N. Mizuguchi

Dynamics of spherical tokamak plasma on the internal reconnection event

Nonlinear magnetohydrodynamic (MHD) simulations are executed to investigate the dynamical behavior of the relaxation phenomenon observed in spherical tokamak (ST) plasma that is known as the Internal Reconnection Event (IRE). The simulation results successfully reproduce several key features of IRE, and the physical mechanisms are revealed. A sudden collapse of the pressure profile takes place as a result of a nonlinear time development of a pressure-driven instability. A magnetic reconnection induced between the internal and the external magnetic field is found to play a crucial role in determining the nature of the overall process, namely, the rapid expulsion of the plasma heat energy due to the pressure imbalance along the reconnected field lines, and the large distortion in the overall shape. The resultant deformations in overall shape of the plasma are in good agreement with the experimental observations.

Dynamics of the ballooning mode and the relation to edge-localized modes in a spherical tokamak

Nonlinear simulations based on the magnetohydrodynamic model have been executed to reveal the dynamics of the ballooning mode in the spherical tokamak plasma. The simulation results have reproduced the characteristic features of the edge-localized mode crash phase, where the filamentary structures are formed along the magnetic field in the edge region, and separated from the core plasma. Moreover, the finite Larmor radius effect is addressed.

Nonlinear simulation of edge-localized mode in a spherical tokamak

A numerical modelling of the dynamics of an edge-localized mode (ELM) crash in the spherical tokamak is proposed by means of a three-dimensional nonlinear magnetohydrodynamic simulation. The simulation result shows a crash of the edge pressure profile due to the spontaneous growth of the ballooning mode instability. The simulation result shows good agreement in several characteristic features of the experimental observation of large scale ELMs in an appropriate time scale: (1) relation to the ballooning instability, (2) intermediate-n precursors, (3) low-n structure on the crash, (4) formation and separation of the filament and (5) considerable amount of convective loss of plasma, where n is the toroidal mode number. Furthermore, the model is verified by examining the effect of diamagnetic stabilization and by comparing the nonlinear behaviour with that of the peeling modes. The ion diamagnetic drift terms are found to stabilize some specific components linearly; nevertheless they are not so effective in the nonlinear dynamics such as the filament formation and the amount of loss. For the peeling mode case, no prominent filament structure is found to appear in contrast to the ballooning mode case.

Non-linear simulations of internal reconnection events in spherical tokamaks

Three dimensional MHD simulations are executed in a full toroidal geometry to clarify the physical mechanisms of the internal reconnection event (IRE) which is observed in spherical tokamak experiments. The simulation results reproduce several main properties of IREs. A notable property in the linear growth of modes is the simultaneous excitation of multiple low n modes, which grow together with similar growth rates. A spontaneous phase alignment mechanism among these excited modes is found in the non-linear stage of the development, which causes a bulge-like deformation of the torus and subsequent expulsion of heat energy to the external region through the occurrence of external reconnection.

Nonlinear dynamics of a collapse phenomenon in heliotron plasma with large pressure gradient

We have executed nonlinear magnetohydrodynamic simulations in a heliotron-type configuration with a large pressure gradient to reveal the nonlinear dynamics of a collapse phenomenon. The simulation results reproduce the qualitative characteristics of the experimental observation on the so-called core density collapse events in the Large Helical Device plasma with the super-dense core profile. A long-term nonlinear behaviour on the event, including the flushing mechanism of the core pressure, is clarified. The simulation result shows the linear growth of the ballooning-like resistive instability modes with the intermediate poloidal wavenumbers. The growth of the modes are eventually saturated, and the system experiences the energy relaxation in about 1 ms. It should be noted that the linear mode structures are localized in the edge region, whereas the core pressure rapidly falls as the system reaches the relaxed state. Such coexistence of the edge perturbation and the core collapse is consistent with the experimental observations. The lost pressure forms a wide base in the peripheral region. The core pressure is, on the other hand, remarkably reduced at a certain period, although it had well withstood the disturbance before it. The most salient feature on this period is the disordering of the magnetic field structure. The system keeps the nested-flux-surface structure well at the beginning, whereas part of them are abruptly lost in this period. Such a situation can induce a flattening of the pressure profile along the reconnected field lines. By checking the place where the plasma loss due to this mechanism occurs, such plasma outlets are found to be located mainly on the disordered region. Thus, one can conclude that the core collapse can be caused by the disturbance of the magnetic field.

2D electron temperature diagnostic using soft x-ray imaging technique

We have developed a two-dimensional (2D) electron temperature (T(e)) diagnostic system for thermal structure studies in a low-aspect-ratio reversed field pinch (RFP). The system consists of a soft x-ray (SXR) camera with two pin holes for two-kinds of absorber foils, combined with a high-speed camera. Two SXR images with almost the same viewing area are formed through different absorber foils on a single micro-channel plate (MCP). A 2D Te image can then be obtained by calculating the intensity ratio for each element of the images. We have succeeded in distinguishing T(e) image in quasi-single helicity (QSH) from that in multi-helicity (MH) RFP states, where the former is characterized by concentrated magnetic fluctuation spectrum and the latter, by broad spectrum of edge magnetic fluctuations.

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.

Simulation of edge-localized modes in a spherical tokamak : effects of ion diamagnetic drift

This paper presents the numerical modeling of nonlinear magnetohydro-dynamic (MHD) instabilities containing the effects of finite Larmor radius (FLR) in spherical tokamak geometry. Our results contain the comparison of small, inter-mediate, and large effects of the FLR. For small values, the linear and nonlinear behavior is not greatly modified, but with intermediate and large FLR, stabilizing effects on the higher wave number components are observed in the linear phase but in the nonlinear phase the overall properties are dominated by MHD effects. Similar filamentary structures are reproduced as found in some of the experimental observations as a result of the development of ballooning modes. The formation and the properties of filamentary type structures during edge-localized modes are also discussed.

Theoretical MHD Analyses of LHD Plasmas

Abstract Theoretical MHD analyses of the Large Helical Device (LHD) plasmas have progressed extensively. Recent results with respect to three-dimensional equilibrium, linear stability, and nonlinear dynamics are presented. Magnetic island variation due to finite-beta effect, the second stability of ballooning modes and related features, self-organization in nonlinear evolution of interchange modes, high-accuracy nonlinear calculation for ballooning modes up to high toroidal wave number, and plasma collapse in the ballooning mode dynamics are discussed.

Heating and particle build-up of field-reversed configuration due to neutral particle injection in a translation process

The magnetic configuration of a field-reversed configuration (FRC) has a center closed region and open field region. Because of this geometrical property, an FRC can be translated from a formation theta-pinch region into a confinement region while it keeps the closed magnetic field. In this work, the “equivalent” neutral beam injection (NBI) in the translation process has been proposed as a heating and particle injection method for an FRC. Translating an FRC plasma through neutral gas background is equivalent to an end-on NBI into the FRC. For the neutral particle density of 1 × 1020m-3, translation velocity of 100km/s and translation length of 1m, the number of particles ˜3 × 1018 and the kinetic energy of ˜40J are supplied due to the equivalent NBI.