G. Kurita

Nonlinear evolution of double tearing modes

The linear and nonlinear behaviors of the tearing mode is systematically studied for nonmonotonic q-profiles on the basis of the reduced magnetohydrodynamics (MHD) equations in cylindrical geometry, and some new features about the double tearing mode are revealed. The linear eigenmode scales as a resistive internal mode for a small distance between two rational surfaces with the same q-value, Δrs, and as the tearing one for large Δrs. New nonlinear phenomena appear in the tearing mode regime and for shorter Δrs. The linear eigenfunction shows sharply localized fluid motion at both resonant surfaces and small but global convective motion between the resonant surfaces; consequently the mode goes through a Rutherford-type regime. When the islands have grown enough, the mode shows explosive growth. This results from the nonlinear coupling among the higher harmonics, so that the inner and outer magnetic islands interact with each other leading to an internal disruption.

Runaway current termination in JT-60U

Termination of the runaway electron current generated during plasma disruptions is found in JT-60U during simulated vertical plasma displacement events where the safety factor at the plasma surface qs decreases. For all discharges with runaway electron generation, the runaway current disappears for qs≥2 with the appearance of spikes in the magnetic fluctuations. The growth rate of the spikes in the magnetic fluctuations decreases by an order of magnitude during the termination of runaway current. Corresponding to the loss of runaway electrons by magnetic fluctuations, heat flux pulses are measured at the inner divertor plates, which indicates interaction of the wall with the runaway electrons. The halo current during runaway termination is small and increases after runaway termination with a dominant toroidal mode of n = 1.

Ferromagnetic and resistive wall effects on the beta limit in a tokamak

The ferromagnetic and resistive wall effects on the beta limit in a high aspect ratio tokamak are investigated. It is shown that the beta limit is reduced to 90% of that without ferromagnetic effect for a high aspect ratio tokamak, when the relative permeability of the ferromagnetic wall is 2. In these analyses, parabolic profiles for both plasma current and pressure are employed and the radius and thickness of the resistive wall are rw = 1.43a and d = 0.07a (a is plasma minor radius), respectively. The stability window with respect to the external kink modes is shown to be reduced by a high permeability effect in the case of a uniform current tokamak plasma, which is different from the case where only the finite resistivity effect is considered. The effect of toroidal plasma flow is also investigated, and it is shown that the toroidal background flow velocity of 0.3vpa, vpa is poloidal Alfven velocity, is sufficient for the resistive wall to have the stability effect of an ideal wall. The ferromagnetic effect of the wall destabilizes both resistive wall and ideal kink modes.

Advanced fusion technologies developed for JT-60 superconducting tokamak

Modification of JT-60 as a full superconducting tokamak (JT-60SC) is planned. The objectives of the JT-60SC programme are to establish scientific and technological bases for steady-state operation of high performance plasmas and utilization of reduced-activation materials in an economically and environmentally attractive DEMO reactor. Advanced fusion technologies relevant to the DEMO reactor have been developed for the superconducting magnet technology and plasma facing components of the JT-60SC design. To achieve a high current density in a superconducting strand, Nb3Al strands with a high copper ratio of 4 have been newly developed for the toroidal field coils (TFCs) of JT-60SC. The R&D to demonstrate the applicability of the Nb3Al conductor to TFCs by a react-and-wind technique has been carried out using a full-size Nb3Al conductor. A full-size NbTi conductor with low ac loss using Ni-coated strands has been successfully developed. A forced cooling divertor component with high heat transfer using screw tubes has been developed for the first time. The heat removal performance of the carbon fibre composite target was successfully demonstrated on an electron beam irradiation stand.

A matrix method for resistive MHD stability analysis of axisymmetric toroidal plasma

Abstract A matrix method to solve a resistive MHD stability problem has been developed. The equations are reduced to an eigenvalue problem of block tridiagonal matrices. The inverse iteration method is employed as a solution method of the eigenvalue problem.

Design and analysis of plasma position and shape control in superconducting tokamak JT-60SC

Abstract The analyses of the plasma position and shape control in the superconducting tokamak JT-60SC in JAERI are presented. The vacuum vessel and stabilizing plates located closely to the plasma are modeled in 3 dimension, and we can take into account the large ports in the vacuum vessel. The linear numerical model used in the design for the plasma feedback control system is based on Grad–Shafranov equation, which allows the plasma surface deformation. For a slower control of the plasma shape, the superconducting equilibrium field (EF) coils outside toroidal field coils are used, while for a fast control of the plasma position, in-vessel normal conducting coils (IV coil) are used. It is shown that the available loop voltages of the EF and IV coils are very limited, but there are sufficient accuracy and acceptable response time of plasma position and shape control.

Convergence of solutions of the MHD stability code ERATO

Abstract Convergence of solutions of the ERATO code is studied numerically. Some irregularities of the computed data points on a convergence curve are observed in our investigation. It is conjectured that these irregularities are due to inaccuracy of the equilibrium or to inappropriate choice of the equilibrium calculation meshes.

Effect of electron and ion viscosity on sawtooth crash in a tokamak

The effect of anomalous electron and ion viscosity induced by the stochastization of magnetic field line on sawtooth crash in a tokamak is investigated by using the reduced set of resistive MHD equations. As the perturbation grows beyond some amplitude, the most unstable mode is turned from the pure resistive mode into the one induced by the anomalous electron viscosity, and the growth rate suddenly increases, as observed in some experiments. After that, the growth of perturbation is decreased due to the anomalous ion viscosity. The sawtooth crash time is prolonged in spite of the explosive growth due to the electron viscosity. However, it is not completely suppressed by the effect of anomalous ion viscosity.

Plasma simulator METIS for tokamak confinement and heating studies

Abstract To fill up a theoretical database necessary for the fusion reactor development of program a plasma simulator METIS was designed and a prototype plasma simulator ProtoMETIS was constructed. METIS is projected on the basis of a MIMD type parallel computer composed of 250 processor elements with distributed memories and optimized for analyses of the nonlinear MHD behavior of a plasma and the loss of alpha particles due to magnetic field ripples in a tokamak. By using ProtoMETIS performance of the METIS architecture was investigated for the above problems and satisfactory results were attained. It was also confirmed that a simulation of a free electron laser used for plasma heating and an MHD equilibrium computation of a tokamak plasma were carried out efficiently on the plasma simulator.

Numerical study of M=1 resistive internal kink mode in a cylindrical Tokamak

A numerical analysis of nonlinear behavior of m=1 resistive internal kink instability in a Tokamak is presented in a cylindrical approximation. A two-dimensional nonlinear code which solves incompressible resistive MHD equations with helical symmetry has been developed. For the case of small longitudinal wave number k, the magnetic axis moves outward until it reaches the critical surface where the value of helical magnetic flux has the same value as the one at the magnetic axis. On the other hand, as k is increased, the shift of the magnetic axis saturates before touching this critical surface. This result gives a possible explanation of suppression of internal disruptions observed in high power neutral beam injection experiment to Tokamaks.