Mode Frequencies Research Articles

Experimental and analytical studies on the influence of rocking ground motions on the frame structure

The characteristics of rocking ground motions and their influence on structural response are rarely considered. This study scrutinizes the structural response solely under the influence of rocking components, utilizing data derived from shaking table tests. Notably, a robust correlation is lacking among the peak parameters of rotational components. The maximum rocking response of each floor is similar to the input peak values of the rocking components. The interstory drift and base force of the structure are closely related to the rotational angle of the rotational components. The mentioned responses are mainly controlled by the structure's 1st mode(X-axis), positively correlated with the wavelet amplitude corresponding to the structure's first mode frequency. The structural acceleration response is closely related to the rotational acceleration of the rocking components. The proportion of higher order modes in acceleration response is usually large. Lower floor acceleration responses are greatly influenced by the high-frequency components of the rocking components, while upper floor acceleration responses are more affected by the low-frequency components.

Flexural–torsional modal interaction in MEMS actuators initiated by minuscule asymmetry

AbstractAn efficient actuation technique for electrostatic MEMS actuators exploiting electro-mechanical-mechanical modal interactions is proposed. The flexural–torsional equations of motion are established, and we manifest that the initiation of a 2:1 autoparametric modal interaction between in-plane bending and torsional modes of the actuator that is supposed to be symmetrical with respect to its axis of rotation is contingent upon the presence of a quadratic stiffness term, which arises from the existence of non-zero first moments of area of the actual cross-section in prismatic microbeams. In order to efficiently reduce the AC voltage value required to reach the activation of the 2:1 mechanical modal interaction, the electrical resonant frequency is syntonized to half of the natural frequency of the in-plane bending mode. The results indicate that the amplitude of the in-plane motion saturates upon the initiation of an energy exchange between the bending and torsional motions. Through suitable tuning of the AC frequency, the amplitude of the in-plane motion is minimized, while the amplitude of the torsional motion, the indirectly excited mode, is maximized. Our results demonstrate that the actuator's torsional motion, when subjected to a 1:2:1 electro-flexural–torsional modal interactions, is triggered by applying a maximum voltage of 10 V, resulting in about 20 degrees rotational angle. Furthermore, prolific frequency combs are generated as a result of secondary Hopf bifurcations along the large-amplitude response branches, inducing quasi-periodicity in the MEMS dynamics.

Quantification of electron–phonon interaction in bismuth telluride under hydrostatic pressure via ultrafast spectroscopy

Here, we demonstrate a strategy for the quantification of electron–phonon interaction (EPI) of bismuth telluride (Bi2Te3) under hydrostatic pressure through systematic femtosecond pump-probe spectroscopy. Two optical phonon modes, namely A1g and Eg with frequencies of 1.87 and 3.71 THz at ambient pressure, are detected using time-resolved transient reflection (TR) measurement. Frequencies of both coherent phonon oscillations increase monotonically by around 33% and 17%, respectively, with the rising pressure up to 5.67 GPa, indicating pressure-induced phonon-hardening effect. The mode-specific electron–phonon coupling constant λ of Bi2Te3 under different pressures are calculated with the frequency of the A1g mode. It turns out that the variation of phonon lifetime and the corresponding phonon dephasing rate of the A1g mode may result from the pressure modification of λ. Our findings reveal the significant role of EPI in phonon transport and shed light on further manipulation on thermoelectric efficiency of Bi2Te3 with external strain.

Infrared signature of the hydroperoxyalkyl intermediate (·QOOH) in cyclohexane oxidation: An isomer-resolved spectroscopic study.

