Finite orbit width effects in large aspect ratio stellarators

New orbit-averaged equations for low collisionality neoclassical fluxes in large aspect ratio stellarators with mirror ratios close to unity are derived. The retain finite orbit width effects by employing the second adiabatic invariant $J$ as a velocity-space coordinate and they have been implemented code KNOSOS (Velasco et al. , J. Comput. Phys. vol. 418, 2020, 109512; Velasco Nucl. Fusion 61, 2021, 116013). used study $1/\nu$ regime lower regimes. For generic unity, collision frequency decreases, transitions directly into $\nu$ regime, without passing through $\sqrt {\nu }$ regime. An explicit formula is obtained. includes effect of particles that transition between different types wells. While these produce stochastic scattering independent value velocity space, diffusion real space remains proportional frequency. only recovered omnigeneity: optimized unity. Neoclassical transport can be calculated derived Calvo ( Plasma Control. 59, 2017, 055014). In stellarators, exists interval $(a/R) \ln (R/a) \ll \nu _{ii} R a/\rho _i v_{ti} R/a$ . occurs an depends on deviation from omnigeneity $\delta$ : $\delta ^{2} |\ln \delta | 1$ Here, $\nu _{ii}$ ion–ion frequency, $\rho _i$ $v_{ti}$ ion gyroradius thermal speed, $a$ $R$ minor major radii.