Decrease In Mode Frequency Research Articles

Error field penetration in J-TEXT tokamak based on two-fluid drift-MHD model

An externally generated resonant magnetic perturbation can induce complex non-ideal MHD responses in their resonant surfaces. We have studied the plasma responses using Fitzpatrick’s improved two-fluid model and program LAYER. We calculated the error field penetration threshold for J-TEXT. In addition, we find that the island width increases slightly as the error field amplitude increases when the error field amplitude is below the critical penetration value. However, the island width suddenly jumps to a large value because the shielding effect of the plasma against the error field disappears after the penetration. By scanning the natural mode frequency, we find that the shielding effect of the plasma decreases as the natural mode frequency decreases. Finally, we obtain the m/n = 2/1 penetration threshold scaling on density and temperature.

Impact of toroidal rotation on the resistive ballooning modes in ASDEX Upgrade tokamak

In this work, we investigate the behavior of instabilities appearing between type-I edge localized modes (ELMs), with increasing neutral beam injection (NBI) power concomitant increase in toroidal rotation, and compare it to the modeling result of the linear magneto-hydrodynamic (MHD) code CASTOR3D. An injection of one NBI beam, increasing toroidal rotation, results in the mode slowing down from 12 kHz to 7 kHz, and its associated radial displacement decreases from 5 mm to 3.5 mm. In addition, modes shift radially outwards towards higher q, decreasing their poloidal mode numbers. The mode velocity is measured to be close to the E × B velocity with significant uncertainties. Through a set of CASTOR3D simulations with varying profiles, resistivity has been identified as the primary contributor to the growth rates. Only a small stabilizing effect due to toroidal rotation has been observed. While experimental results show a decrease of mode frequency with rotation, the opposite trend is observed in modeling. Reasons for discrepancies between modeling and experiment are discussed. Nevertheless, a main contributor to the mode frequency has been identified to be rotation velocity. CASTOR3D classifies modes as resistive ballooning modes as they do not appear unstable in ideal MHD.

Influence of adiabatic response of passing energetic ions on high-frequency fishbone

Adiabatic response effects on high-frequency fishbone instability driven by passing energetic ions are studied. With finite orbit width effects, the adiabatic contribution is derived analytically for purely passing energetic ions. By approximating the adiabatic contribution to the first order of the reverse aspect ratio we derive one of its analytic expressions, which is expected to be more accurate than that in a previous work (Graves J P 2004 Phys. Rev. Lett. 92 185003). For high-frequency fishbone instability, nonadiabatic response is usually dominant over adiabatic response, but under certain circumstances the latter plays an important role, comparable to the former. With a more generalized distribution function by introducing Gaussian-type factors representing their pitch and radial dependences and using a slowing-down equilibrium distribution for the energy of energetic ions, numerical analysis indicates that the adiabatic contribution is conducive to stabilization of the mode and causes a decrease in mode frequency. In addition, we find that the adiabatic contribution has a weak stabilizing effect on the fishbone instability when the finite orbit width effect is taken into account. We further analyze the dependence of the adiabatic contribution on the characteristic parameters of the distribution function. When the neutral beam has either a larger deviation from the plasma axis or a larger radial profile, the adiabatic contribution has a more evident effect on the fishbone instability. When the neutral beam has a relatively small critical energy, the adiabatic contribution has a greater effect on the mode instability.

Numerical analysis of the effect of train length on train aerodynamic performance

The improved delayed detached eddy simulation is adopted in the present study to investigate the influence of the train length on its aerodynamic performance. The low y+ wall treatment and the cubic constitutive relation are adopted to resolve the viscous flows and model the anisotropic turbulence within the boundary layer. The analysis implied that the distribution region and intensity of velocity fluctuation are strengthened, resulting in a larger turbulence kinetic energy distribution and a higher boundary layer thickness as the train length increases. A reduction in the streamwise velocity and the negative pressure with the increasing train length on the tail train is observed, resulting in lower drag and lift coefficients. As the length of the train increases, both the mean and instantaneous slipstream velocities are increased. The boundary layer thickness and the skin friction coefficient are compared with flat plate theory, reduced-scale, and full-scale experiments, proving the ability of numerical simulation to model the boundary layer velocity profile and skin friction coefficient distribution correctly. The wake structures are identified by the Spectral Proper Orthogonal Decomposition method, the dominant mode frequency decreases, and the wavelength becomes larger as the length of the train becomes longer due to the thickening boundary layer.

