Fig. 8. Free electron laser. wall maintains a 2000 K blackbody distribution with energy derived from the focused sunlight. Thus, the indirect SPL harnesses the energy of the entire solar spectrum and not just the energy in a narrow pumping band as with the direct SPL. However, not all of the energy delivered into the cavity by the focused sunlight is absorbed by the lasant. Some radiates out through the input window, and some leaks through the cavity walls and radiates away. Unlike the EDL, the indirect SPL does not need to convert sunlight to electricity to accomplish lasant pumping. Further, the indirect SPL can, in contrast to the direct SPL, utilize all the wavelengths in the solar spectrum. Thus, the indirect SPL is a particularly attractive option. It is, however, a fairly new option and has not been investigated extensively (9,13,14). To date, an operational indirect SPL has not been built. However, amplification has been demonstrated with a lasant pumped by blackbody radiation (14). 3.4 Free Electron Laser The free electron laser is shown in Fig. 8. A stream of electrons enters the laser cavity from the left, travels through a periodic magnetic field, and exits to the right. During this process, some of the energy of the electrons is converted into electromagnetic (EM) wave energy, i.e., photons are created. The wave is reflected back and forth between the two mirrors. It acquires energy from the electron stream as it travels from left to right but neither gains nor loses energy as it travels from right to left. The photons that leak out through the partially reflecting mirror constitute the laser beam. Some photons are needed in the tube in order for more to be created. The parameter Xo in Fig. 8 is the magnet wavelength, i.e., the spatial period of the
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