An improved formulation of the ripple-averaged kinetic theory of neoclassical transport in stellarators

The ripple-averaged kinetic theory of neoclassical transport in multiple-helicity stellarators is presented anew, using a formulation which ameliorates several shortcomings of the conventional approach. In particular, during the averaging procedure the usual simplifying assumptions of symmetric local ripples and 'small' rotational transform per field period are avoided through an appropriate choice of coordinate system and local model for the magnetic field strength; averages are truly performed along a field line and not along the toroidal-angle coordinate, as is common. Phase space divides naturally into three portions in which particles are (1) locally trapped, (2) locally reflected but not trapped, and (3) locally passing; the second of these is lacking in the conventional description. As an example of the theory's many possible applications, the monoenergetic radial transport coefficient in the asymptotic l/nu regime is derived and radial profiles of this quantity are determined for the Large Helical Device and Wendelstein 7-X. The results are compared with those obtained using the conventional approach; significant differences are found in the case of Wendelstein 7-X and verified numerically using the Drift Kinetic Equation Solver (DYES).