Low-frequency Oscillations Research Articles

Effect of doping of organo-soluble carbon dots on ionic relaxation and conductivity of planar anchored cyanobiphenyl based nematic liquid crystal

We demonstrate here the striking effect of organo-soluble carbon dots (CDs, diameter ∼7- 8 nm) on the tunability of ionic relaxation and conductivity of the host, planar anchored thermotropic nematic liquid crystal (5CB, 4-Cyano-4′-pentylbiphenyl). The optical texture, frequency, temperature, and concentration dependent dielectric studies of pristine 5CB and CDs-5CB composites have been performed using the polarising optical microscope and high-resolution dielectric spectroscopy, respectively. The doping of 0.5 wt% CDs into planar anchored 5CB is resulted into the vertical alignment confirmed through the cross-polarised optical microscope. The doping of CDs in 5CB liquid crystal is resulted into an unprecedented faster ionic relaxation with an abatement of the ionic amplitude at room temperature. The shift in the relaxation frequency for 0.5 wt% CDs-5CB composite (induced vertical alignment) as compared to pristine 5CB (planar aligned) at 28 °C (nematic) is found to be ∼13 folds which is further increased to ∼18 folds at 38 °C (isotropic). A similar trend in the conductivity curve is also observed which further confirms the significant enhancement in conductivity value with increase in temperature and concentration of dopant CDs. The observation of enhanced ionic conductivity and faster ionic relaxation in CDs-5CB composites as compared to pristine 5CB are attributed to the complete change of planar to vertical alignment by doping of CDs on planar anchored liquid crystal sample cells. It appears that the plausible strong interaction between hydrocarbon chains of CDs and 5CB molecules through anchored substrate surfaces with increasing concentration of CDs and decrease in rotational viscosity of host 5CB with increasing temperature certainly plays a pivotal role in the tuning of ionic relaxation and conductivity in the liquid crystal sample cells. We certainly believe that our results would catalyze the cognizance of the ionic relaxation and conductivity in CDs-5CB composites and other related systems. Moreover, such composites with tunable ionic relaxation/conductivity would be certainly helpful in the fabrication of various tuneable devices such as wave front corrector, low frequency oscillation generators, dynamic light scattering, and so on.

Visual Stimuli Modulate Local Field Potentials But Drive No High-Frequency Activity in Human Auditory Cortex.

Neuroimaging studies suggest cross-sensory visual influences in human auditory cortices (ACs). Whether these influences reflect active visual processing in human ACs, which drives neuronal firing and concurrent broadband high-frequency activity (BHFA; >70 Hz), or whether they merely modulate sound processing is still debatable. Here, we presented auditory, visual, and audiovisual stimuli to 16 participants (7 women, 9 men) with stereo-EEG depth electrodes implanted near ACs for presurgical monitoring. Anatomically normalized group analyses were facilitated by inverse modeling of intracranial source currents. Analyses of intracranial event-related potentials (iERPs) suggested cross-sensory responses to visual stimuli in ACs, which lagged the earliest auditory responses by several tens of milliseconds. Visual stimuli also modulated the phase of intrinsic low-frequency oscillations and triggered 15-30 Hz event-related desynchronization in ACs. However, BHFA, a putative correlate of neuronal firing, was not significantly increased in ACs after visual stimuli, not even when they coincided with auditory stimuli. Intracranial recordings demonstrate cross-sensory modulations, but no indication of active visual processing in human ACs.

Evidence for a dynamic corona in the short-term time lags of black hole X-ray binary MAXI J1820+070