Infrared (IR) action spectroscopy is utilized to characterize carbon-centered hydroperoxy-cyclohexyl radicals (·QOOH) transiently formed in cyclohexane oxidation. The oxidation pathway leads to three nearly degenerate ·QOOH isomers, β-, γ-, and δ-QOOH, which are generated in the laboratory by H-atom abstraction from the corresponding ring sites of the cyclohexyl hydroperoxide (CHHP) precursor. The IR spectral features of jet-cooled and stabilized ·QOOH radicals are observed from 3590 to 7010cm-1 (∼10-20kcal mol-1) at energies in the vicinity of the transition state (TS) barrier leading to OH radicals that are detected by ultraviolet laser-induced fluorescence. The experimental approach affords selective detection of β-QOOH, arising from its significantly lower TS barrier to OH products compared to γ and δ isomers, which results in rapid unimolecular decay and near unity branching to OH products. The observed IR spectrum of β-QOOH includes fundamental and overtone OH stretch transitions, overtone CH stretch transitions, and combination bands involving OH or CH stretch with lower frequency modes. The assignment of β-QOOH spectral features is guided by anharmonic frequencies and intensities computed using second-order vibrational perturbation theory. The overtone OH stretch (2νOH) of β-QOOH is shifted only a few wavenumbers from that observed for the CHHP precursor, yet they are readily distinguished by their prompt vs slow dissociation rates to OH products.

Dominant higher-order vortex gyromodes in circular magnetic nanodots.

The transition to the third dimension enables the creation of spintronic nanodevices with significantly enhanced functionality compared to traditional 2D magnetic applications. In this study, we extend common two-dimensional magnetic vortex configurations, which are known for their efficient dynamical response to external stimuli without a bias magnetic field, into the third dimension. This extension results in a substantial increase in vortex frequency, reaching up to 5 GHz, compared to the typical sub-GHz range observed in planar vortex oscillators. A systematic study reveals a complex pattern of vortex excitation modes, explaining the decrease in the lowest gyrotropic mode frequency, the inversion of vortex mode intensities, and the nontrivial spatial distribution of vortex dynamical magnetization noted in previous research. These phenomena enable the optimization of both oscillation frequency and frequency reproducibility, minimizing the impact of uncontrolled size variations in those magnetic nanodevices.

Split-type seismic metamaterials with pseudo-surface wave weakening mechanism

Pseudo surface waves, as a type of propagating wave, are detrimental to the attenuation of seismic Rayleigh waves. A split seismic metamaterial (SSM) that can weaken pseudo surface wave modes and generate ultra wideband gaps of 12.92Hz is proposed. The split configuration of SSM can be obtained by separating the modes of different waves within the energy band curve, which can cause the surface waves inside the sound cone to move outward and weaken the pseudo surface wave modes inside the band gap. Combined with the frequency response curve, it is further demonstrated that the attenuation amplitude of SSM is larger than that of monolithic seismic metamaterials (MSM) within the same bandgap range. Afterwards, by studying the changes in bandgap through structural parameters, the method of a gradient array was used to fuse the characteristic frequency that can generate larger attenuation with the weakened pseudo surface wave mode frequency, resulting in greater attenuation. Finally, it is verified by experiments that SSM can provide ultra-wide low-frequency band gaps while also generating large attenuation accuracy, which provides a practical solution for the application of seismic metamaterials.

Weak time-scale separation at the onset of oscillatory magnetoconvection in rapidly rotating fluids

Convective instabilities are one of the integral parts of the dynamics of flows driven by thermal buoyancy. Naturally, physical phenomena exhibit a wide disparity in the length and timescales of the field variables in numerical simulations and experimental observations. Such variations are not represented in the traditional normal mode stability analysis attempting to understand the onset of convection. This study attempts to incorporate different time constants for different perturbation variables in the linear stability analysis with the help of a Taylor series expansion. The infinite horizontal layer model is chosen for simplicity. Apart from the classical Rayleigh-Bénard system, additional physical effects such as background rotation and magnetic field have been considered with plausible implications for geophysical flow applications. The time scale separation is implemented by considering a slight change in the frequency of temperature perturbation compared to that for other physical quantities. Both analytical and numerical methods have been utilised for the investigation. The threshold buoyancy force is reduced when the temperature perturbation has a smaller frequency than the frequencies of other variables. Besides that, the onset wavenumber and frequency of the oscillatory modes are modified due to weak scale separation from the onset characteristics of the reference case. In particular, enhanced frequency of temperature perturbations leads to smaller-scaled magnetically controlled convective rolls and larger-scaled viscously controlled instabilities at the onset. A robust dependence of the onset characteristics with the parameter quantifying the timescale separation is obtained. Additionally, two transitions in the convective onset modes with scale separation have been identified.

