Takaya Hayashi

Externally driven magnetic reconnection and a powerful magnetic energy converter

Numerical simulation of two‐dimensional compressible magnetic reconnection is carried out for more than a dozen cases with different anomalous resistivities and boundary conditions. After a quiescent stage of magnetic energy buildup, anomalous resistivity leads to an abrupt conversion of the stored magnetic energy into the plasma bulk motion and heat. Consequently, plasma jets as high as the local Alfven speed are generated downstream of the magnetic separatrix. Slow shocks formed just downstream of the separatrix and fast mode expansion in the upstream region play a leading role in the formation of strong plasma jets. Anomalous resistivity is the primary cause of the abrupt onset of reconnection. Once reconnection preceeds, however, its ultimate fate no longer seems to be dependent on the resistivity, but is largely controlled by the boundary conditions.

Nonlinear three-dimensional simulation for self-organization and flow generation in two-fluid plasmas

A three-dimensional Hall-Magnetohydrodynamics (Hall-MHD) simulation code has been developed to study the self-organization process in two-fluid plasmas. An appreciable amount of flow is created in the direction perpendicular to the magnetic field, which is in a sharp contrast with the relaxed states in single-fluid MHD.

Self-organizing plasmas

The primary purpose of this paper is to extract a grand view of self-organization through an extensive computer simulation of plasmas. The assertion is made that self-organization is governed by three key processes, i.e. the existence of an open complex system, the existence of information (energy) sources and the existence of entropy generation and expulsion processes. We find that self-organization takes place in an intermittent fashion when energy is supplied continuously from outside. In contrast, when the system state is suddenly changed into a non-equilibrium state externally, the system evolves stepwise and reaches a minimum energy state. We also find that the entropy production rate is maximized whenever a new ordered structure is created and that if the entropy generated during the self-organizing process is expelled from the system, then the self-organized structure becomes more prominent and clear.

Modeling of magnetic island formation in magnetic reconnection experiment

Formation of a magnetic island found in the Magnetic Reconnection Experiment (MRX) [M. Yamada, H. Ji, S. Hsu, et al., Phys. Plasmas 4, 1936 (1997)] is investigated by a magnetohydrodynamic (MHD) relaxation theory and a numerical simulation. In the cohelicity injection with a mean toroidal field, the growing process of the island into a spheromak-type configuration is explained by quasistatic transition of the force-free and minimum energy state to a state with larger normalized helicity. It also turns out that no magnetic island would be generated in the counterhelicity case. The MHD simulation with inhomogeneous electric resistivity agrees with experimental results, which clearly shows formation and growth of the magnetic island in a diffusion region where the reconnection takes place.

Magnetic reconnection and relaxation phenomena in Spherical Tokamak

Reconnection is a transient process in essence, and causality is a key point in dealing with reconnection. The driven concept came from this viewpoint. Computer simulation is a powerful tool to understand the overall processes in a self-consistent manner. One example of global scale nonlinear processes observed in laboratory plasmas, where the driven magnetic reconnection plays important roles, is described.

Deceleration mechanism of spheromak-like compact toroid penetrating into magnetized plasmas

To understand the fueling process in a fusion device by a spheromak-like compact toroid (SCT) injection method, magnetohydrodynamic numerical simulations, where a SCT is injected into magnetized target plasmas, have been carried out so far. As a result, it has been found that the SCT penetration into magnetized target plasmas is accompanied by complex physical dynamics, which is not adequately described by the conventional simple theoretical model. In this study, based on the previous simulation results, a new theoretical model to determine the penetration depth of the SCT is represented. Here, the SCT is considered to be decelerated not only by the magnetic pressure force but also by the magnetic tension force, which is generated by the bending of the target magnetic field as a result of the SCT penetration. Furthermore, by comparing the penetration depth of the SCT estimated from the theoretical model with that in the simulation, the accuracy of the model is examined. Finally, the effect of magnetic reconnection on the SCT penetration is discussed.

Effect of magnetic reconnection on CT penetration into magnetized plasmas

To understand the fuelling process in a fusion device by a compact toroid (CT) injection method, three dimensional MHD numerical simulations, where a spheromak-like CT (SCT) is injected into magnetized target plasmas, has been carried out. It has been found that the SCT penetration into magnetized target plasmas is accompanied by complex physical dynamics, which is not simply described by the conventional simple theoretical model. One of the most remarkable phenomena is magnetic reconnection. Magnetic reconnection plays a role in supplying the high density plasma, initially confined in the SCT magnetic field, to the target region. Furthermore, it is suggested that magnetic reconnection relaxes the deceleration of the SCT.

Computer simulation of nonlinear phenomena in plasmas

Abstract Simulation Science is explored by developing Man-machine Interactive System for Simulation (MISSION), in which an advanced virtual reality visualization system is an important element. Plasma is an exemplar of a medium governed by complexity, and plasma physics research is evolving from a “static science” into a “dynamic science”. In this paper, a topic in Simulation Science on nonlinear behavior of plasma physics is picked up, specifically, energy relaxation phenomena. Three-dimensional simulations are executed to clarify physical mechanisms of relaxation activities that are observed in the spherical tokamak experiments. The simulation results predict an occurrence of self-organizing transition from an axisymmetric state to a stable n =2 helical state.