ABSTRACT In X-ray observations of hard state black hole X-ray binaries (BHXRBs), rapid variations in accretion disc and coronal power-law emission are correlated and show Fourier-frequency-dependent time lags. On short ($\sim$0.1 s) time-scales, these lags are thought to be due to reverberation and therefore may depend strongly on the geometry of the corona. Low-frequency quasi-periodic oscillations (QPOs) are variations in X-ray flux that have been suggested to arise because of geometric changes in the corona, possibly due to general relativistic Lense–Thirring precession. Therefore, one might expect the short-term time lags to vary on the QPO time-scale. We performed novel spectral-timing analyses on Neutron Star Interior Composition ExploreR observations of the BHXRB MAXI J1820+070 during the hard state of its outburst in 2018 to investigate how the short-term time lags between a disc-dominated and a coronal power-law-dominated energy band vary on different time-scales. Our method can distinguish between variability due to the QPO and broad-band noise, and we find a linear correlation between the power-law flux and lag amplitude that is strongest at the QPO frequency. We also introduce a new method to resolve the QPO signal and determine the QPO phase dependence of the flux and lag variations, finding that both are very similar. Our results are consistent with a geometric origin of QPOs, but also provide evidence for a dynamic corona with a geometry varying in a similar way over a broad range of time-scales, not just the QPO time-scale.

Does Proprioception Involve Synchronization with Theta Rhythms by a Novel Piezo2 Initiated Ultrafast VGLUT2 Signaling?

This opinion manuscript outlines how the hippocampal theta rhythm could receive two novel peripheral inputs. One of the ways this could be achieved is through Piezo2 channels and atypical hippocampal-like metabotropic glutamate receptors coupled to phospholipase D containing proprioceptive primary afferent terminals. Accordingly, activated proprioceptive terminal Piezo2 on Type Ia fibers synchronizes to the theta rhythm with the help of hippocampal Piezo2 and medial septal glutamatergic neurons. Second, after baroreceptor Piezo2 is entrained to activated proprioceptive Piezo2, it could turn on the Cav1.3 channels, which pace the heart rhythm and regulate pacemaker cells during cardiac sympathetic activation. This would allow the Cav1.3 channels to synchronize to theta rhythm pacemaker hippocampal parvalbumin-expressing GABAergic neurons. This novel Piezo2-initiated proton–proton frequency coupling through VGLUT2 may provide the ultrafast long-range signaling pathway for the proposed Piezo2 synchronization of the low-frequency glutamatergic cell surface membrane oscillations in order to provide peripheral spatial and speed inputs to the space and speed coding of the hippocampal theta rhythm, supporting locomotion, learning and memory. Moreover, it provides an ultrafast signaling for postural and orthostatic control. Finally, suggestions are made as to how Piezo2 channelopathy could impair this ultrafast communication in many conditions and diseases with not entirely known etiology, leading to impaired proprioception and/or autonomic disbalance.

Effects of clamping end-tidal CO2 on neurofluidic low-frequency oscillations.

In recent years, low-frequency oscillations (LFOs) (0.01-0.1 Hz) have been a subject of interest in resting-state functional magnetic resonance imaging research. They are believed to have many possible driving mechanisms, from both regional and global sources. Internal fluctuations in the partial pressure of CO2 (PCO2) has long been thought of as one of these major driving forces, but its exact contributions compared with other mechanisms have yet to be fully understood. This study examined the effects of end-tidal PCO2 (PetCO2) oscillations on LF cerebral hemodynamics and cerebrospinal fluid (CSF) dynamics under "clamped PetCO2" and "free-breathing" conditions. Under clamped PetCO2, a participant's PetCO2 levels were fixed to their baseline average, whereas PetCO2 was not controlled in free breathing. Under clamped PetCO2, the fractional amplitude of hemodynamic LFOs in the occipital and sensorimotor cortex and temporal lobes were found to be significantly reduced. Additionally, the fractional amplitude of CSF LFOs, measured at the fourth ventricle, was found to be reduced by almost one-half. However, the spatiotemporal distributions of blood and CSF delay times, as measured by cross-correlation in the LF domain, were not significantly altered between conditions. This study demonstrates that, while PCO2 oscillations significantly mediate LFOs, especially those observed in the CSF, other mechanisms are able to maintain LFOs, with high correlation, even in their absence.

