the electron transport remains anomalous and not well described by existing quasilinear transport models. A key feature of these plasmas is the large value of the normalized pressure gradient, which is shown to completely stabilize conventional drift-wave and kinetic ballooning mode instabilities in the ITB, but destabilizes the microtearing mode. Nonlinear gyrokinetic simulations of the ITB region performed with the cgyro code demonstrate that the microtearing modes are robustly unstable and capable of driving electron energy transport levels comparable to experimental levels for input parameters consistent with the experimental measurements. These simulations uniformly predict that the microtearing mode fluctuation and flux spectra extend to significantly shorter wavelengths than the range of linear instability, representing significantly different nonlinear dynamics and saturation mechanisms than conventional drift-wave turbulence, which is also consistent with the fundamental tearing nature of the instability. The predicted transport levels are found to be most sensitive to the magnetic shear, rather than the temperature gradients more typically identified as driving turbulent plasma transport.