The solar pumped laser is a good alternative because it do not need any solar energy converter. However it requires quite large reflectors. The indirect solar pumped laser can operate with much smaller reflector because it uses larger fraction of solar spectrum. The overall mass of this laser type can be smaller than its competitors. So far this laser type has been only investigated in initial state. Free Electron Laser The Free Electron Laser (FEL) uses an electron beam passing through periodically modulated magnetic field. In this magnetic field some of the energy of the electrons is converted to electromagnetic wave energy. This wave is reflected between two mirrors so the wave travels along and against the electron beam. The wave acquires energy while traveling along the electron beam but it does not acquire nor loss energy while traveling against electron beam. The total effect of reflections is photon amplification Figure 7.35 Operation of Indirect Solar Pumped Laser. [Walbridge, 1980] The frequency of the amplified wave depends on the speed of entering electrons into the magnetic field. This phenomenon means that the FEL is tunable. Another advantage is its efficiency. Over 50% extraction of light energy from the electron beam should be possible as explained in paper from Boeing, 1980. The lasant of the FEL is an electron beam contained by magnetic field. Due to this there is no need for an output window. The output radiation can exit through a hole in one of the mirrors and both mirrors may be 100% reflective and thus avoid absorption in partially reflective mirror used in other types of lasers. Although this laser type has many advantages, it is not yet fully developed. The equipment for a magnetic field required to keep the electron beam under control may be very heavy. Development of superconductive materials may help to develop also this laser. [Walbridge, 1980] [Boeing Aerospace, 1980]
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