Low-frequency vibrations of bacteriochlorophyll oligomers in chlorosomes of photosynthetic green bacteria

In green photosynthetic bacteria, light absorption occurs in bacteriochlorophyll (BChl) c/d/e oligomers, which are located in chlorosomes - unique structures created by nature to collect the energy of very weak light fluxes. Using coherent femtosecond spectroscopy at cryogenic temperature, we detected and studied low-frequency vibrational motions of BChl c oligomers in the chlorosomes of the green bacteria Chloroflexus (Cfx.) aurantiacus. The objects of the study were chlorosomes isolated from bacterial cultures grown under different light intensity. It was found that the Fourier spectrum of low-frequency coherent oscillations in the Qy band of BChl c oligomers depends on the light intensity used for the growth of bacteria. It turned out that the number of low-frequency vibrational modes of chlorosomes increases as the illumination under which they were cultivated decreases. Also, the frequency range within which these modes are observed expands, and the frequencies of most modes change. Theoretical modeling of the obtained data and analysis of the literature led to the conclusion that the structural basis of Cfx. aurantiacus chlorosomes are short linear chains of BChl c combined into more complex structures. An increase in the length of these chains in chlorosomes grown under weaker light leads to the observed changes in the spectrum of vibrations of BChl c oligomers. This increase is an effective mechanism for the adaptation of bacteria to changing external conditions.

Multifractal characteristics of neuronal activity of the <i> globus pallidus </i> in patients with dystonia

Dystonia is a movement disorder characterized by involuntary muscle contractions resulting in anomalous postures, often accompanied by dystonic tremors. Among the indicative dystonic postures is an involuntary tonic tilt or rotation of the head. Currently, the etiology and pathogenesis of the disease remain theories. Currently, only low-frequency (theta-alpha) oscillations in the globus pallidus are regarded as the possible biomarker of pathological activity in dystonia. In many patients with dystonia, the absence of rhythmic neuronal activity in the basal ganglia renders this biomarker unusable for surgical treatment of the disease. Additionally, the mechanisms underlying the rearrangement of temporary patterns of neural activity that result in pathological symptoms remain unclear. We propose that the appearance of these pathological rhythms is associated with the reduction of dynamic complexity in the pattern of neural activity within the globus pallidus. We aim to use multifractal analysis in this study to evaluate changes in the globus pallidus neural activity pattern’s dynamic complexity and their correlation with the clinical manifestations of dystonia. The study examined the electrical activity of the globus pallidus in 23 patients receiving deep brain stimulation (DBS) treatment. We used the microelectrode registration technique and local field potential (LFP) registration during intraoperative neuron monitoring to precisely identify the specific locations of the external and internal segments of the globus pallidus (GPe and GPi). In four patients, an additional postoperative recording of local field potentials (LFP) was conducted while performing DBS and vibrational impact on the neck muscles. A neurologist evaluated the patients to determine the severity of the disease using the Burke-Fahn-Mardsen Dystonia Rating Scale (BFMDRS). The LEAD DBS software package was employed for postoperative reconstruction of the DBS stimulating electrode positions and for simulating the optimal stimulation zone, selected by the neurologist (i.e., volume of tissue activated, VTA). Subsequently, we identified the region that presented the most significant variation in multifractal spectrum parameters following DBS to understand its correlation with the optimal VTA. A total of 745 local potential records were analyzed, with 433 in GPe and 312 in GPi. The severity of dystonia was found to be correlated with the parameters of the multifractal spectrum (rho: –0.5630882, p=0.005149 for GPe and rho: –0.5180609, p=0.01133 for GPi). Moreover, as the dystonia severity increased along the BFMDRS scale, the multifractal spectrum became narrower and more asymmetric. In post-surgery recordings, we observed changes in the shape of the multifractal spectrum during DBS stimulation or vibration. In particular, when exposed, the multifractal spectrum significantly widened, and its symmetry was restored. We identified the region with the greatest alteration in the width of the multifractal spectrum following DBS and discovered that this region considerably intersected or fully matched the neurologist’s chosen zone for optimal DBS stimulation. We demonstrated that multifractal analysis can serve as an ancillary approach to evaluate the extent of proprioceptive feedback impairment and as a biomarker of dystonia. The noteworthy and substantial correlation with dystonia severity underscores the considerable potential of multifractal spectrum features as a biomarker of dystonia. Additionally, the overlapping areas with the most significant DBS impact and the widest variation in the multifractal spectrum in DBS can be used as a tool for identifying the most favorable region for DBS stimulation.

