Magnetic turnstiles in the two-mode nonresonant stellarator divertor*

Nonresonant stellarator divertor is very important for the development of stellarator concept. To date, in our study of nonresonant stellarator divertor, we have found three distinct types of stellarator divertors – the nonresonant divertor, the hybrid divertor, and the two-mode divertor. The magnetic configuration of the two-mode divertor is obtained when one of the three shape parameters in the Hamiltonian function for the trajectory of magnetic field lines in the nonresonant stellarator divertor that controls the sharpness of the edges on the outermost confining surface vanishes. We have been studying the two-mode divertor for some time. It appears to have some interesting properties. We have found that the outermost surface is surrounded by a layer of chains of small islands. It takes a very long times for the field lines starting just outside the outermost surface to escape this layer and to funnel into the magnetic turnstiles. Two important parameters to calculate the 3D structure of magnetic turnstiles in nonresonant stellarator divertors are the maximum random radial shifts from the outermost surface for the starting positions of field lines, and the number of toroidal circuits of one period of the stellarators through which the field lines are integrated. Generally, the maximum shift is 0.01b, and the lines are integrated through 200,000 toroidal transits. b is the minor radius. However, these standard choices used to uncover the full 3D structure of the magnetic turnstiles do not work for the two-mode divertor. Numerical experiments take long compute times. We are conducting numerical experiments to find the optimal selection of the two parameters that will give us the full 3D structure of magnetic turnstiles. We will give results of our search for optimal choice of these parameters in two-mode divertor.