Combination of quasi-isodynamic and piecewise omnigenous magnetic fields

Abstract

Due to their simultaneous optimization for radial and parallel neoclassical transport, quasi-isodynamic fields have been the main choice of stellarator magnetic configuration for most fusion reactor candidates in recent years. However, achieving a high degree of quasi-isodynamicity often comes at the cost of a strong shaping of the flux surfaces of the stellarator and complex coil geometries. In this work, the concepts of quasi-isodynamicity and piecewise omnigenity are combined to form QI-pwO fields. These fields are quasi-isodynamic in the low-field region of the magnetic surface, whereas they significantly depart from quasi-isodynamicity in the high-field region without sacrificing the neoclassical transport properties of quasi-isodynamic fields. This departure could make it easier to integrate the optimization of neoclassical transport with other physical and technological aspects of a stellarator reactor.

Figures (10)

• Figure 1: Magnetic field strength of a QI field (left) and of a pwO field far from quasi-isodynamicity (right).
• Figure 2: Magnetic field strength of W7-X high-mirror at $s=0.25$ (left) and a QI-pwO field (right). For W7-X, the toroidal range is shifted by half a period, since $B=B_\mathrm{max}$ lies at $\zeta=0$ instead of $\zeta=\pi/N_{fp}$.
• Figure 3: Radial (left) and bootstrap (right) transport coefficients for the fields of figure \ref{['FIG_MODEL']}.
• Figure 4: Scan in $w_2$: from left to right and top to bottom, $w_2=\pi$, $w_2=0.8\pi$, $w_2=0.6\pi$, and $w_2=0.4\pi$.
• Figure 5: Bootstrap transport coefficient as a function of $w_2$ for the scan of figure \ref{['FIG_W2']} (orange: $\pm D_{31}$ for W7-X).