Tomofumi Kobayashi

Quench simulation analysis of a superconducting coil

Abstract A quench simulation program has been developed with the intention of identifying a quench position in a superconducting coil and it has been applied to behaviour analysis of an artificially quenched superconducting coil. The superconducting coil is a solenoidal, bobbinless, and densely wound impregnated coil with many voltage terminals, thermocouples and heaters. The propagation of a normal zone was made two-dimensional by a one-turn-heater or three-dimensional by a point-heater set in the coil. The time-spatial changes in terminal voltages and temperatures agree well with those obtained from the simulation analysis within an accuracy of ± 10 ms or ± 1 mm. If accurate thermal properties of the materials constituting the superconducting coil are obtained, identification of the quench position in the coil should be possible. Normal front velocities and effective thermal conductivities of the radial, azimuthal and axial directions are also analysed from experimental results.

Analysis of Eddy Currents Induced in the Vacuum Vessel of a Tokamak Device

The structural design of vacuum vessels containing a plasma requires a knowledge of the eddy currents induced by the pulsed magnetic fields employed in these vessels. Consideration of the eddy currents is also indispensable for design of a feedback control system for the plasma position. A method is presented for analyzing the transient behavior of the eddy currents induced in a vacuum vessel consisting of several shell sections each of which is composed of thick plates and bellows made up with thin sheets. This method is based on integro-differential equations with magnetic vector and electrostatic scalar potentials. The charges accumulating at each bellows edge play an important role, and create axially non-symmetric components of the eddy currents.

Bifurcated confinement solutions for diverted tokamaks

Divertor effects on the particle confinement time in a tokamak plasma are investigated by modelling of particle, momentum and energy transports in the divertor plasma and particle transport in the main plasma, with a simplified model for neutral-particle transport. The divertor plasma has three equilibrium states in a limited range of the ion flux entering the divertor. It is found that the existence of the three equilibrium states might lead to mode conversion of the particle confinement characteristics during additional heating.

Numerical analyses of cold and dense divertor plasmas realized in Doublet III poloidal divertor experiments and their application to a near-term tokamak reactor

Abstract The cold and dense divertor plasmas observed in the Doublet III experiments are numerically reproduced based on a self-consistent modelling of the divertor plasma and neutral particles. Especially it is found that the electron density near the divertor plate is proportional to the 3rd power of the electron density at the divertor throat, which is in good conformity to the experimentally observed scaling law. The numerical simulation using the same divertor modelling strongly indicates the possibility of the cold and dense divertor operation for the near-term tokamak reactor.

A steady-state tokamak reactor using the compressional Alfvén wave

A tokamak reactor concept for steady-state operation has been studied. This reactor uses the compressional Alfven wave to sustain the plasma current. Its structure is simplified by eliminating the poloidal coils in the torus centre. The start-up scenario is obtained by minimizing the wave and electric powers of the coils through solving the power balance and Grad-Shafranov equations. The steady-state operating point is obtained from the viewpoint of maximizing the Q-value (Q = fusion power/current drive power). The largest Q-value is obtained by using compressional Alfven wave rather than other RF waves.

Determination of Optimum Positions of the Poloidal Field Coils of a Large Tokamak

A set of two numerical methods is presented for determining optimum configuration of the poloidal field coils of a large tokamak. The first method is an application of the virtual-casing principle developed by Zakharov and determines both the currents and the positions of a system of the coils. The second one used a simplex method of non-linear programming for optimization calculations of the coil positions in a given area in the two dimensional space. This is needed to meet various practical requirements in the design of tokamaks. These methods have been applied in the design of JT-60 (JAERI large tokamak) to determine the positions of the ohmic heating coils, and shown to be effective when they are used in combination.

Plasma position and shape control analysis of tokamak plasma by 2D equilibrium model.

The method to analyze the dynamics of 2D plasma equilibrium was formulated. By this method, we can examine the plasma shape control of the noncircular tokamak. Effective plasma position, ellipticity and current control by the conventional PI control law was obtained in computer simulations for a tokamak with a major radius of 0.4 m which had a hybrid poloidal coil system with 3 pairs of coils.

Equilibrium Plasma Control for a Large Tokamak with Many Poloidal Coils to Produce the Ohmic and Equilibrium Field

Plasma equilibrium control methods, based on a multivariable control technique, are formulated in the hybrid poloidal coil system, where all of the many coils are designed for ohmic heating and controlling the plasma equilibria of a large tokamak. This formulation is divided into a program control method during the buildup phase and a feedback method for controlling the plasma current and the equilibrium during the steady-state phase. The former gives programming coil currents and voltages in order to reduce the required peak power by adjusting the magnetic field profile inside the plasma, keeping the plasma equilibrium fixed. The latter includes a proportional-integral (PI) control law in the conventional optimal control theory. Effective plasma equilibrium control by these methods was obtained in computer simulations for a fusion reactor with a major radius of 5.5 m.

Energy transport of beam-heated high-temperature and high-density plasmas in Doublet-III

The empirical scaling of the electron thermal diffusivity, ?e, is investigated for more than 100 beam-heated discharges. These discharges include two major features: 1) high-temperature and high-density plasma (Te(0)?Ti(0)?2.5?3 keV at e?(5?7) ? 1013 cm?3, PNBI 4 MW), which will be the basis for the breakeven experiments in the next-generation tokamaks, 2) three types of discharges, i.e. good and poor confinement divertor discharges and limiter discharges. ? All kinds of discharges (good heating and poor heating divertor discharges, limiter discharges) have fhe same functional form in ?e within ~ 40% at 0.25 a < r < 0.65 a, where ?e in the simplest expression scales as ?e ? 1/ne. The ion thermal diffusivity, ?i, is consistent with the assumption that the neoclassical ?i can be applicable to our three kinds of discharges. For discharges with different heating efficiency, there is no systematic difference, in the adjustable multiplier, to the neoclassical theory by Hinton-Hazeltine for an ion collisionality of ?i* = 0.02?0.5.

Plasma Shapes for Achieving High Heating Efficiency during Neutral Beam Injection in Doublet III

Three plasma configurations have been compared for achieving an improved heating efficiency (H-mode) during the neutral beam injection phase. The H-mode plasmas are observed only in divertor configurations. The highly elongated divertor shape has achieved H-mode plasmas with the best heating efficiency. Required beam power for achieving these plasmas increases from ~1 MW to ~4 MW as the plasma current increases from 360 kA to 740 kA. The stored plasma energy increases as the plasma current increases and improves 50% over that in low heating efficiency (L-mode) at the best H-mode discharges.