Enhancing the transient stability of interconnected power systems by designing an adaptive fuzzy-based fractional order PID controller

The reliable and efficient operation of the electrical energy transmission system heavily relies on maintaining stability as a key requirement. Interconnected power systems often experience low-frequency oscillations (LFOs), which have the potential to initiate instability and, subsequently, necessitate careful attention and investigation. To tackle this issue, researchers have focused on various controllers, including Fractional-Order Proportional-Integral-Derivative (FOPID) controllers. The FOPID controller, with its increased number of design parameters, has proven to be more successful than the traditional PID controller in various engineering applications. However, determining the optimal parameters for the controller becomes more challenging due to the increased design space. Moreover, the performance of fixed-gain FOPID controllers may be degraded when confronted with highly nonlinear and uncertain controlled objects such as power systems. This issue is primarily attributed to the static nature of fixed-gain FOPID controllers. To overcome these challenges associated with fixed-gain FOPID controllers and to instill an adaptive quality, this article introduces a novel power system control methodology. This adaptive approach combines a FOPID control strategy with a fuzzy logic system and is denoted as EOA-AFFOPID. The proposed adaptive controller dynamically tunes its coefficients through a fuzzy block, which significantly improves the overall performance of the control system. To establish a performance benchmark, we have also created an alternative controller, named EOA-FOPID, a version of the optima FOPID controller with fixed coefficients. The proposed methodology is applied to multi-machine interconnected power systems equipped with Static Synchronous Series Compensators (SSSCs), and comparative evaluations are conducted with other popular recent control strategies. The comparisons demonstrate the effectiveness of the EOA-AFFOPID control strategy in damping system fluctuations and achieving significant performance improvements in terms of different metrics including overshoot, undershoot, and settling time.

An NEA/OECD benchmark-experiment for the validation of CFD for mixing and thermal fatigue in T-junction dead leg flows

The current screening criteria for thermal fatigue used in isolated branch lines at nuclear power plants are based on simplified approaches, which, on one hand, tend to be overly conservative, and on the other hand, appear to miss important physics. This is demonstrated by the fact that a number of affected pipes were missed by the current screening methods.The purpose of the present work is to produce novel and accurate experimental data, which can be used for a benchmark of Computational Fluid Dynamics (CFD) in order to advance the prediction capabilities of thermal mixing and fatigue in dead leg T-junctions and improve the accuracy of the screening criteria. The ultimate value creation is increased safety and availability at nuclear power plants. This is of great interest and utmost importance to the nuclear reactor safety community.A new pressurized experimental setup of a T-junction has been designed and constructed for the purpose. The experimental setup, with a temperature difference of 120 °C, more closely resembles plant conditions than in earlier experiments. The boundary conditions in terms of the flowrates and temperatures are controlled and stable within their measurement uncertainty – a prerequisite for CFD-grade experiments.The novelty of the present work is measurements of the temperature fluctuations both in the near wall fluid mixing region and throughout the solid pipe wall material.It has been shown that the interaction between the swirl penetration of hot water and the stratified layer in the cold dead leg has an intermittent character and that random clusters of bursts of hot water penetrate into the stratification and the stagnant cold water below. The average time scale of this process is of the order of hundreds of seconds.The stratification between the hot and the cold water in the dead leg has also shown to have a low frequency oscillation of the order of 0.1 Hz, which wiggles in the circumferential direction. After hot water has penetrated downward and excited the instability, the temperature falls back to the initial one in a manner similar to a damped oscillation.The vertical temperature profiles have shown to be self-similar, for moderate temperatures, when scaled with the temperature difference between the hot and the cold water and with the penetration length.The penetration length increases with the velocity in the hot leg confirming earlier reported correlations.A small (less than 0.1 % of the hot main flow) cold leakage flow into the T-junction is sufficient to suppress the swirl penetration and substantially reduces the penetration length. However, the temperature fluctuations have a larger amplitude and range across the whole mixing zone. The fluid temperature fluctuations are shifted towards a more high frequency content, however, the low frequency nature of the swirl penetration is still present.The obtained novel dataset will be provided for a blind CFD benchmark exercise as part of an OECD/NEA project in 2023 and 2024.

