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Z. X. Liu

Publications and source records attributed to Z. X. Liu.

7 recordsLinked to original sources
arXiv2020

Edge Temperature Ring Oscillation Modulated by Turbulence Transition for Sustaining Stationary Improved Energy Confinement Plasmas

A reproducible stationary improved confinement mode (I-mode) has been achieved recently in the Experimental Advanced Superconducting Tokamak, featuring good confinement without particle transport barrier, which could be beneficial to solving the heat flux problem caused by edge localized modes (ELM) and the helium ash problem for future fusion reactors. The microscopic mechanism of sustaining stationary I-mode, based on the coupling between turbulence transition and the edge temperature oscillation, has been discovered for the first time. A radially localized edge temperature ring oscillation (ETRO) with azimuthally symmetric structure ($n=0$,$m=0$) has been identified and it is caused by alternative turbulence transitions between ion temperature gradient modes (ITG) and trapped electron modes (TEM). The ITG-TEM transition is controlled by local electron temperature gradient and consistent with the gyrokinetic simulations. The self-organizing system consisting with ETRO, turbulence and transport transitions plays the key role in sustaining the I-mode confinement. These results provide a novel physics basis for accessing, maintaining and controlling stationary I-mode in the future.

physics.plasm-ph↗
arXiv2019

I-mode investigation on the Experimental Advanced Superconducting Tokamak

By analyzing large quantities of discharges in the unfavorable ion $ \vec B\times \nabla B $ drift direction, the I-mode operation has been confirmed in EAST tokamak. During the L-mode to I-mode transition, the energy confinement has a prominent improvement by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with the I-mode observation on other devices, the $ E_r $ profiles obtained by the eight-channel Doppler backscattering system (DBS8)\cite{J.Q.Hu} show a deeper edge $ E_r $ well in the I-mode than that in the L-mode. And a weak coherent mode (WCM) with the frequency range of 40-150 kHz is observed at the edge plasma with the radial extend of about 2-3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the $ E_r $ fluctuation and the caused flow shear from WCM should play an important role during I-mode. In addition, a low-frequency oscillation with a frequency range of 5-10 kHz is always accompanied with WCM, where GAM intensity is decreased or disappeared. Many evidences show that the a low-frequency oscillation may be a novel kind of limited cycle oscillation but further investigations are needed to explain the new properties such as the harmonics and obvious magnetical perturbations.

physics.plasm-ph↗
arXiv2010

Observation of Anomalous Phonons in E-Type Orthorhombic REMnO3

We observe the appearance of a phonon near the lock-in temperature in orthorhombic REMnO3 (RE: Lu and Ho) and anomalous phonon hardening in orthorhombic LuMnO3. The anomalous phonon occurs at the onset of spontaneous polarization. No such changes were found in incommensurate orthorhombic DyMnO3. These observations directly reveal different electric polarization mechanisms in the E-type and IC-type REMnO3.

cond-mat.str-el↗
arXiv2010

Anomalous Hall Effect in Fe/Gd Bilayers

Non-monotonic dependence of anomalous Hall resistivity on temperature and magnetization, including a sign change, was observed in Fe/Gd bilayers. To understand the intriguing observations, we fabricated the Fe/Gd bilayers and single layers of Fe and Gd simultaneously. The temperature and field dependences of longitudinal resistivity, Hall resistivity and magnetization in these films have also been carefully measured. The analysis of these data reveals that these intriguing features are due to the opposite signs of Hall resistivity/or spin polarization and different Curie temperatures of Fe and Gd single-layer films.

cond-mat.mtrl-sci↗
arXiv2007

Satellite Observations of Separator Line Geometry of Three-Dimensional Magnetic Reconnection

Detection of a separator line that connects magnetic nulls and the determination of the dynamics and plasma environment of such a structure can improve our understanding of the three-dimensional (3D) magnetic reconnection process. However, this type of field and particle configuration has not been directly observed in space plasmas. Here we report the identification of a pair of nulls, the null-null line that connects them, and associated fans and spines in the magnetotail of Earth using data from the four Cluster spacecraft. With di and de designating the ion and electron inertial lengths, respectively, the separation between the nulls is found to be ~0.7di and an associated oscillation is identified as a lower hybrid wave with wavelength ~ de. This in situ evidence of the full 3D reconnection geometry and associated dynamics provides an important step toward to establishing an observational framework of 3D reconnection.

physics.plasm-ph↗
arXiv2007

In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail

Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September, 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.

physics.plasm-ph↗