The author considers how the collisionless detrapping of particles from the magnetic mirrors formed between discrete longitudinal field coils affects the thermal conductivity of trapped ions, . The effect is significant when the distance over which a trapped ion drifts during the time of confinement in a local mirror (ripple well) , becomes comparable with the extent of the zone in which local magnetic mirrors exist, z0∼Nq Rδ (δ is the ripple depth, R the major radius of the torus, q the stability factor, N the number of coils, and ωB the cyclotron frequency). At temperatures for which Δ ≫ Z0, becomes proportional to νi. The range of operating temperatures in a tokamak reactor corresponds to Δ ≲ z0. In this range the value of is 5–30 times lower than the value predicted previously and the dependence .