Задача о колебаниях в цепи связанных линейных осцилляторов с демпфирующей и антидемпфирующей границами

The problem of oscillations in a finite homogeneous chain of coupled harmonic oscillators is considered under special boundary conditions that ensure a stable flow of energy from one end of the chain to the other. The problem covers, as special cases, the classical problem of oscillations in a chain with free and fixed ends, as well as the problem of oscillations in a chain with absorbing and anti-absorbing boundaries. Absorbing boundary conditions are used in numerical simulation of wave propagation to minimize the influence of non-physical boundaries. An exact analytical solution to the considered problem is obtained. The dynamical system of the problem is studied. In particular, a description of the invariant subspaces of the system is given. The oscillatory properties of the system are investigated. The phenomenon of a significant increase in the amplitude of low-frequency oscillations has been discovered and studied. The problem is solved in Schrodinger variables. The solution is presented both in terms of infinite series of Bessel functions and in terms of natural oscillations (eigenmodes) of the system.

Low-Frequency Oscillations of Bacteriochlorophyll Oligomers in Chlorosomes of Photosynthetic Green Bacteria.

In green photosynthetic bacteria, light is absorbed by bacteriochlorophyll (BChl) c/d/e oligomers, which are located in chlorosomes - unique structures created by Nature to collect the energy of very weak light fluxes. Using coherent femtosecond spectroscopy at cryogenic temperature, we detected and studied low-frequency vibrational motions of BChl c oligomers in chlorosomes of the green bacteria Chloroflexus (Cfx.) aurantiacus. The objects of the study were chlorosomes isolated from the bacterial cultures grown under different light intensity. It was found that the Fourier spectrum of low-frequency coherent oscillations in the Qy band of BChl c oligomers depends on the light intensity used for the growth of bacteria. It turned out that the number of low-frequency vibrational modes of chlorosomes increases as illumination under which they were cultivated decreases. Also, the frequency range within which these modes are observed expands, and frequencies of the most modes change. Theoretical modeling of the obtained data and analysis of the literature led to conclusion that the structural basis of Cfx. aurantiacus chlorosomes are short linear chains of BChl c combined into more complex structures. Increase in the length of these chains in chlorosomes grown under weaker light leads to the observed changes in the spectrum of vibrations of BChl c oligomers. This increase is an effective mechanism for bacteria adaptation to changing external conditions.

Small-Signal Synchronization Stability of Grid-Forming Converters With Regulated DC-Link Dynamics

The dc-link voltage dynamics can be directly used to synchronize grid-forming (GFM) converters with ac grids. However, such control scheme is prone to unstable low-frequency oscillations (LFO), owing to the constant-power dynamics at the dc link. This paper analyzes first the small-signal stability of dc-link voltage-synchronized GFM converters, and proposes a control method for enhancing the system stability under different operating scenarios. In the approach, a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis voltage-feedforward control is introduced in parallel with the dc-link voltage control for the grid-synchronization purpose. The LFO problem with the conventional dc-link voltage-synchronization control is mitigated. Compared to conventional cascaded/paralleled dc-link voltage- and active power-based synchronization, the method mitigates the synchronous oscillation problem. Experimental tests are performed for the GFM converter operating in both rectifier and inverter modes and with different grid strengths. The results corroborate theoretical analyses and validate the effectiveness of the approach.

Modified Chameleon Swarm Optimization Algorithm to Improve the Power System Stability

Power system stability has been difficult due to the occurrence of low frequency oscillation in the modern power system. Oscillation is mainly due to sudden changes of frequency, load, voltage, active power and reactive power. In this paper we provide a damping solution to oscillate the low frequency by using modified chameleon optimization algorithm. Damping performance and stability analysis of system is done by proposed modified chameleon Swarm Algorithm compared with conventional chameleon Swarm algorithm and Genetic algorithm with different operating condition in terms of Real power, Reactive power and Load disturbances. The comparative results confirmed that the proposed controller exhibit higher damping ratio, and better damping of deviations in speed and power angle to improve the stability of the system.

