energy must be invested to break apart the polymerized fragments and reconstitute the original molecule. The overall efficiency of this type of laser, not including the energy needed for reconsitution of the lasant, is approximately 0.5% (laser power v solar power). Indirect Optically Pumped Lasers. Three indirect optically pumped lasers have been examined: • Static C02 laser • Flowing CO laser (subsonic) • Mixing C0/C02 laser (subsonic) Of these three only the last one combines the features of scalability, high efficiency, and laser wavelength suitable for atmospheric transmission. At this stage CO lasers appear to transmit successfully only on isolated lines and, since the CO laser is relatively inefficient when operated in a single line mode, it has not been considered further. The indirect optically pumped laser (IOPL) uses a solar heated blackbody cavity to pump the lasant. The advantages of the cavity are reduced radiative losses, a downshift in the peak of the pump radiation toward the infra-red absorption lines of CO and C02, an increase in the irradiation of the lasant to a full 4H steradians, and refilling the spectral lines absorbed by the lasant through radiative re-emmission of a full blackbody spectra by the cavity walls. As in the direct optically pumped case, the IOPL needs electricity or mechanical power only for circulating coolants and moving the lasant through the laser optical cavity. From Table 1 it appears initially that this class of laser will be quite lightweight. The mixing gas version is shown in Figure 1. Initial gain experiments have been performed indicating the viability of the blackbody radiation pump method. Further research is needed to demonstrate the complete laser concept. Free Electron Lasers. We have also investigated three possible versions of the free electron laser: the CATALAC FEL, the Double FEL, and the Storage Ring FEL. The CATALEC FEL, illustrated schematically in Figure 2, is based on a concept developed at LASL to help recapture some of the energy left over in the electrons as they exit from the laser cavity. These electrons are recirculated through the rf-linac 180° out of phase with the next bunches of electrons to be accelerated. The electrons are declerated and return most of their remaining energy to the accelerating field. The linac therefore behaves as a catalyst for transferring the energy of decelerating electrons to those being accelerated. The spent electrons are collected at the other end of the linac with approximately 8 MeV energies and the accelerated electrons emerge with energies on the order of 50 MeV. No high power FELS have been built in the wavelength range suitable for atmospheric propagation so that this laser technology must be regarded as extremely tentative. Elementary gain and oscillator experiments have been performed by Madey and his co-workers at Stanford which indicate that the principle will work. Several larger FEL experiments for Ip lasers are now in the planning stage and are due to come on line in late 1980 or 1981. Nevertheless, a substantial amount of theoretical analysis has been performed which permits us to carry out elementary scaling calculations; the results of these are included in Table 1 for the CATALAC FEL which operates essentially as a once-through device with good energy recover, and for the storage ring FEL. The double FEL is, at present, too sensitive to assumptions made regarding low losses of the standing EM wave used as the virtual wiggler field.
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