A free-electron laser (fel) driven by a very compact electrostatic accelerator and capable of operating continuous wave (cw) with high efficiently is being investigated. After passing through undulator, the current driving the fel is recovered in an electron collector. The electron beam-line is being designed in such a way as to maximize the current recovery which is expected to be higher than 99.5*. The small amount of current lost is being replaced by a power supply. With a careful design of the system components, the overall efficiency is anticipated to be better than 50%. This first-ever cw fel is being operated in the 5mm range withh 10kw of continuous power, with an easy continuous tunability over its operation range. Extensions of the concept towards fels capable of operating cw at shorter wavelengths and higher powers also are being considered. The cw fel, when successful, would operate at a single mode with a high degree of coherence. New concepts in radar, or more generally, sensor technology are likely to make better use of the coherence properties of the radar beams. For that type of technology the cw fel would prove to be a much welcomed source of radiation.