A Passivity-Based Small-Signal DEF Analysis for Low-Frequency Oscillation Source Characterization of VSC-HVdc

A Passivity-Based Small-Signal DEF Analysis for Low-Frequency Oscillation Source Characterization of VSC-HVdc

Zeeman Splitting of Torsional Oscillation Frequencies of Magnetars

Magnetars form a special class of neutron stars possessing superstrong magnetic fields and demonstrating power flares triggered by these fields. Observations of such flares reveal the presence of quasi-periodic oscillations (QPOs) at certain frequencies; they are thought to be excited in the flares. QPOs carry potentially important information on magnetar structure, magnetic field, and mechanisms of magnetar activity. We calculate frequencies of torsional (magneto-elastic) oscillations of the magnetar crust treating the magnetic field effects in the first order of perturbation theory. The theory predicts the splitting of non-magnetic oscillation frequencies into Zeeman components. Zeeman splitting of the torsional oscillation spectrum of magnetars was suggested, clearly described and estimated by Shaisultanov and Eichler (2009), but their work has not been given considerable attention. To extend it, we suggest the technique of calculating oscillation frequencies, including Zeeman splitting at not too strong magnetic fields for arbitrary magnetic field configuration. Zeeman splitting enriches the oscillation spectrum and simplifies the theoretical interpretation of observations. We calculate several low-frequency oscillations of magnetars with a pure dipole magnetic field in the crust. The results qualitatively agree with the low-frequency QPOs detected in the hyperflare of SGR 1806–20 and in the giant flare of SGR 1900+14.

Experimental Study on Combustion Modes and Oscillations in a Cavity-Based Scramjet Combustor

This study conducted experiments to investigate flame behaviors in a cavity-based scramjet combustor operating under Mach 2.92 supersonic inflow. Pressure measurements and flame chemiluminescence observations were combined to study the initial flame formation and combustion mode transitions. In addition, flame luminosity standard deviation and fast Fourier transform power spectral density (PSD) were post-processed to assess the impact of equivalence ratio and aft wall height on combustion modes as well as oscillations. It is indicated that increasing the equivalence ratio leads to a transition from lifted shear-layer mode to ramjet mode. Elevating the cavity aft wall height improves the combustor’s self-ignition capability and promotes the transition from lifted shear-layer mode to ramjet mode as long as flame stabilization is achieved. It is found that increasing the equivalence ratio and aft wall height also results in more intense combustion oscillations; besides, raising the cavity aft wall height could accentuate the predominant frequency of low-frequency oscillations in the combustor. Finally, it is demonstrated that the application of spark plasma effectively suppresses these low-frequency combustion oscillations, which is a promising active control method in the scramjet engine.

Fast-varying time lags in the quasi-periodic oscillation in GRS 1915 + 105

ABSTRACT The properties of subsecond time variability of the X-ray emission of the black hole binary GRS 1915 + 105 are very complex and strictly connected to its patterns of variability observed on long time-scales. A key aspect for determining the geometry of the accretion flow is the study of time lags between emission at different energies, as they are associated to key time-scales of the system. In particular, it is important to examine the lags associated to the strong low-frequency quasi-periodic oscillations (QPOs), as the QPOs provide unambiguous special frequencies to sample the variability. We have analysed data from an observation with the AstroSat satellite, in which the frequency of the low-frequency QPO varies smoothly between 2.5 and 6.6 Hz on a time-scale of ∼10 h. The derived phase lags show the same properties and evolution of those observed on time-scales of a few hundred days, indicating that changes in the system geometry can take place on times below one day. We fit selected energy spectra of the source and rms and phase-lag spectra of the QPO with a time-variable Comptonization model, as done previously to RossiXTE data of the same source, and find that indeed the derived parameters match those obtained for variations on much longer time-scales.

