Benchmark of the Local Drift-kinetic Models for Neoclassical Transport Simulation in Helical Plasmas

TL;DR

This paper benchmarks various local drift-kinetic models for neoclassical transport in helical plasmas, highlighting the importance of tangential magnetic drift and the advantages of the ZOW model in accuracy and computational efficiency.

Contribution

It provides a comprehensive comparison of local drift-kinetic models, demonstrating the ZOW model's effectiveness in reducing unphysical behaviors and efficiently evaluating bootstrap current.

Findings

Tangential magnetic drift suppresses unphysical radial flux peaking.

ZOW model mitigates unphysical behaviors across magnetic geometries.

ZOW efficiently evaluates bootstrap current with low computational cost.

Abstract

The benchmarks of the neoclassical transport codes based on the several local drift-kinetic models are reported here. Here, the drift-kinetic models are ZOW, ZMD, DKES-like, and global, as classified in [Matsuoka et al., Physics of Plasmas 22, 072511 (2015)]. The magnetic geometries of HSX, LHD, and W7-X are employed in the benchmarks. It is found that the assumption of $\boldsymbol E \times \boldsymbol B$ incompressibility causes discrepancy of neoclassical radial flux and parallel flow among the models, when $\boldsymbol E \times \boldsymbol B$ is sufficiently large compared to the magnetic drift velocities. On the other hand, when $\boldsymbol E \times \boldsymbol B$ and the magnetic drift velocities are comparable, the tangential magnetic drift, which is included in both the global and ZOW models, fills the role of suppressing unphysical peaking of neoclassical radial-fluxes found in the other local models at $E_r \simeq 0$. In low collisionality plasmas, in particular, the tangential drift effect works well to suppress such unphysical behavior of the radial transport caused in the simulations. It is demonstrated that the ZOW model has the advantage of mitigating the unphysical behavior in the several magnetic geometries, and that it also implements evaluation of bootstrap current in LHD with the low computation cost compared to the global model.