NewTrans1.txt[9/15/2024 8:15:35 PM] However, pumping dyes requires considerable fluxes (50 to 100 kW/cm2), which the sun, even after concentration, cannot generate (since these fluxes are greater than those emitted on the surface of the sun), and continuous dye lasers can only be pumped by other lasers. Nevertheless, there does not seem to be any obstacle in principle to the production of amplifiers in a liquid medium, and this path appears to be one of the only ones allowing, in the long term, to hope to arrive at a laser pumped directly by the sun with a not too low efficiency. In the meantime, it must be recognized that the direct use of the sun is not very promising, which brings us to what is, after optical pumping, the second major method of pumping lasers. 3. DISCHARGE PUMPING IN GASES When a gas is subjected to an intense electric field, some atoms are ionized. The electrons thus released acquire kinetic energy at the expense of the field, and come to strike the atoms. During the shock, part of the energy is given to them, and brings them into an excited state. This mode of excitation is very interesting in the case of gases. As we have seen, the excitation of these by a source, whose energy is distributed throughout the spectrum (flash lamp, sun, electric arc, etc.) only leads to a very low efficiency. On the other hand, the efficiency of excitation by electric discharge can be high. This is, in particular, the case for CO2 and N2 as illustrated in Fig. 6. However, the CO2-N2 mixture is, moreover, a favorable laser medium, and if we combine, in particular, the pumping efficiency Rt, reported in Fig. 6, with the intrinsic efficiency R, = 0.4 of the CO2 laser (see Fig. 2), we find that the efficiency of such a laser could reach approximately 35%. The same reasoning, concerning the CO laser whose intrinsic efficiency Rt reaches 0.8 for certain series of transitions, leads to a theoretical efficiency of nearly 70%. In fact, the observed efficiencies only approach these values in favorable circumstances, generally at low temperatures. Thus, 24% was reached in a CO2 laser (6) at room temperature. In CO lasers, efficiencies of 63% were obtained around 65 °K (7), but the efficiency falls very quickly when the temperature increases, returning to a few % around 300 °K. On the other hand, a key advantage of these lasers is that their size does not seem to be limited, provided that the gas heated by the discharge is cooled by sufficiently rapid circulation (through an exchanger). Continuous powers of several hundred kilowatts have thus been obtained for CO2 lasers, with flow rates of 0.4 kg of CO2 per kilojoule “laser” emitted. In the case of CO lasers, the same methods would be applicable, and these two lasers (CO and CO2) are the only ones, among all those currently existing, to allow the emission of high powers (up to several hundred kW, perhaps 1 MW) with a significant efficiency. We will therefore seek to define more precisely the configuration of such a laser, focusing particularly on the CO laser, which on the one hand offers a better efficiency, and on the other hand presents, in terms of the transmission of the laser beam to the ground and in the atmosphere, various advantages to which we will return. 4. CONFIGURATION OF AN EMBEDDED CO LASER 4.1 Electrically Pumped Laser The efficiency of a CO laser depends primarily on the temperature. It is possible to consider maintaining the
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