In order to realize fusion energy it is necessary to effectively confine high-temperature high-density plasma. In order to then generate and maintain high-temperature high-density plasma, in many instances we heat the plasma through high-energy ions. For that reason, it is important to investigate the behavior of high-energy ions and the plasma heating efficiency, and analyses of experiment results using calculation estimates based upon theories are being undertaken. Here we will report on the development through the Large Helical Device (LHD) of an analysis code for evaluating the heating efficiency of plasma through high energy ions.
In the LHD, we primarily generate high-energy ions by injecting high-energy hydrogen atoms into a plasma. The high energy ions, just as they are named, have a larger amount of energy than the plasma ions and move quickly. The plasma is confined in a doughnut-type magnetic field lines container, and while the high-energy ions are circulating around the magnetic line of force they move about inside the container and heat the plasma. For that reason, investigating the heating efficiency of the high-energy ions and what places they are heating is important for analyzing the performance of the plasma confinement. We call the course taken by the high-energy ions the “trajectory,” and through computer simulations of the trajectories of high-energy ions inside plasma we can predict whether the high-energy ions are heating the plasma efficiently and where the heating places will be located. In the future fusion reactor, predicting the movement of high-energy ions is also important for the fusion reactor’s design because high-energy helium ions that are generated by a nuclear reaction will heat and maintain the plasma.
Prediction of the trajectories of high-energy ions is being undertaken through numerical calculations using household personal computers and high-speed supercomputers. But in order to be able to make better predictions, the development of high-speed analysis codes is being undertaken. In the LHD, the phenomenon in which some of the high-energy ions that escaped from the magnetic field container re-enter the container is being measured. In code used heretofore, it was thought that high-energy ions that escaped from the container all were lost. But because of higher precision analysis, we are developing evaluation codes for heating efficiency which consider the heating efficiency in the trajectory of the high-energy ions in the area outside the container. When having used this evaluation code and conducted analyses, in cases in which high-energy hydrogen atoms were injected in the tangential direction into a doughnut-shape plasma, when reducing the strength of the magnetic field for the plasma confinement, the high-energy ions that re-enter even if they had left the magnetic field container increase. And in cases in which the high-energy ions are injected vertically, when the plasma pressure increases the high-energy ions that re-enter even if they have left the magnetic container increase. Further, compared to evaluations to date, we have learned that in plasma in which exist numerous high-energy ions the heating efficiency increases as much as 20-30 percent.
In this way, regarding code that evaluates heating efficiency, through enlarging the calculation area based upon the observations from the experiments and enhancing precision, it has become possible to analyze experiment results in finer detail. In the future, we will improve preparation of the analysis code and further improve prediction accuracy regarding the efficiency of plasma confinement looking forward toward the fusion reactor of the future.