Hybrid Stellarator Divertor

Recently an efficient method for simulation of stellarator divertor was developed by Boozer and Punjabi [A. H. Boozer and A. Punjabi, Phys. Plasmas \textbf{25}, 092505 (2018)]. This method was used to simulate nonresonant stellarator divertor [A. Punjabi and A. H. Boozer, Phys. Plasmas \textbf{27}, 012503 (2020)]. The three parameters denoted by $\varepsilon _{\mathrm{0}}$, $\varepsilon_{t}$, and $\varepsilon_{x}$ in the Hamiltonian for the trajectories of magnetic field lines control the shape of the outermost confining surface in nonresonant stellarator divertor. These parameters are called shape parameters. They control the elongation, triangularity, and the sharp edges on the outermost confining surface, respectively. In the 2020 Punjabi and Boozer paper on simulation of nonresonant stellarator divertor in PoP, the shape parameters had the values $\varepsilon_{\mathrm{0}}=\varepsilon_{t}=$0.5, and $\varepsilon_{x}=$-0.31. It is found that in for $\varepsilon _{\mathrm{0}}=\varepsilon_{t}=$0.5, when $\varepsilon_{x}$ is varied in the range -0.25 to -0.1, a new kind of divertor is formed. We call this hybrid divertor. Hybrid divertor has features of both the nonresonant divertor as well as the resonant divertor. The size of the resonant islands varies as $\varepsilon_{x}$ is varied in this range; islands become smaller as $\varepsilon_{x}\to $-1/4. In this paper, we give the rotational transforms and sizes of the islands for different values of the parameter $\varepsilon_{x}$ in this range. Work supported by the DOE OFES grants DE-SC0020107 and DE-FG02-07ER54937 to Hampton University, and DE-FG02-03ER54696 to Columbia University. Research used resources of the NERSC, supported by the Office of Science, U.S. DOE, under Contract No. DE-AC02-05CH11231.