Temperature and layer number dependence of the G and 2D phonon energy and damping in graphene

We have studied the temperature and size dependence of the G and 2D phonon modes in graphene. It is shown that in a graphene monolayer the phonon energy decreases whereas the phonon damping increases with increasing temperature. The electron–phonon interaction leads to hardening whereas the fourth-order anharmonic phonon–phonon processes lead to softening of the phonon energy with increasing temperature. We have shown that the electron–phonon interaction plays an important role also by the dispersion dependence of the phonon G mode, by the observation of the Kohn anomaly. The G mode frequency decreases and damping increases, whereas the 2D phonon frequency and damping increase with increasing layer number. The temperature and size effects of the 2D mode are much stronger than those of the G mode.

Raman studies of GaN/sapphire thin film heterostructures

Using Raman spectroscopy, we have studied the optical phonon modes of GaN nucleation layers with the thicknesses of 7 and 45nm, grown on sapphire (0001) substrates by metal organic chemical vapor deposition at low temperatures (500–600°C). These layers consisted of mixed hexagonal and cubic phases. The Raman results from mixed phases were compared with those from pure hexagonal layers which were grown at higher temperatures over 1000°C. The E2H and A1(LO) phonon modes are observed at 548 and 733cm−1 for 45nm thick nucleation layer, while the silent low-frequency B1 mode which is forbidden in good quality hexagonal GaN is observed at 314cm−1. The presence of the strong hexagonal modes for GaN nucleation layers of 45nm thick confirms the crystalline nature of the GaN nucleation layer and dominant hexagonal phase in this mixed cubic-hexagonal nucleation layer. The observed frequencies are shifted with respect to the corresponding A1 and E2 phonon modes in hexagonal GaN. The decrease in mode frequency implies the presence of in-plane tensile strain in these GaN nucleation layers of 45nm thick. The Raman scattering spectra taken from different positions on the sample show similar spectral features, indicating that the GaN nucleation layers of 45nm thick are homogeneous in micron scale.

Breathing mode frequencies of a rotating Fermi gas in the BCS-BEC crossover region

We study the breathing mode frequencies of a rotating Fermi gas trapped in a harmonic plus radial quartic potential. We find that as the radial anharmonicity increases, the lowest order radial mode frequency increases while the next lowest order radial mode frequency decreases. Then at a critical anharmonicity, these two modes merge and beyond this merge the cloud is unstable against the oscillations. The critical anharmonicity depends on both rotational frequency and the chemical potential. As a result of the large chemical potential in the BCS regime, even with a weak anharmonicity the lowest order mode frequency increases with decreasing the attractive interaction. For large enough anharmonicities in the weak-coupling BCS limit, we find that the excitation of the breathing mode frequencies make the atomic cloud unstable.

Oscillation and radiation of a superconducting ring in a Helmholtz coil

We discuss oscillation and radiation of a superconducting ring in a nonuniform magneticfield produced by a Helmholtz coil. This is a simple example of the oscillation of asuperconductor between two magnets. Two oscillation modes (the parallel mode and theantiparallel mode) exist, depending on the directions in which the currents flow in the twocoils of the Helmholtz coil. Calculations show the following facts. (i) A characteristicradius-to-height ratio of the Helmholtz coil where a maximum frequency can be producedexists. (ii) As the temperature increases, both the parallel and the antiparallel modefrequencies are fixed, but the parallel mode frequency starts to decrease and falls to 0 in atemperature region below the critical temperature, indicating an oscillation in theintermediate state. (iii) When the superconducting ring oscillates with trapped flux in it,as the trapped flux increases, the parallel mode frequency decreases, but theantiparallel mode frequency increases. When the frictional force is not present,radiation alone reduces the motion energy. We show that the oscillations in bothmodes stop as time approaches infinity. Over time, the amplitude decreases inthe form of a root inverse for the parallel mode and decreases exponentially forthe antiparallel mode. The half-lives of the amplitudes have been calculated as2.1 × 1023 years for theparallel mode and 6.9 × 1023 years for the antiparallel mode.

