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We present the first 3D, global, two-fluid, flux-driven simulations of plasma turbulence in stellarators with different configurations: one with an island divertor; another one corresponding to the TJ-K stellarator; and a set of equilibria with increasing torsion and ellipticity. The simulations were carried out with the GBS code [1], which solves the two-fluid drift-reduced Braginskii equations.
The vacuum magnetic field of the island divertor configuration corresponds to a 5-field period stellarator and was constructed using the Dommaschk potentials [2]. It was found that the radial particle and heat transport is mainly driven by a field-aligned mode with low poloidal wavenumber, whose origin is investigated theoretically [3]. Transport is observed to be larger on the high-field side of the device and this is explained by means of a non-local linear theory. In contrast to tokamak simulations and experiments, but in agreement with edge measurements in W7-X [4], radial propagation of coherent filamentary structures (blobs) is not observed, revealing important differences between stellarator and tokamak edge transport mechanisms.
We further present the first validation of a simulation of plasma turbulence in a stellarator configuration against experimental measurements in the TJ-K stellarator [5]. The comparison shows that GBS retrieves the main turbulence properties observed in the device, namely the fact that transport is dominated by fluctuations with low poloidal mode number.
Finally we present simulations in a set of equilibria with increasing ellipticitiy and increasing torsion generated by VMEC. The limit of zero ellipticity and zero torsion corresponds to a tokamak with circular flux surfaces, allowing to study edge turbulence in the transition between a tokamak and a stellarator. The role of ellipticity and torsion as well as of magnetic shear is discussed.
[1] P. Ricci et al., Plasma Physics and Controlled Fusion 54, 124047 (2012)
[2] W. Dommaschk, Computer Physics Communications 40, 203 (1986)
[3] A. J. Coelho et al, Nuclear Fusion 62, 074004 (2022)
[4] C. Killer et al, Plasma Physics and Controlled Fusion 62, 085003 (2020)
[5] A. J. Coelho et al, Plasma Physics and Controlled Fusion 65, 085