Consequences of Time-Reversal Symmetry for the Electric Field Scaling of Transport in Stellarators

Most three-dimensional fusion-plasma-confinement geometries possess symmetric magnetic fields satisfying B(θ,ζ)=B(-θ,-ζ), where θ and ζ are poloidal and toroidal flux coordinate angles, respectively. For such configurations with large aspect ratios, the time-reversal symmetry of the collisionless orbits is sufficient to prove that the local diffusion coefficient, D(Eᵣ), is independent of the sign of the radial electric field, Eᵣ. Consequences of this property for neutral-beam injection and for generic root-jumping models for stellarators are discussed.