La 0.67 Ca 0.33 Mn 1Ϫx Cu x O 3 (xϭ0 and 0.15͒ epitaxial thin films were grown on the ͑100͒ LaAlO 3 substrates, and the temperature dependence of their resistivity was measured in magnetic fields up to 12 T by a four-probe technique. We found that the competition between the ferromagnetic metallic ͑FM͒ and paramagnetic insulating ͑PI͒ phases plays an important role in the observed colossal magnetoresistance ͑CMR͒ effect. Based on a scenario that the doped manganites approximately consist of phase-separated FM and PI regions, a simple phenomenological model was proposed to describe the CMR effect. Using this model, we calculated the resistivity as functions of temperature and magnetic field. The model not only qualitatively accounts for some main features related to the CMR effect, but also quantitatively agrees with the experimental observations.

[1] Using a 2.5-dimensional, time-dependent ideal magnetohydrodynamic model in spherical coordinates, we present a numerical study of the property of magnetostatic equilibria associated with a coronal magnetic flux rope embedded in an axisymmetrical background magnetic field. The background field is potential (either closed or partly opened), a magnetic flux rope emerges out of the solar surface, and the resultant system is allowed to relax to equilibrium through numerical simulation. It is shown that the flux rope either sticks to the solar surface so that the whole magnetic configuration stays in equilibrium or escapes from the top of the computational domain, leading to the opening of the background field. Whether the rope remains attached to the solar surface or escapes to infinity depends on the magnetic energy of the system. The rope sticks to the solar surface when the magnetic energy of the system is less than a certain threshold, and it escapes otherwise. The threshold is slightly larger than the open limit, i.e., the magnetic energy of the corresponding fully opened field. The gravity, say, associated with the prominence mass, will raise the threshold by an amount that is approximately equal to the magnitude of the excess gravitational energy associated with the prominence. It implies that a catastrophe occurs when the magnetic energy of the system exceeds the threshold. The implication of such a catastrophe in coronal mass ejections is briefly discussed.

No abstract

Mastering nuclear fusion, which is an abundant, safe, and environmentally competitive energy, is a great challenge for humanity. Tokamak represents one of the most promising paths toward controlled fusion. Obtaining a high-performance, steady-state, and long-pulse plasma regime remains a critical issue. Recently, a big breakthrough in steady-state operation was made on the Experimental Advanced Superconducting Tokamak (EAST). A steady-state plasma with a world-record pulse length of 1056 s was obtained, where the density and the divertor peak heat flux were well controlled, with no core impurity accumulation, and a new high-confinement and self-organizing regime (Super I-mode = I-mode + e-ITB) was discovered and demonstrated. These achievements contribute to the integration of fusion plasma technology and physics, which is essential to operate next-step devices.

The effects of the cooperative Jahn-Teller effect on the crystal structure and the stability of the charge ordered (CO) state were studied by measurements of powder X-ray diffraction, resistivity, and ultrasound for Pr 1-x Ca x MnO 3 (0.5≤x≤0.875). Powder X-ray diffraction revealed a change of the crystal structure from tetragonally compressed to tetragonally elongated orthorhombic between x=0.75 and x=0.8 in the CO state, resulting from the crossover of the cooperative Jahn-Teller vibration mode from Q 2 to Q 3 . The relative stiffening of the ultrasound (∆V/V) reflecting the magnitude of the cooperative Jahn-Teller lattice distortion in the CO state increases with increasing x from 0.5 to 0.625, reaching the largest and being almost x-independence for 0.625≤x≤0.8, and drops steeply with further increase of x.Coincident with the variation of the ∆V/V with x, the stability of the CO state reflected by the magnetoresistance effect increases with increasing x from 0.5 to 0.625, reaching the most stable for 0.625

Investigation on the equilibrium operation regime, its ideal magnetohydrodynamics (MHD) stability and edge localized modes (ELM) characteristics is performed for the China Fusion Engineering Test Reactor (CFETR). The CFETR operation regime study starts with a baseline scenario (R = 5.7 m, B T = 5 T) derived from multi-code integrated modeling, with key parameters β N , β T , β p varied to build a systematic database. These parameters, under profile and pedestal constraints, provide the foundation for the engineering design. The long wavelength low-n global ideal MHD stability of the CFETR baseline scenario, including the wall stabilization effect, is evaluated by GATO. It is found that the low-n core modes are stable with a wall at r/a = 1.2. An investigation of intermediate wavelength ideal MHD modes (peeling ballooning modes) is also carried out by multi-code benchmarking, including GATO, ELITE, BOUT++ and NIMROD. A good agreement is achieved in predicting edge-localized instabilities. Nonlinear behavior of ELMs for the baseline scenario is simulated using BOUT++. A mix of grassy and type I ELMs is identified. When the size and magnetic field of CFETR are increased (R = 6.6 m, B T = 6 T), collisionality correspondingly increases and the instability is expected to shift to grassy ELMs.