Charge (in)stability and superradiance of Topological Stars

We study linear massive scalar charged perturbations of Topological Stars in the fuzzball and in the black hole (Black String) regimes. The objects that naturally couple to the electric 3-form field strength of these solutions are charged strings, wound around the compact direction. We explore the possibility of instabilities of these solutions, in analogy with the charge instability already highlighted for other non-BPS geometries like JMaRT. This issue is addressed by calculating quasi-normal mode frequencies with a variety of techniques: WKB approximation, direct integration, Leaver method and by exploiting the recently discovered correspondence between black hole/fuzzball perturbation theory and quantum Seiberg-Witten curves. All mode frequencies we find have negative imaginary parts, implying an exponential decay in time. This suggests a linear stability of Topological Stars also in this new scenario. In addition, we study the charge superradiance for the Black String. We compute the amplification factor with the numerical integration method and a quantum Seiberg-Witten motivated definition including instantonic corrections.

Numerical Investigation on the Influence of Continuous Rotating Backpropagation Pressure Wave on Turbine Performance

Abstract Researches have shown that the use of a continuous detonation afterburner can improve the propulsion performance of aero engine. However, backpropagation pressure waves (BPW) generated by the pressure gain of detonation will affect the internal flow and performance of turbine. This article simulates BPW through a custom function, and investigates the effects of BPW amplitude, rotation frequency, and propagation mode on turbine performance through three-dimensional simulation. The results show that as the pressure amplitude of the BPW increases, the pressure oscillation at each section of the turbine increases and a local subcritical flow state will appear, leading to the decrease of turbine flowrate and turbine power, as well as an intensification of instantaneous turbine power fluctuations. As the rotation frequency of the BPW increases, the pressure oscillation at each section of the turbine gradually decreases. The flowrate and power of the turbine do not change much, but turbine efficiency gradually decreases. Compared to the aligned mode, the turbine performs better under the influence of BPW in misaligned mode. Compared to the single-wave mode, the fluctuation of transient turbine power is lower under the influence of BPW in the multiwave mode excluding collision mode. Finally, the constraints of equal flowrate region and equal turbine power line on the peak-to-peak value of the BPW were analyzed when the joint operation of the turbine and compressor was not affected. The rotation frequency and mode of BPW will affect the flowrate region and power line.

Resistive wall mode and fishbone mode in ITER steady state scenario: roles of fusion-born alphas and plasma flow

Abstract Drift-kinetic effects of fusion-born alpha particles on the n=1 (n is the toroidal mode number) resistive wall mode (RWM) is numerically investigated for a recent design of the ITER 10 MA steady state plasma scenario, utilizing a magneto-hydrodynamic (MHD)-kinetic hybrid toroidal model. While the fluid theory predicts unstable RWM as the normalized plasma pressure βN exceeds the no-wall Troyon limit and with the mode growth rate monotonically increasing with βN, inclusion of the drift-kinetic contribution of trapped alphas qualitatively modifies the behavior by stabilizing the mode at high βN. In fact, a complete stabilization of the n=1 RWM up to the ideal-wall Troyon limit is found. On the other hand, another unstable branch - the alpha-driven n=1 fishbone mode (FB) – is identified in the high-βN regime, with the mode frequency matching that of the toroidal precession frequency of trapped alphas. Fast plasma toroidal flow however helps mitigate the FB instability. Kinetic stabilization of the RWM and flow stabilization of the (alpha-triggered) FB result in an enhancement of βN from the design value of 3.22 to 3.52 for the ITER scenario considered, while still maintaining stable plasma operation against the aforementioned MHD instabilities.

