High-efficiency high-order harmonic generation without tunneling

High-order harmonic emission is calculated for a highly charged ion exposed to an ultra-intense laser field with an intensity near saturation. Low-order harmonics having an energy lower than the ionization potential $I_p$ are emitted with extremely high efficiency. In contrast to their usual behavior, the low-order harmonics may exhibit a plateau that is amenable to a classical mechanism or a resonancelike maximum, depending on the shape of the binding potential. Tunneling is not involved.