Self-generation of hollow current profile and tilt instability in field-reversed configuration

Two-dimensional electromagnetic particle simulation is performed to investigate the profile relaxation from a magnetohydrodynamic (MHD) equilibrium to a kinetic one and the physical property of the kinetic equilibrium in the field-reversed configuration. The radial oscillation is excited in order to relax an excess energy in the MHD equilibrium. After this profile oscillation, the system spontaneously relaxes toward a kinetic equilibrium, in which the electron current profile becomes hollow as a result of the combined effects of the gradient-B drift near the field-null line and the E×B drift generated by the ion finite Larmor radius effect near the magnetic separatrix. On the other hand, the ion current profile becomes peaked due to the effect of the ion meandering orbit near the field-null line. The stability of the obtained kinetic equilibrium against the tilt mode is also studied by means of three-dimensional full electromagnetic particle simulation. It is found that the growth rate of the tilt instability in the case of the hollow current profile and high separatrix beta value is smaller than that in the case of the peaked current profile

Dynamical Evolution of Thin Current Sheets in a Three-Dimensional Open System

Dynamical behavior of thin current sheets under the influence of collisionless reconnection in an open system is investigated by using newly developed electromagnetic (EM) particle simulation codes. In a three-dimensional open system collisionless driven reconnection evolves dynamically under the influence of an external driving flow and three different types of plasma instabilities excited in a thin current sheet. Driving electric field imposed at the boundary penetrates into the current sheet in accordance with the propagation of the lower hybrid drift wave excited in the periphery. When the electric field reaches the neutral sheet, collisionless reconnection is triggered. The current sheet is split as a result of collisionless reconnection, and thus small islands appear in the downstream. The accumulation of current density inside the islands excites the kink instability leading to the destruction of the island structure. A low frequency EM instability is triggered in the current sheet after the island structure disappears in the system

Development of Multi-Hierarchy Simulation Model with Non-Uniform Space Grids for Collisionless Driven Reconnection

A multi-hierarchy simulation model aimed at magnetic reconnection studies has been developed, in which macroscopic and microscopic physics are solved self-consistently and simultaneously. In this work, the previous multi-hierarchy model by these authors is extended to a more realistic one with non-uniform space grids. Based on the domain decomposition method, the multi-hierarchy model consists of three parts: a magnetohydrodynamics algorithm to express the macroscopic global dynamics, a particle-in-cell algorithm to describe the microscopic kinetic physics, and an interface algorithm to interlock macro and micro hierarchies. For its verification, plasma flow injection is simulated in this multi-hierarchy model and it is confirmed that the interlocking method can describe the correct physics. Furthermore, this model is applied to collisionless driven reconnection in an open system. Magnetic reconnection is found to occur in a micro hierarchy by injecting plasma from a macro hierarchy

Effective heating of nonadiabatic protons in magnetic reconnection with a guide field

The mechanism of plasma heating through magnetic reconnection with a guide magnetic field is investigated by means of two-dimensional electromagnetic particle simulations. These simulations mimic the dynamics of two torus plasmas merging through magnetic reconnection in a spherical tokamak (ST) device. It is found that a large part of protons, which behave as nonadiabatic, are effectively heated in the downstream because a ring-like structure of proton velocity distribution is observed at a local point in the downstream. The characteristic features of the velocity distribution can be explained as the following proton motion. Upon entering the downstream across the separatrix, nonadiabatic protons suddenly feel the strong electromagnetic field in the downstream and move in the outflow direction while rotating mainly around the guide magnetic field. The protons gain kinetic energy not only on the separatrix but also in the downstream. This effective heating process can be interpreted as the “pickup,” which, however, was thought to be responsible for only heavy ions. In this work, it is demonstrated that the pickup of protons is compatible with the known pickup theory in the cases in which the plasma beta is much less than 1, which is satisfied in STs

Equivalent-Circuit Model for Axisymmetric High-Temperature Superconducting Film: Application to Contactless jC Measurement System and Pellet Injection System

A high-temperature superconducting (HTS) film is used for numerous engineering devices. The analysis of the shielding current density in the HTS is essential to develop the HTS devices. By using the equivalent-circuit model (ECM) [1] , the analysis of the shielding current density becomes equivalent to solving the initial-value problem of the 1st-order ordinary differential system

Current Distribution Optimization by Using Genetic-Algorithm Based On-Off Method: Application to Pellet Injection System

The current distribution in the electromagnet is optimized by using the genetic-algorithm based on-off method so as to maximize the acceleration performance of the Superconducting Linear Acceleration (SLA) system. In the SLA system, a pellet container is accelerated by the interaction between a shielding current density and an applied magnetic field. By using the equivalent-circuit model, the distribution of the shielding current density is approximated as a set of the multiple current loops. In contrast, the current distribution in the electromagnet is represented by means of the on-off method. As the method for optimizing the current distribution in the electromagnet, two types of genetic algorithms are adopted. The results of computations show that the pellet velocity for the optimized current distribution is 1.3 times as fast as that for the homogeneous current distribution

The investigation of back-transformation mechanisms of ringwoodite and majorite in the Yamato 75267 H6

The Tenth Symposium on Polar Science/Special session: [OA] Antarctic meteorites, Thur. 5 Dec. / 3F Multipurpose conference room, National Institute of Polar Researc

Simulation Data Analysis by Virtual Reality System

We introduce new software for analysis of time-varying simulation data and new approach for contribution of simulation to experiment by virtual reality (VR) technology. In the new software, the objects of time-varying field are visualized in VR space and the particle trajectories in the time-varying electromagnetic field are also traced. In the new approach, both simulation results and experimental device data are simultaneously visualized in VR space. These developments enhance the study of the phenomena in plasma physics and fusion plasmas

Dependence of the Pickup-Like Ion Effective Heating on the Poloidal and Toroidal Magnetic Fields during Magnetic Reconnection

The dependence of the ion effective heating on the poloidal (reconnection) and toroidal (guide) magnetic fields during magnetic reconnection in the presence of a guide magnetic field is investigated by means of particle simulations, which mimic merging plasmas in a spherical tokamak. In previous works, our simulations demonstrated that the ion temperature perpendicular to the magnetic field grows mainly in the downstream, in which ring-shaped velocity distributions are formed. This means that ions are effectively heated. The basic theory explains that the ring-shaped distribution is formed by the ions which rotate around the guide magnetic field while E × B drifting. In this work, the basic theory is extended to a more general theory including not only a ring-shaped distribution, but also a circular-arc-shaped distribution. The generalized theory explains that the effective temperature changes by the radius and the central angle of the arc-shaped velocity distribution and conjectures the dependence of the ion effective heating on the poloidal and toroidal magnetic fields. The simulations show that the ion heating energy is proportional to the square of the poloidal magnetic field, whereas the ion temperature decreases as the toroidal field is larger, but the toroidal field dependence becomes small for the regime of high toroidal field. These tendencies are consistent with those observed in experiments