• low Ohm losses on the cavity walls permitting continuous operation: average laser powers of several megawatts appear possible. • high electric fields (10 MV/m) allowing compact LINACs (an electron energy of 100 MeV is necessary to obtain 1 pm wavelength), which is essential for space use. • possibility of recovering the unspent energy of the electrons after the wiggler exit by recirculating the beam through the same RF superconducting cavities, converting it back to RF energy. By recovering unspent energy, overall efficiency including klystron efficiency could reach 30% or more (see Fig.7). It is very important to note that superconducting cavity technology is well known and the fabrication process is reliable. Moreover CERN makes cavities in Niobium- Titanium, with a higher critical temperature. In conclusion, RF LINACs with superconducting cavities seem to be a good concept for future FELs with average power of several megawatts and overall efficiency of 30% or more. For lower average power, compact FELs are a possibility. For example, racetrack microtrons is a path to be explored. Conclusion Due to its short wavelength, laser transmission of energy in space can be envisaged over distances of many thousands of kilometers. We hope that within several decades, lasers, optics and pointing systems will be technologically advanced enough to meet space requirements. We think that such networks offer many advantages on the satellite design level. The study and development of such projects however, would require international agreement and cooperation.
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