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

The ripple-averaged kinetic theory of neoclassical transport in multiple-he licity stellarators is presented anew, using a formulation which ameliorate s several shortcomings of the conventional approach. In particular, during the averaging procedure the usual simplifying assumptions of symmetric loca l ripples and 'small' rotational transform per field period are avoided thr ough an appropriate choice of coordinate system and local model for the mag netic field strength; averages are truly performed along a field line and n ot along the toroidal-angle coordinate, as is common. Phase space divides n aturally 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 the ory's many possible applications, the monoenergetic radial transport coeffi cient 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. T he results are compared with those obtained using the conventional approach ; significant differences are found in the case of Wendelstein 7-X and veri fied numerically using the Drift Kinetic Equation Solver (DYES).