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内容記述 |
It is well known that relaxation phenomena which take place in thermonuclear plasma have great influence on the confinement in not only that it is usually harmful to sustainment of configuration, but sometimes that the confinement is improved to much better state by relaxation. The Taylor's theory [J.B. Taylor, Rev. Mod. Phys. 58, 741(1986)] is one of the most successful and useful theory describing relaxation phenomena, in which the total magnetic energy is minimized under the constraint that the total magnetic helicity is conserved. However, it is not sufficient to be applied for real fusion plasma because it ignores the existence of plasma pressure. On the other hand, the achievable plasma beta value in experiments has been increasing in recent years. In particular, the spherical tokamak concept is attracting a great deal of attention because it has excellent properties in the confinement and the stability at high β, and the engineering economy. Therefore, a generalized theory on relaxation phenomena including the effect of plasma pressure is needed. In this thesis, we shall try to reveal the dynamics of pressure-driven relaxation phenomena observed in spherical tokamak plasma, which is so-called IRE (Internal Reconnection Event), by means of a numerical simulation. IRE is observed as a rapid fall in the soft X-ray signal and an increase in the net toroidal current, together with a drastic deformation in overall shape. However, the physical mechanism of IRE had not been clarified at all until this study was carried out. The simulation results are full of suggestion for both improving the confinement of spherical tokamak plasma and understanding the general nature of the pressure-driven relaxation phenomena.<br /> The simulation is based on a nonlinear three-dimensional magnetohydrodynamic equations. It is executed in a full toroidal spherical tokamak geometry including an open external magnetic field region, so that the dynamical behavior of an IRE, such as a large distortion in overall shape, can be properly treated. The spontaneous time development of tiny perturbations applied on an initial unstable equilibrium is pursued by a high-accuracy scheme.<br /> The simulation results successfully reproduce the key features of IRE in good agreement with experimental observations. The dominant linear eigenmodes for an initial condition including a q = 1 rational surface are found to be a combination of several low-n modes. Especially, the m = 2/n = 2 and the m = 1/n = 1 pressure-driven interchange modes grow simultaneously with almost the same large growth rate. As a result of the nonlinear development of such low-n modes, which induce an elliptically elongating and shifting convection flow in the poloidal cross section, a pressure bulge appears on the surface of the torus in a toroidally localized region, where the radial positive displacements of each mode are aligned to each other. The localized deformation generates a current sheet structure near the separatrix, which induces magnetic reconnection between the internal and external magnetic fields. The reconnected field line links the core region at high pressure and the peripheral region at low pressure, so that a large pressure gradient is formed along the reconnected field line. The confined plasma is rapidly expelled out of the torus due to the flows induced by the pressure gradient. The expelled plasma extends in the periphery, forming a characteristic helically twisted conical layers at the top and the bottom of the torus, which is in good agreement with experimental results observed by using a CCD camera. On the other hand, the plasma pressure at the center of the torus falls into about 40% of that at the initial state in a short time scale of several tens of Alfven transit time, which also agrees well with experiments. After releasing a part of heat energy out of the separatrix, the system is once stabilized with respect to ideal modes. However, another kind of an instability, which has a nature of a resistive mode, is excited. This instability includes an m = 2/n = 1 component dominantly, and can be destructive compared to the original ideal modes. Due to the growth of this insta- bility, the overall shape of the torus is largely distorted, which agrees with experimental observations, such as the tilting and the axis-asymmetric elongation of the torus. <br /> More detailed analyses of the simulation results show several interesting dynamics on pressure-driven relaxation phenomena, such as an almost parallel magnetic reconnec- tion, formation of a tunnel-like convective loss channel due to the magnetic reconnection between the internal and the external fields, and spontaneous phase alignment among multiple modes on weakly nonlinear development of pressure-driven instability. |
要旨・審査要旨 / Abstract, Screening Result (327.8 kB)
