Thoughts on the new technologies for Compact FEL devices

The possibility to merge different solutions exploiting wave undulators, laser plasma acceleration and other schemes, allows the design of compact Free Electron Laser (FEL) sources operating in the extreme UV-X-ray region of the spectrum. Newly developed high peak power lasers have opened indeed effective possibilities of driving coherent light sources operating with laser plasma accelerated beams and wave undulators. We combine different ideas including also the concept of sheared beam configuration to achieve compact and reliable FEL devices. We finally comment on the perspective use of Radio Frequency (RF) undulators, which seems to be fairly good candidates for opening a new technological strategy for the design of FEL oscillators, including bi-harmonic and multi-frequency operation.