Masao Kasai

Development of a two-dimensional fluid code and its application to the Doublet III divertor experiment

A two-dimensional time dependent fluid code has been developed for transport processes in the edge plasma of a tokamak, coupled with a Monte Carlo method for neutral gas behaviour. The code employs a particle-incell method for the numerical solution of fluid equations. A simulation of the Doublet III divertor experiment has been performed with this code. It has been confirmed that the radial profiles of temperature and density in the scrape-off and divertor region can be simulated fairly well

Simulation studies on plasma position control for the next generation tokamak machines with up-down asymmetry

The effect of coupling between vertical and radial plasma motions, originating from the asymmetric features of plasma configuration, provides new and interesting problems. A plasma column moves vertically even when it is disturbed radially by some kinds of perturbations. The tokamak with single-null poloidal divertor configuration and up—down asymmetry, has two types of coupling effects: one is due to the asymmetry of external equilibrium field and the other is due to the asymmetry of eddy currents induced in structures surrounding the plasma, such as first wall and blanket. Stability analysis and simulations on plasma position control have been carried out for the tokamak reactor with up—down asymmetric configuration peculiar to the single-null poloidal divertor. The results indicate that the effect of the asymmetry of the equilibrium field is governing; the up—down asymmetry enhances a growth rate of plasma vertical instability compared to a symmetric case; and the stability criteria for the asymmetric system should be modified. Large vertical displacements toward the divertor plates are observed in the simulations on plasma position control when large disturbances are applied to the plasma, e.g. 40% reduction of poloidal beta, βp, and normalized internal plasma inductance, li. It is quite difficult to suppress the vertical movement with a practical control power level, although the radial movement could be controlled. Plasma with the single-null divertor would therefore interact strongly with the divertor plates, or the first wall near the divertor plates, and would be disrupted at some level of radial perturbations, while for such perturbations a plasma with up—down symmetry (e.g. double-null divertor configuration) could attain equilibrium and be shut down in a controlled manner.

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.