(a) Dispersion and attenuation of spin wave excited on top of helical configuration. The solid curve represents the dispersion relation with damping, while the dashed curve without damping. The dotted curve denotes the attenuation length as a function of wave number. (b) Group velocity. The solid and dashed curves correspond to the cases with and without damping, respectively. The inset in (b) shows the dependence of ρ_u/ρ_v on k. Here the same parameters are used as those in Fig. 2 except the DMI parameter D = 3 × 10^–3 J/m² which ensures d² > κ.