The following table shows the global efficiency r of different kinds of laser systems assuming a 20% solar cell efficiency ( except for sun-pumped lasers which don’t use these cells). Diode laser pumped solid laser Laser diode arrays Solar pumped solid state laser Free electron laser A more complete analysis would take into account collector or solar cell and radiator masses. The paper “Problemes poses par la transmission d’energie par faisceaux laser” given by M. Gaillard and A. Laurens at the last SPS symposium, discussed different types of lasers, including FEL. At that time, FEL performance was being studied, and progress in this field would change many concepts in microwaves and optics. Since then, FELs have indeed advanced and could be good candidates for energy transmission from Earth to satellites and, possibly in the distant future, for big orbital stations. The main advantages of FELs are: • tunability of the wavelength; • high laser beam quality; • high efficiency. Wavelength tunability is very advantageous for propagation through the atmosphere and photovoltaic cell conversion efficiency. There are good transmission windows around 1 and 2 micrometers. The advantage of a wavelength of about 1 micrometer is associating high transmission capacity with relatively small optics. Shorter wavelength FELs, however have decreased efficiency and require higher qualities of the electron beam. For high energy, there were two candidates at the time of this paper: • induction LINAC, • radio-frequency LINAC. Previous years electron beam qualities were poor (normalized emittance eN defined as yrrpO where p is the radius, 0 the half-angle at any beam focus and y the relativist y factor; and n energy dispersion) compared to the requirements of short wavelengths.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==