Synthesis, characterization and structure-property relations in mullite-type Pb2(Pb1−x Sn x )O4 solid solution

Abstract The crystal structures of both Pb2PbO4 (Pb3O4) and Pb2SnO4 at room temperature can be described using mullite-type setting in the space groups P42/mbc and Pbam, respectively. At what chemical extend the crystal structure prefers either of the space groups would be an excellent playground in the Pb2(Pb1−x Sn x )O4 solid solution. Members of the solid solutions have been prepared by solid-state reactions carried out in sealed quartz tubes. Each sample has been found to be phase pure confirmed by X-ray powder diffraction data Rietveld refinement. Samples with higher tin content require higher synthesis temperatures, and controlled decomposition of Pb3O4 serves as the source for both Pb2+ and Pb4+ cations. Since the Pb4+ cation is larger than Sn4+, the MO6 polyhedral volume decreases with increasing Sn-content. As such, each metric parameter shows a linear trend following Vegard’s rule. The concomitant contraction of the MO6 octahedra and the high stereo-chemical activity of the 6s 2 lone electron pairs of lead in the Pb2+O4 distorted pyramid results in symmetry reduction. DFT suggests dynamical instability of the tetragonal Pb3O4 while Pb2SnO4 keeps orthorhombic symmetry at low temperatures, which agrees well with the experimental findings. The global blue shift of the vibrational mode frequencies is explained by the quasi-harmonic approach. The indirect band-gap linearly increases from 2.1(1) eV (x = 0) to a maximum value of 2.5(1) eV for x = 0.8 followed by a sharp drop towards Pb2SnO4. Thermogravimetric analysis demonstrates higher thermal stability with increasing Sn-content, which is explained in terms of higher bond strength of Sn–O than that of Pb–O in the MO6 octahedra.

Rayleigh waves in viscoelastic piezoelectric half-space with cladding structures: An analytic Legendre-Laguerre polynomial method

Piezoelectric materials are widely used in surface acoustic wave devices. Many piezoelectric materials themselves have viscoelastic properties, and their surface wave characteristics, especially the attenuation characteristics still needs to be explored. This article proposes a Legendre-Laguerre orthogonal polynomial method to solve the Rayleigh wave problems in a viscoelastic piezoelectric half space with a covering layer. The proposed method compensates for the inherent shortcomings of traditional Laguerre polynomials in solving layered half spaces: the normal stress and electric displacement are discontinuous. The correctness of the method was verified through literature comparison and finite element simulation. At the same time, by utilizing the orthogonality of the Legendre-Laguerre polynomial, the integral analytical formula encountered in the solution process is derived, which improves the computational efficiency by more than ten times. Through the analysis and discussion of the dispersion and attenuation curves, it is found that the piezoelectric effect can suppress the attenuation of Rayleigh waves; the piezoelectric and viscous properties of the covering layer mainly affect attenuation at high frequencies, while those of the half space layer mainly affect attenuation at low frequencies of high-order modes.

Deletion of the α2δ-1 calcium channel subunit increases excitability of mouse chromaffin cells.

