Fig. 1: Plasma model based on the JET tokamak.

a Toroidal geometry in cylinder coordinates (R, z, ζ). The black curve represents a magnetic field line with helicity q = 1. b Shape of the plasma cross-section in a poloidal (R, z) plane. The gray contours are magnetic flux surfaces. The color contours show the plasma beta, which measures the ratio of thermal to magnetic pressure as \(\beta =2{\mu }_{0}P/{B}_{0}^{2}\) with μ₀ = 4π × 10⁻⁷ H m⁻¹. c Central portion of a field helicity profile \(q(\overline{r})\) that facilitates energy-selective alpha particle confinement. The dashed black line marks the q = 1 surface. The red line is the boundary of a reduced simulation domain.