One of the most important areas of research towards the realization of a fusion
reactor is the research of confinement of the reactor core plasma. For this purpose
a research project of the computational science called the project of "Numerical
EXperiment of Tokamak (NEXT)" is being carried out in line with the experimental
and theoretical researches. The NEXT project aims at understanding the extremely
complicated plasma behavior which cannot be treated theoretically, by solving
it on the basis of computer simulation.
A stable confinement is attained by establishing a plasma current profile which increases from the plasma surface towards the plasma center but begins to decrease from a certain radial position. By increasing the total plasma current in order to improve the confinement property, however, the current distribution near the plasma center is expected to flatten due to the occurrence of instability. Even in such a situation, however, the plasma current concentrates near the off-center region of the plasma cross section by taking into account of the fact that friction is small in a high temperature plasma and the magnetic lines of force are reconnected, and then the current near the central part reduces again.
In this way a series of plasma occurrences is related to various phenomena with different time scales. Therefore, the complete truth of plasma behaviour is understandable by executing a long term numerical simulation of the complex system by a particle model simulation code. We have performed such a simulation as a part of the NEXT project and clarify the above-mentioned self-restoring process of plasma confinement. It is clearly seen from Fig. 3-3 that the plasma itself restores the confinement magnetic field distorted by instability. |