High voltage-gated Ca2+ channels (HVCCs) shape the electrical activity and control hormone release in most endocrine cells. HVCCs are multi-subunit protein complexes formed by the pore-forming α1 and the auxiliary β, α2δ and γ subunits. Four genes code for the α2δ isoforms. At the mRNA level, mouse chromaffin cells (MCCs) express predominantly the CACNA2D1 gene coding for the α2δ-1 isoform. Here we show that α2δ-1 deletion led to ∼60% reduced HVCC Ca2+ influx with slower inactivation kinetics. Pharmacological dissection showed that HVCC composition remained similar in α2δ-1-/- MCCs compared to wild-type (WT), demonstrating that α2δ-1 exerts similar functional effects on all HVCC isoforms. Consistent with reduced HVCC Ca2+ influx, α2δ-1-/- MCCs showed reduced spontaneous electrical activity with action potentials (APs) having a shorter half-maximal duration caused by faster rising and decay slopes. However, the induced electrical activity showed opposite effects with α2δ-1-/- MCCs displaying significantly higher AP frequency in the tonic firing mode as well as an increase in the number of cells firing AP bursts compared to WT. This gain-of-function phenotype was caused by reduced functional activation of Ca2+-dependent K+ currents. Additionally, despite the reduced HVCC Ca2+ influx, the intracellular Ca2+ transients and vesicle exocytosis or endocytosis were unaltered in α2δ-1-/- MCCs compared to WT during sustained stimulation. In conclusion, our study shows that α2δ-1 genetic deletion reduces Ca2+ influx in cultured MCCs but leads to a paradoxical increase in catecholamine secretion due to increased excitability. KEY POINTS: Deletion of the α2δ-1 high voltage-gated Ca2+ channel (HVCC) subunit reduces mouse chromaffin cell (MCC) Ca2+ influx by ∼60% but causes a paradoxical increase in induced excitability. MCC intracellular Ca2+ transients are unaffected by the reduced HVCC Ca2+ influx. Deletion of α2δ-1 reduces the immediately releasable pool vesicle exocytosis but has no effect on catecholamine (CA) release in response to sustained stimuli. The increased electrical activity and CA release from MCCs might contribute to the previously reported cardiovascular phenotype of patients carrying α2δ-1 loss-of-function mutations.

Metalite, a new class of composite laminates with unique properties

Metalite is a new class of antisymmetric composite laminates composed of angle-plies, 0-degree, and 90-degree plies, presenting unique properties. These include extremely thin laminates suitable for minimum gauge applications, remarkable weight savings compared to conventional quads, adjustable zero and negative coefficients of thermal expansion (CTE), ease of manufacturing, excellent ability to adjust mode frequency, change sound radiation characteristics, and high tunability. In this study, Metalite laminates ranging from 3 to 8 plies are described using their feasible spaces and compared with quads, detailing the weight savings achieved for hard, soft, and neutral laminates. Through an experimental study, the CTE value of a hybrid Metalite is correlated with theory, demonstrating how to tune zero and negative CTE values. The proposed work offers significant benefits through practical solutions for designing and manufacturing lightweight composite laminates with unique properties.

Design and simulation of a tunable parity-time symmetric optoelectronic oscillator utilizing integrated components

Non-Hermitian photonics, relaying on parity-time (PT) symmetry, have shown promise in achieving mode selection for optical or microwave single-mode oscillation. Typically, a PT-symmetric system is constructed using two coupled loops with identical geometry. This article utilizes the PT-symmetry property to select a single frequency mode in an optoelectronic oscillator (OEO). However, traditional OEO implementations often involve discrete components, limiting widespread adoption due to factors such as size, weight, power consumption, and cost. Our aim in this paper is to leverage integrated components within the OEO loop. The proposed structure incorporates an integrated micro-ring resonator (MRR) with a high-quality factor (Q-factor) that serves both as a modulator and a resonator. Additionally, we suggest employing an adjustable integrated power splitter utilizing a micro heater to balance the gain and loss of two mutually coupled OEO loops. In this configuration, two integrated photo detectors (PD) are also utilized. In this setup, the single-frequency mode can be easily identified by simultaneously utilizing the properties of PT-symmetry and an integrated high-Q-factor resonator, obviating the need for a narrowband microwave filter. By adjusting the center frequency of the microwave photonic filter (MPF), the frequency of the generated signal can be tuned over a wide range. For instance, setting the generated frequency of the microwave signal to 11.5 GHz results in a measured phase noise of − 76.5 dBc/Hz at a 10-kHz offset frequency, with a side mode suppression ratio (SMSR) of 40 dB.

Computational biomechanics for a standing human body: Modal analysis and simulation.

