Chaotic behavior of high energy ions in a helical system

Abstract

The collisionless drift orbit of high energy ions in the Large Helical Device (LHD) is studied numerically with high accuracy. Due to lack of axial symmetry, phase space of the particles in LHD is composed of regular drift surfaces and chaotic regions. The structure of this phase space is predicted theoretically and confirmed by numerical computations of drift orbits for high energy ions. It is found that banana shape drift surface spread out between inner part of magnetic surface and chaos field line regions outside of the outermost magnetic surface. Life time of chaotic orbit particles are investigated numerically by orbit trace until each particle reaches at vacuum vessel walls. It is found that particle life times for chaotic orbits in the LHD is depend on the particle energy and are controllable by magnetic field intensity.The positions of lost particles at the vacuum vessel are compared with the divertor chart for the field lines. In LHD, lost particles starting from within the magnetic surfaces are...