Numerical Study of Whisker Field Lines in the Periphery of the Large Helical Device

The behavior of periphery magnetic field lines in the standard Large Helical Device ( l =2 heliotron/torsatron type) configuration is studied numerically. Three different types of behavior are found, corresponding to three different regions: the stochastic region near the outermost magnetic surface, the whisker region, and the inter-whisker region outside the stochastic region. The behavior of whisker and inter-whisker field lines is specifically analyzed. It is found that whisker field lines exhibit both regular and irregular types of behavior, whereas inter-whisker field lines exhibit only regular behavior. The connection lengths for the whisker field lines are usually as long as several tens of toroidal pitches, whereas those for the inter-whisker field lines are usually less than ten toroidal pitch lengths. Whisker field lines are characterized by three fundamental processes: stretching, folding, and nesting associated with the motion of a residual X-point of the separatrix. Simple modeling is performed to reproduce these three fundamental processes.