Spontaneous structure formation in a network of chaotic units with variable connection strengths

As a model of temporally evolving networks, we consider a globally coupled logistic map with variable connection weights. The model exhibits self-organization of network structure, reflected by the collective behavior of units. Structural order emerges even without any inter-unit synchronization of dynamics. Within this structure, units spontaneously separate into two groups whose distinguishing feature is that the first group possesses many outwardly-directed connections to the second group, while the second group possesses only few outwardly-directed connections to the first. The relevance of the results to structure formation in neural networks is briefly discussed.Comment: 4 pages, 3 figures, REVTe

High Performance Operation of Negative-Ion-Based Neutral Beam Injection System for the Large Helical Device

It is a touchstone for the success of ITER and future fusion reactor whether the present high performance negative-ion-based NBI (N-NBI) heating systems work properly. The LHD and JT-60U are only two facilities where N-NBI systems are working for high power plasma heating / current drive in the world. Because handling of negative hydrogen / deuterium ions was amateur technology, it has taken a long time to improve its skill. In LHD, we succeeded in improving the performance of one of three beam lines dramatically in 2003 by adopting a multi-slot grounded grid for the accelerator of ion source. The effort on improving the performance was also done in other beam lines with conventional ion sources in parallel. The guidelines of improving are optimization of magnetic multi-cusp configuration for efficient negative ion production, and increasing the transparency of the grounded grid for reduction of heat load on it. As a result the available beam power has been increased, that is, successive injection power level more than 10MW became possible throughout four-month long experimental campaign, although the maximum injection power has been almost the same. The averaged negative ion beam current density at the exit of ion source, which was evaluated from the port-through injected power, was achieved up to 350A/m^2 up0 nosupersub which is larger than the required value of ITER NBI in hydrogen beam operation. Pulse length at high beam power level has also been extended owing to the reduction of heat load on the grounded grid. These results (increase in available power and pulse length) have contributed to expand the operation region of LHD. By continuous R&D, we also have found the way of solving an associated problem of multi-slot grounded grid system, that is, mismatched conditions of optimum beam optics in vertical and horizontal directions. According to this result, better beam divergence can be realized, and the increase in the total injection power is expected in the next experimental campaign

Stability diagrams for bursting neurons modeled by three-variable maps

We study a simple map as a minimal model of excitable cells. The map has two fast variables which mimic the behavior of class I neurons, undergoing a sub-critical Hopf bifurcation. Adding a third slow variable allows the system to present bursts and other interesting biological behaviors. Bifurcation lines which locate the excitability region are obtained for different planes in parameter space.Comment: 7 pages, 3 figures, accepted for publicatio

Random global coupling induces synchronization and nontrivial collective behavior in networks of chaotic maps

The phenomena of synchronization and nontrivial collective behavior are studied in a model of coupled chaotic maps with random global coupling. The mean field of the system is coupled to a fraction of elements randomly chosen at any given time. It is shown that the reinjection of the mean field to a fraction of randomly selected elements can induce synchronization and nontrivial collective behavior in the system. The regions where these collective states emerge on the space of parameters of the system are calculated.Comment: 2 pages, 2 figs, accepted in The European Physical Journa

Magnetic Phase Diagrams with Possible Field-induced Antiferroquadrupolar Order in TbB2_2C2_2

Magnetic phase diagrams of a tetragonal antiferromagnet TbB$_2$C$_2$ were clarified by temperature and field dependence of magnetization. It is noticeable that the N{\'e}el temperature in TbB$_2$C$_2$ is anomalously enhanced with magnetic fields, in particular the enhancement reaches 13.5 K for the ${}$ direction at 10 T. The magnetization processes as well as the phase diagrams are well interpreted assuming that there appear field-induced antiferroquadrupolar ordered phases in TbB$_2$C$_2$. The phase diagrams of the AFQ compounds in RB$_2$C$_2$ are systematically understood in terms of the competition with AFQ and AFM interactions.Comment: 4 pages, 4 figures, RevTeX

Magnetic-field enhanced aniferromagnetism in non-centrosymmetric heavy-fermion superconductor CePt3_3Si

The effect of magnetic field on the static and dynamic spin correlations in the non-centrosymmetric heavy-fermion superconductor CePt$_3$Si was investigated by neutron scattering. The application of a magnetic field B increases the antiferromagnetic (AFM) peak intensity. This increase depends strongly on the field direction: for B${\parallel}$[0 0 1] the intensity increases by a factor of 4.6 at a field of 6.6 T, which corresponds to more than a doubling of the AFM moment, while the moment increases by only 10 % for B${\parallel}$[1 0 0] at 5 T. This is in strong contrast to the inelastic response near the antiferromagnetic ordering vector, where no marked field variations are observed for B${\parallel}$[0 0 1] up to 3.8 T. The results reveal that the AFM state in CePt$_3$Si, which coexists with superconductivity, is distinctly different from other unconventional superconductors.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.

Condensation in Globally Coupled Populations of Chaotic Dynamical Systems

The condensation transition, leading to complete mutual synchronization in large populations of globally coupled chaotic Roessler oscillators, is investigated. Statistical properties of this transition and the cluster structure of partially condensed states are analyzed.Comment: 11 pages, 4 figures, revte

Amplitude death in coupled chaotic oscillators

Amplitude death can occur in chaotic dynamical systems with time-delay coupling, similar to the case of coupled limit cycles. The coupling leads to stabilization of fixed points of the subsystems. This phenomenon is quite general, and occurs for identical as well as nonidentical coupled chaotic systems. Using the Lorenz and R\"ossler chaotic oscillators to construct representative systems, various possible transitions from chaotic dynamics to fixed points are discussed.Comment: To be published in PR