3D calculation of magnetic turnstiles in nonresonant stellarator divertor

  A very important result on the magnetic turnstiles in the nonresonant stellarator divertor [A. Punjabi and A. H. Boozer, Phys. Plasmas 27, 012503 (2020)] is obtained. The exiting and entering magnetic turnstiles of a family of magnetic turnstiles in the nonresonant stellarator divertor do not leave and enter the last confining surface at the same location but on the opposite sides of the last confining surface. This result is contrary to the conventional understanding of the turnstiles in Hamiltonian mechanics [R. S. Mackay, J. D. Meiss, and I. C. Percival, Physica D 13, 55 (1984); J. D. Meiss, Chaos 25, 097602 (2015)]. A method is developed to calculate the 3D structure of the magnetic turnstiles in the annulus between the last confining surface and the wall in topological torii. This method is used to calculate the 3D structure of the magnetic turnstiles in the nonresonant stellarator divertor. It is found that there are two families of magnetic turnstiles with probability exponents 9/5 and 9/4. The questions of the nature of continuous toroidal stripes and the secondary family having a negative probability exponent from our simulation of nonresonant stellarator divertor [A. Punjabi and A. H. Boozer, Phys. Plasmas 27, 012503 (2020)] are resolved. The toroidal stripes are a distinct family of turnstiles; and the secondary family with negative exponent are not magnetic turnstiles; they are the positions of the largest radial excursion of the last confining surface. The method can be useful in the designing and optimization of divertors in toroidal fusion plasmas. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences under Awards DE-SC0020107 to Hampton University and DEFG02-03ER54696 to Columbia University. This research used resources of the NERSC, supported by the Office of Science, US DOE, under Contract No. DE-AC02-05CH11231.