Short-lead seasonal precipitation forecast in northeastern Brazil using an ensemble of artificial neural networks

This study assesses the deterministic and probabilistic forecasting skill of a 1-month-lead ensemble of Artificial Neural Networks (EANN) based on low-frequency climate oscillation indices. The predictand is the February-April (FMA) rainfall in the Brazilian state of Ceará, which is a prominent subject in climate forecasting studies due to its high seasonal predictability. Additionally, the study proposes combining the EANN with dynamical models into a hybrid multi-model ensemble (MME). The forecast verification is carried out through a leave-one-out cross-validation based on 40 years of data. The EANN forecasting skill is compared with traditional statistical models and the dynamical models that compose Ceará’s operational seasonal forecasting system. A spatial comparison showed that the EANN was among the models with the smallest Root Mean Squared Error (RMSE) and Ranked Probability Score (RPS) in most regions. Moreover, the analysis of the area-aggregated reliability showed that the EANN is better calibrated than the individual dynamical models and has better resolution than Multinomial Logistic Regression for above-normal (AN) and below-normal (BN) categories. It is also shown that combining the EANN and dynamical models into a hybrid MME reduces the overconfidence of the extreme categories observed in a dynamically-based MME, improving the reliability of the forecasting system.

Characteristics of edge temperature ring oscillation during the stationary improved confinement mode in EAST

The I-mode is a natural edge localized mode (ELM)-free regime with H-mode-like improved energy confinement and L-mode-like particle confinement, making it an attractive scenario for future tokamak-based fusion reactors. A kind of low-frequency oscillation has been widely observed, with a frequency between stationary zonal flow and geodesic-acoustic mode (GAM) zonal flow. In EAST, most stationary I-mode shots have such a mode, called edge temperature ring oscillation (ETRO). This mode probably plays an important role in development and maintenance of the I-mode , while investigations are needed to clarify the differences between ETRO and similar mode low-frequency oscillation in other devices, such as limit cycle oscillation (LCO). In this paper, the properties of ETRO are described in detail, including the structure of its magnetic components, its radial propagation characteristics, statistics of its central frequency, a linear analysis of the alternating transition turbulences and a comparison with GAM and LCO. Although some similarities can be found between ETRO and both GAM and LCO, the main features are not identical. ETRO is probably a novel type of finite frequency zonal flow or pressure gradient-induced drift that is unique to the I-mode. It is found that modest fueling can reduce ETRO intensity while maintaining I-mode confinement, suggesting that supersonic molecular beam injection could be used as an effective tool to control ETRO.

Periodic repolarization dynamics: Different methods for quantifying low-frequency oscillations of repolarization

BackgroundPeriodic repolarization dynamics (PRD) is an electrocardiographic biomarker that quantifies low-frequency (LF) instabilities of repolarization. PRD is a strong predictor of mortality in patients with ischaemic and non-ischaemic cardiomyopathy. Until recently, two methods for calculating PRD have been proposed. The wavelet analysis has been widely tested and quantifies PRD in deg2 units by application of continuous wavelet transformation (PRDwavelet). The phase rectified signal averaging method (PRDPRSA) is an algebraic method, which quantifies PRD in deg. units. The correlation, as well as a conversion formula between the two methods remain unknown. MethodThe first step for quantifying PRD is to calculate the beat-to-beat change in the direction of repolarization, called dT°. PRD is subsequently quantified by means of either wavelet or PRSA-analysis. We simulated 1.000.000 dT°-signals. For each simulated signal we calculated PRD using the wavelet and PRSA-method. We calculated the ratio between PRDwavelet and PRDPRSA for different values of dT° and RR-intervals and applied this ratio in a real-ECG validation cohort of 455 patients after myocardial infarction (MI). We finally calculated the correlation coefficient between real and calculated PRDwavelet. PRDwavelet was dichotomized at the established cut-off value of ≥5.75 deg2. ResultsThe ratio between PRDwavelet and PRDPRSA increased with increasing heart-rate and mean dT°-values (p < 0.001 for both). The correlation coefficient between PRDwavelet and PRDPRSA in the validation cohort was 0.908 (95% CI 0.891–0.923), which significantly (p < 0.001) improved to 0.945 (95% CI 0.935–0.955) after applying the formula considering the ratio between PRDwavelet and PRDPRSA obtained from the simulation cohort. The calculated PRDwavelet correctly classified 98% of the patients as low-risk and 87% of the patients as high-risk and correctly identified 97% of high-risk patients, who died within the follow-up period. ConclusionThis is the first analytical investigation of the different methods used to calculate PRD using simulated and clinical data. In this article we propose a novel algorithm for converting PRDPRSA to the widely validated PRDwavelet, which could unify the calculation methods and cut-offs for PRD.