We develop computational mechanical modeling and methods for the analysis and simulation of the motions of a human body. This type of work is crucial in many aspects of human life, ranging from comfort in riding, the motion of aged persons, sports performance and injuries, and many ergonomic issues. A prevailing approach for human motion studies is through lumped parameter models containing discrete masses for the parts of the human body with empirically determined spring, mass, damping coefficients. Such models have been effective to some extent; however, a much more faithful modeling method is to model the human body as it is, namely, as a continuum. We present this approach, and for comparison, we choose two digital CAD models of mannequins for a standing human body, one from the versatile software package LS-DYNA and another from open resources with some of our own adaptations. Our basic view in this paper is to regard human motion as a perturbation and vibration from an equilibrium position which is upright standing. A linear elastodynamic model is chosen for modal analysis, but a full nonlinear viscoelastoplastic extension is possible for full-body simulation. The motion and vibration of these two mannequin models is analyzed by modal analysis, where the normal vibration modes are determined. LS-DYNA is used as the supercomputing and simulation platform. Four sets of low-frequency modes are tabulated, discussed, visualized, and compared. Higher frequency modes are also selectively displayed. We have found that these modes of motion and vibration form intrinsic basic modes of biomechanical motion of the human body. This view is supported by our finding of the upright walking motion as a low-frequency mode in modal analysis. Such a "walking mode" shows the in-phase and out-of-phase movements between the legs and arms on the left and right sides of a human body, implying that this walking motion is spontaneous, likely not requiring any directives from the brain. Dynamic motions of CAD mannequins are also simulated by drop tests for comparisons and the validity of the models is discussed through Fourier frequency analysis. All computed modes of motion are collected in several sets of video animations for ease of visualization. Samples of LS-DYNA computer codes are also included for possible use by other researchers.

Patient preferences for preventive migraine treatments among Canadian adults: A discrete choice experiment.

To evaluate preferences for key attributes of injected or infused preventive migraine treatments and assess heterogeneity in preferences among Canadian participants with migraine. Current treatment options for migraine prevention differ in their attributes, including mode of administration, efficacy, and dosing frequency; preferences for such attributes can vary among patients. With the advent of new therapies, evidence demonstrating patient preferences for injected or infused preventive migraine treatments is necessary. Canadian adults self-reporting a diagnosis of migraine completed a cross-sectional, internet-based survey that included a discrete choice experiment. Participants were presented with attributes of preventive migraine treatments, including speed of onset, durability of efficacy, mode of administration, administration setting, and dosing frequency. Latent class analysis (LCA) was used to identify subgroups of patients who differed in their treatment preferences. In total, 200 participants completed the survey. Participants' treatment preferences were most sensitive to improvements in the durability of effectiveness from "wears off 2 weeks before next dose" to "does not wear off before the next dose" (absolute difference in weights = |-0.95 to 1.07| = 2.02) and improvements from "cranial injections" to "intravenous infusions" (|-1.04 to 0.58| = 1.62); participants equally preferred self-injection and intravenous infusion from a health-care provider (mean weight = 0.58 and 0.47, respectively) as a route of administration over cranial injections (mean weight = -1.04). Three subgroups were identified with LCA: group one (n = 103) prioritized fast-acting and durable therapies, group two (n = 54) expressed aversion to cranial injections, and group three (n = 43) favored treatments administered in a health-care provider setting. In this sample of Canadian adults with migraine, we showed that durability of effectiveness and mode of administration are key attributes influencing patient preferences for preventive migraine treatments; however, certain groups of patients may differ in their treatment priorities. Our results highlight the need for patient-provider discussions regarding treatment attributes and consideration of patients' preferences when selecting a preventive migraine treatment.

Patient preferences for preventive migraine treatments among Canadian adults: A discrete choice experiment.

