''Hybrid'' Monte Carlo simulation of ripple transport in stellarators

A rapid simulation has been developed to accurately describe ripple effects on particle motion in stellarators at low collision frequencies. The majority of ripple trapped particles are followed through the iterative conservation of their bounce action J, while those particles in the region of phase space on either side of the ripple trap/detrap boundary are followed using guiding center equations. This ''hybrid'' formulation provides the most accurate numerical description of ripple trapped particle orbits to date, short of a fully guiding center code. This is important since these orbits are often substantially different from those allowed for in analytic theory and in other transport codes. Further, the hybrid simulation is much faster than a fully guiding center treatment, making all collision frequency regimes of interest accessible at reasonable costs of computer time. The methods employed allow the examination of stellarator configurations for which the magnitude of the toroidal well, ε/sub t/, is larger than that of the helical well, ε/sub h/, as well as configurations more often treated in which ε/sub h/ > ε/sub t/. Results are obtained for the usual analytic model of the helical ripple, ε/sub h/ = ε/sub h/(r), as well as a more realistic model for which ε/sub h/ = ε/sub h/(r,θ). The results are largely explainable in terms of existing analytic theories, although some slight modifications seem to be necessary