Raman investigations on nitrogen ion implantation effects on semi-insulating InP

Raman scattering measurements on liquid-encapsulated Czochralski-grown Fe-doped semi-insulating InP(1 0 0) single crystal substrates have been carried out before and after 120 keV N + implantation for various doses from 10 13 to 10 15 cm −2 and also after post-implantation rapid thermal annealing of these samples. It is observed that LO phonon mode frequency decreases and full width at half maximum (FWHM) increases with fluence due to implantation-induced lattice damage. Forbidden Raman TO mode in (1 0 0) cut InP is observed at the doses of 5×10 13 and 5×10 14 cm −2. This might have appeared due to the polycrystalline and/or misoriented regions created during implantation. TO mode is not observed for high doses in as-implanted samples due to excessive lattice damage induced by the implantation. On rapid thermal annealing at 573 K for 30 s, the implanted samples show a partial recovery of LO phonon mode frequency and FWHM due to partial annealing of the damage. On partial annealing of the implantation-induced defects, TO mode FWHM decreases and area under the peak increases for all the doses from 10 13 to 10 15 cm −2.

Absence of a PbTiO3 phase transition in PbTiO3/BaTiO3 superlattices

Abstract We have performed X-ray diffraction and Raman spectroscopy on three PbTiO 3 /BaTiO 3 superlattices (PTO/BTO) from room temperature (RT) to 600°C. The superlattices were 4000 A thick and the modulation wavelengths Λ, with equally thick layers of PTO and BTO, were 136, 187 and 357 A. The samples were laser deposited onto single crystal MgO substrates buffered with 50 A of SrTiO 3 (STO). We observe no X-ray evidence of a phase transition in the PTO layers of these three superlattice samples up to 600°C. This result was reinforced by X-ray modeling of the samples. We also monitored the E(1TO) or “soft “ mode by Raman spectroscopy up to 550°C and found only a linear decrease of mode frequency with temperature for all three superlattices: another sign that no phase transition is taking place in this temperature range.

Effect of mode locking on toroidal flow evolution

The response of the toroidal flow velocity to being abruptly locked in the vicinity of a magnetohydrodynamic (MHD) tearing-mode-induced magnetic island is examined analytically and numerically using a diffusive transport model in a cylindrical plasma. The resultant toroidal momentum confinement is shown to degrade significantly after mode locking, as is often observed on many tokamaks and reversed field pinches (RFPs). The degraded toroidal momentum confinement time in the core and edge regions of the plasma are evaluated and compared with experimental observations. The mode locking time-scale itself is also investigated by using a model toroidal torque balance equation. The decrease of mode frequency during mode locking seems to be governed mainly by the electromagnetic torque exerted on the resistive layer. For tokamak plasmas, the mode locking time-scale is found to be much shorter than the diffusion time-scale, which is in general agreement with experimental observations

Capillary bridge modes driven with modulated acoustic radiation pressure

A liquid bridge between two solid surfaces is known as a capillary bridge. In the experiments to be described, the bridge liquid is a mixture of PDMS with a dense organic liquid. The bridge has the same density as the surrounding water bath such that its equilibrium shape between circular posts is a circular cylinder of length L and radius R. Low‐frequency modes of such a bridge are known to exist where surface tension provides the restoring force. Even neglecting viscosity, the modal characteristic equation is complicated by the boundary conditions. It is demonstrated that low‐frequency modulation of an ultrasonic standing wave in the surrounding bath can be used to excite specific axisymmetric or nonaxisymmetric capillary modes. Coupling to each mode depends on the location of the bridge in the standing wave. The coupling is a consequence of the space‐time modulation of the radiation pressure as discussed previously for drops and bubbles [P. L. Martson, 15–26 (1980)]. The predicted decrease of mode frequency with increasing slenderness S=L/2R is confirmed as is the vanishing of the frequency of the most unstable mode as S approaches π. [Work supported by NASA.]