To evaluate preferences for key attributes of injected or infused preventive migraine treatments and assess heterogeneity in preferences among Canadian participants with migraine. Current treatment options for migraine prevention differ in their attributes, including mode of administration, efficacy, and dosing frequency; preferences for such attributes can vary among patients. With the advent of new therapies, evidence demonstrating patient preferences for injected or infused preventive migraine treatments is necessary. Canadian adults self-reporting a diagnosis of migraine completed a cross-sectional, internet-based survey that included a discrete choice experiment. Participants were presented with attributes of preventive migraine treatments, including speed of onset, durability of efficacy, mode of administration, administration setting, and dosing frequency. Latent class analysis (LCA) was used to identify subgroups of patients who differed in their treatment preferences. In total, 200 participants completed the survey. Participants' treatment preferences were most sensitive to improvements in the durability of effectiveness from "wears off 2 weeks before next dose" to "does not wear off before the next dose" (absolute difference in weights = |-0.95 to 1.07| = 2.02) and improvements from "cranial injections" to "intravenous infusions" (|-1.04 to 0.58| = 1.62); participants equally preferred self-injection and intravenous infusion from a health-care provider (mean weight = 0.58 and 0.47, respectively) as a route of administration over cranial injections (mean weight = -1.04). Three subgroups were identified with LCA: group one (n = 103) prioritized fast-acting and durable therapies, group two (n = 54) expressed aversion to cranial injections, and group three (n = 43) favored treatments administered in a health-care provider setting. In this sample of Canadian adults with migraine, we showed that durability of effectiveness and mode of administration are key attributes influencing patient preferences for preventive migraine treatments; however, certain groups of patients may differ in their treatment priorities. Our results highlight the need for patient-provider discussions regarding treatment attributes and consideration of patients' preferences when selecting a preventive migraine treatment.

The effect of kinetic and photoinduced processes at high pressure on cluster structure of ultrahard fullerite

We have studied the kinetic and photoinduced aspects of the transformation of the cluster structure of ultrahard fullerite in the pressure ranges of a new phase diagram of carbon corresponding to the stability regions of diamond (up to 55 GPa) and onion-like structures of the fullerene type (55–115 GPa). A long exposure (for 3 months) at 25 GPa leads to the beginning of the transformation of the sample of ultrahard fullerite into diamond. A further increase to 70 GPa, under the influence of laser irradiation at wavelengths of 405 and 532 nm at doses of 5*1013 J/m2, initiates the formation of onion-like particles in ultrahard fullerite samples, which is accompanied either by a broadening of the high-frequency mode of the Raman spectrum, or its disappearance under high pressure. After pressure release, the high frequency mode of the Raman spectrum is located at 1570–1605 cm−1 at excitation wavelengths of 532 and 405 nm, respectively, which is typical for ultrahard fullerite. Bulk modulus of such an irradiated sample corresponds to a typical value for ultrahard fullerite of about 600 GPa. In addition to the formation of nanodiamonds and onion–like particles, the synthesis of ultrahard fullerite is also accompanied by the appearance in the sample of systems with double bonds of the C=C=C type with a line around 1935 cm−1, the nature of which is not entirely clear.

Global gyrokinetic study of density gradient driven instability in tokamaks: the ubiquitous mode

The ubiquitous mode is investigated in the linear regime for the first time using a global gyrokinetic model. These modes are driven by the density gradient in trapped electron population but with mode frequency in the ion diamagnetic drift direction, in contrast to the conventional trapped electron mode. The dispersion relation is calculated along with the global mode structure. The ubiquitous mode is quite global although appears at a shorter wavelength ( 12ky2ρi2>1 ). We show that the main driving mechanism is the density gradient and the temperature gradient has only a modest effect; the mode can persist at higher temperature gradient scenarios making it another possible channel of anomalous transport. The magnetic shear reduces the growth of the mode; while the electron to ion temperature ratio has a nonmonotonic effect on the mode growth rate—growth rate increases initially for the ubiquitous branch of the mode and decreases afterwards as the conventional trapped electron mode starts dominating. The role of safety factor and toroidicity is also analyzed. Finally, a mixing length-based estimation of transport is presented.