The latest BOUTþþ studies show an emerging understanding of dynamics of edge localized mode (ELM) crashes and the consistent collisionality scaling of ELM energy losses with the world multitokamak database. A series of BOUTþþ simulations are conducted to investigate the scaling characteristics of the ELM energy losses vs collisionality via a density scan. Linear results demonstrate that as the pedestal collisionality decreases, the growth rate of the peeling-ballooning modes decreases for high n but increases for low n (1 < n < 5), therefore the width of the growth rate spectrum c(n) becomes narrower and the peak growth shifts to lower n. Nonlinear BOUTþþ simulations show a two-stage process of ELM crash evolution of (i) initial bursts of pressure blob and void creation and (ii) inward void propagation. The inward void propagation stirs the top of pedestal plasma and yields an increasing ELM size with decreasing collisionality after a series of microbursts. The pedestal plasma density plays a major role in determining the ELM energy loss through its effect on the edge bootstrap current and ion diamagnetic stabilization. The critical trend emerges as a transition (1) linearly from ballooning-dominated states at high collisionality to peelingdominated states at low collisionality with decreasing density and (2) nonlinearly from turbulence spreading dynamics at high collisionality into avalanche-like dynamics at low collisionality. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4905070]The phenomena of nonlocal transport, such as avalanches or turbulence spreading, are well-known in diverse systems. Examples include, but are not limited to: piles of granular matter, discrete energy dissipating events in earthquakes and solar flares, and turbulent overshoot and penetration in fluid turbulence. Nonlocal dynamics of turbulence, transport, and zonal flows are often observed in plasma turbulence simulations and experiments, such as Edge Localized Modes (ELMs) in tokamaks. ELMs are a common characteristic feature of the tokamak H-mode plasma regime, where the high frequency ELM instability repeats periodically throughout the high confinement mode (H-mode) phase of the discharge. 1 The instability causes quasi-periodic relaxations of the edge pedestal, resulting in a series of hot plasma eruptions on a fast MHD timescale and leading to large energy fluxes to the plasma facing components (PFCs), which will suffer from excessive ablation, fast erosion, or melting. The concern about the survival of PFCs in ITER has sparked intense interest in ELM dynamics and in the parameter scaling of ELM energy loss. Numerous experiments in divertor tokamaks have shown a decrease in the relative type I ELM energy loss with increasing pedestal density (collisionality) over a decade ago. 2 There are attempts to provide an explanation for the dependence of ELM energy loss on collisionality. 3 However, there is as yet no common accepted explanation of the observed scaling neither from analytical theory nor numerical simulations. 4He...

The Jahn-Teller effect in the charge-ordered (CO) state for La 1-x Ca x MnO 3 (0.5≤x≤0.87) was studied by measuring the low-temperature powder x-ray diffraction, internal friction, and shear modulus. We find that the electron-lattice interaction with the static Jahn-Teller distortion is the strongest near x ≈ 0.75 in the CO state. It was particularly observed that a crossover of the Jahn-Teller vibration mode from Q 2 to Q 3 near x=0.75 induces crossovers of the crystal structure from tetragonally compressed to tetragonally elongated orthorhombic, and of the magnetic structure from CE-type to C-type near x=0.75. The experimental results give strong evidence that the Jahn-Teller effect not only plays a key role in stabilizing the CO state, but also determines the magnetic and crystal structures in the CO state for La 1-x Ca x MnO 3 . 1 It is well known that the La 1-x Ca x MnO 3 (0.5≤x≤0.87) manganites show charge, spin, and/or orbital orderings below the charge ordering transition temperature T CO [1], and much efforts have been devoted to this system to disclose the microscopic origin of the charge-ordered (CO) state [2,3]. It has been suggested that when the long-range Coulomb interaction and/or a strong electron-lattice interaction with the Jahn-Teller (JT) distortion overcomes the kinetic energy of e g electrons, real-space charge and concomitant spin and/or orbital orderings occur throughout the crystal structure. Despite many investigations have been done on this aspect, the main driving force of the CO state being the long-range Coulomb interaction or the electron-lattice interaction with the JT effect or both is still a subject of discussion [3,4]. Recent experimental observation of "wigner-crystal" CO state from transmission electron microscopy, synchrotron x-ray and neutron diffractions on La 0.33 Ca 0.67 MnO 3 [2,5,6] demonstrates that the long-range Coulomb interaction might be the main driving force of the CO state, and indeed, some theoretical calculations [7,8] support the Coulombic model. However, if CO state was mainly due to long-range Coulomb interaction, the z-axis stacking of charges [9] and the observed "bi-stripe" CO state [10] which are both energetically penalized by the nearest-neighbor Coulomb repulsion V NN , therefore, can not be fully understood based on the Coulombic model. This shows that other ingredients, especially the electron-lattice interaction with the JT effect, are needed to understand the formation of CO state. The theoretical calculations in Refs. [3,4,[11][12][13][14][15][16] have shown that the JT effect not only stabilizes the CO state, but also strongly affects the magnetic structures. The relative importance of the long-range Coulomb interaction or the electron-lattice interaction with