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September 22, 2014
Changing the Orbit by Collison: The Transport of Particles and Heat Caused by Collisions of Charged Particles

In magnetic fusion devices, beginning with the Large Helical Device (LHD), high-temperature high-density plasma is confined in the magnetic field lines container having a doughnut configuration. The high-temperature and high-density of plasma in the doughnut configuration typically are high in the core region and weaken as the plasma moves toward the edge (that is, the surface of the doughnut). At this time, particles and heat inside the plasma move toward the edge from the core region, and this phenomenon of particle and heat movement is called “transport.” An important cause of transport is the result of collisions among particles such as ions and electrons in plasma. In order to understand this phenomenon, because these particles that bear electric charge show complicated movement in the magnetic field lines container, computer simulation is inevitable. Here we will introduce research on transport caused by the collision of electrically-charged particles.

When ions and electrons that are electrically-charged particles approach one another they sometimes repulse the other electrically and sometimes attract with the other, and the direction of their movement thus changes. This is the collision of electrically-charged particles, and it is not necessarily always a head-on collision. In fusion plasma, while the numerous ions and electrons are flying about in the magnetic field lines container, and while repeatedly colliding together and randomly changing direction (orbit), as a whole they almost become neutral in terms of electricity and disseminate within the magnetic field lines container. Then, in order to steadily confine plasma over a long period of time, it is necessary to understand well the transport that occurs through the collision of electrically charged particles.

Features of transport that occurs through collisions are given characteristics by the electrically-charged particles moving along the magnetic field lines. In measuring the strength of the magnetic field along magnetic field lines in the doughnut configuration, that strength receives influence by the coil configuration that produces the magnetic field, and that strength grows stronger and grows weaker. This change of the magnetic field’s strength along the magnetic field lines causes the movement of electrically-charged particles back and forth along the magnetic field lines. Then, while numerous electrically-charged particles are moving back and forth along the magnetic field lines, by repeatedly colliding, little by little their direction changes and their orbit broadens. As a result, particles and heat move in the direction of low heat and density (that is, toward the edge).

In plasma are included an extremely large number of electrically-charged particles such as ions and electrons. Those electrically-charged particles are flying about in the magnetic field lines container while drawing various orbits. The influence of the orbits of electrically-charged particles upon transport is complicated. Further, in addition to the configuration of the magnetic field lines container, because they change according to temperature and density, for analysis and prediction of experiment results, in a theoretical evaluation of the magnitude of transport we must use large-scale numerical simulations using a supercomputer. Due to advances in supercomputers in recent years, we are now able to conduct more precise simulations, even in cases of ever more complicated configurations of the magnetic field lines container. At present, we are researching extremely complicated particle transport and heat transport in which magnetic field lines containers are in randomly disordered conditions that are seen in edge areas of LHD plasmas. Through such research, we aim to further advance our comprehension of the confinement of particles and heat in steady plasma conditions and to enhance the performance of fusion plasma.