Neoclassical transport in quasi-axially symmetric stellarators

We present a numerical and analytic assessment of the transport in two quasi-axially symmetric stellarators including one variant of the MHH2 class [1] of such devices, and a configuration we refer to as NHH2, closely related to MHH2. Monte Carlo simulation results are compared with expectations from established stellarator neoclassical theory and with some empirical stellarator scalings used as an estimate of the turbulent transport that might be expected. From the standpoint of transport, these may be viewed as either tokamaks with large (delta similar to 1%)but low-Il ripple, or as stellarators with small ripple. For NHH2, numerical results are reasonably well explained by analytic neoclassical theory. MHH2 adheres less to assumptions made in most analytic theory, and its numerical results agree less well with theory than those for NHH2. However, for both, the non-axisymmetric contribution to the heat flux is comparable with the symmetric neoclassical contribution, and also falls into the range of the experimental anomalous (turbulent) contribution. Thus, it appears, that effort to further optimize the thermal transport beyond the particular incarnation studied here would be of at most modest utility. However, the favorable thermal confinement relies heavily on the radial electric field. Thus, the present configuration will have a loss cone for trapped energetic ions, so that further optimization may be indicated for a large device of this type.