抄録
Magnetic field lines and particle orbits were analyzed in the periphery of the Large Helical Device (LHD), which is called the chaotic field line region in this paper. The widths of the chaotic field line region were numerically identified for the standard LHD configuration with the magnetic axis position Rax = 3.75 m and for an improved confinement configuration with Rax = 3.6 m. It was found that the reflected particles include of what we have named chaotic particles and non-chaotic particles. Most of the reflected particles are mirror-confined with strong adiabaticity in the chaotic field line region. The remaining reflected particles, named type-A and type-B particles, are harmful to confinement. We found by detailed analysis of the vacuum magnetic field in the LHD that there exist loss canals that are the open intersections of |B| = const. and B · ∇ B = 0 with (B · ∇)2B > 0. The type-A particles, which are deeply trapped in the intersections and exhibit non-chaotic behavior, are lost along the loss canal due to grad-B drift, and may cause a toroidal drift in the chaotic field line region. The type-B particles, which exhibit chaotic behavior, pass into and out of the last closed magnetic surface (LCMS), leading to cooling down of the core plasma through collisions with cold neutrals in the chaotic field line region. When field strengths at the inner and the outer edge of the LCMS are different, type-B particles are produced through constancy of the kinetic energy and the magnetic moment of the collisionless particles. It was also found that the standard LHD configuration, Rax = 3.75 m, has both type-A and type-B particles, while there are virtually no such particles in the Rax = 3.6 m configuration. In the latter configuration, there is no loss canal within the chaotic field region, and the field strengths at both edges of the LCMS are the same. It is therefore concluded that the improved LHD confinement with Rax = 3.6 m over Rax = 3.75 m is a result of reductions and/or removal of type-A and type-B particles.
