The luminosity of the accelerator scales as the wall-plug-to-beam efficiency. So one needs at the same time a high-gradient acceleration and an efficient energy transfer. The use of high-frequency RF maximizes the electric field in the RF cavities for a given stored energy. However, standard RF sources scale unfavorably to high frequencies, both in maximum delivered power and in efficiency. A way to overcome such a drawback is to use standard low-frequency RF sources to accelerate the drive beam and use it to produce RF power at high frequency. The drive beam is therefore used for intermediate energy storage.