Direct Optimization of Fast-Ion Confinement in Stellarators

Confining energetic ions such as alpha particles is a prime concern in the design of stellarators. However, directly measuring alpha confinement through numerical simulation of guiding-center trajectories has been considered to be too computationally expensive and noisy to include in the design loop, and instead has been most often used only as a tool to assess stellarator designs post hoc. In its place, proxy metrics, simplified measures of confinement, have often been used to design configurations because they are computationally more tractable and have been shown to be effective. Despite the success of proxies, it is unclear what is being sacrificed by using them to design the device rather than relying on direct trajectory calculations. In this study, we optimize stellarator designs for improved alpha particle confinement without the use of proxy metrics. In particular, we numerically optimize an objective function that measures alpha particle losses by simulating alpha particle trajectories. While this method is computationally expensive, we find that it can be used successfully to generate configurations with low losses

Energetic Particle Tracing in Optimized Quasisymmetric Stellarator Equilibria

Recent developments in the design of magnetic confinement fusion devices have allowed the construction of exceptionally optimized stellarator configurations. The near-axis expansion in particular has proven to enable the construction of magnetic configurations with good confinement properties while taking only a fraction of the usual computation time to generate optimized magnetic equilibria. However, not much is known about the overall features of fast-particle orbits computed in such analytical, yet simplified, equilibria when compared to those originating from accurate equilibrium solutions. This work aims to assess and demonstrate the potential of the near-axis expansion to provide accurate information on particle orbits and to compute loss fractions in moderate to high aspect ratios. The configurations used here are all scaled to fusion-relevant parameters and approximate quasisymmetry in various degrees. This allows us to understand how deviations from quasisymmetry affect particle orbits and what are their effects on the estimation of the loss fraction. Guiding-center trajectories of fusion-born alpha particles are traced using gyronimo and SIMPLE codes under the NEAT framework, showing good numerical agreement. Discrepancies between near-axis and MHD fields have minor effects on passing particles but significant effects on trapped particles, especially in quasihelically symmetric magnetic fields. Effective expressions were found for estimating orbit widths and passing-trapped separatrix in quasisymmetric near-axis fields. Loss fractions agree in the prompt losses regime but diverge afterward.Comment: 24 pages, 15 figure

Data for: Symplectic integration with non-canonical quadrature for guiding-center orbits in magnetic confinement devices

This dataset contains code examples for different symplectic integrators with non-canonical quadrature points described in [1]. Hereguiding-center motion is implemented in its axisymmetric variant for tokamak magnetic fields in canonicalized flux coordinates.[1] C. G. Albert, S. V. Kasilov, and W. Kernbichler, Symplectic integration with non-canonical quadrature for guiding-center orbits in magnetic confinement devices, Mar. 2019, arXiv:1903.06885. Submitted to J. Comp. Phy

Data for: Symplectic integration with non-canonical quadrature for guiding-center orbits in magnetic confinement devices

This dataset contains code examples for different symplectic integrators with non-canonical quadrature points described in [1]. Hereguiding-center motion is implemented in its axisymmetric variant for tokamak magnetic fields in canonicalized flux coordinates.[1] C. G. Albert, S. V. Kasilov, and W. Kernbichler, Symplectic integration with non-canonical quadrature for guiding-center orbits in magnetic confinement devices, Mar. 2019, arXiv:1903.06885. Submitted to J. Comp. PhysTHIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV