NewTrans1.txt[9/15/2024 8:15:35 PM] amplifier gas at around 60-100 °K by circulating it in a radiator R (Fig. 7). For a 100 MW laser, the gas flow rate required (to limit its heating to about twenty °C) is 0.5 tonnes per second, which would require a pump P whose power can be estimated at 20 MW. If the efficiency of the laser itself is 60%, 66 MW of heat will have to be evacuated into the vacuum by a radiator whose surface, even if it radiated like a black body at 60°K, should reach nearly 100 km2. We can thus see that the overall efficiency of an electrically pumped laser could exceed 50% (taking into account the circulation pump), but that the need for a heat radiator at 60 °K represents, in terms of the weight and size of the satellite, a constraint that is all the more marked since the surface of the sensors required to produce the electricity for the laser will be significantly lower (1 km2 if the sensors have an efficiency of 20%). (Note that, in the case of a CO2 laser, whose efficiency is about 20%, it would be necessary to evacuate, for the same radiated power, 400 MW of heat. On the other hand, the efficiency of the CO2 laser remains acceptable up to around 400 °K. Under these conditions, for a laser operating between 300 and 400 °K, a flow rate of 1 ton/second of gas would be required, maintained by a 40 MW pump. The efficiency of the complete system would be about 18%, but the surface area of the radiator required is only 1 km2.) On the other hand, the energy collecting surface is a little more than double that required for the CO laser. 4.2 Electrically Pumped and Relaxed Laser There is a way to reconcile the high efficiency of the CO laser and a reduced radiator: it is the electrically pumped and relaxed laser. Its principle uses supersonic expansion in a nozzle to lower the CO temperature. It is therefore necessary to have a compressor P powerful enough to cause the expansion, but, in return, the temperature upstream of the nozzle T can be high, while at the end of the expansion the temperature reaches the required 65-80 °K. Fig. 8 shows the diagram and characteristics of such an assembly, proposed by J.D.G. Rather (9). Still for the same power of 100 MW emitted, a laser efficiency of 65%, the power to be evacuated by the radiator is 46 MW, and the overall efficiency is 47%. The CO stop temperature being approximately 300 °K, the surface of the radiator is lowered to around 0.1 km2. The CO laser with expansion thus appears, in conclusion, as the best transformer currently available between electrical energy and light energy. It now remains to examine the other two links in the chain: the beam and the collection of energy on the ground. 5. THE PROPAGATION OF THE LASER BEAM The beam, crossing the atmosphere, is therefore subject to an absorption which depends on the thickness of the air crossed, that is to say both the altitude and the inclination of the beam. For a vertical beam, we can (10) derive from Fig. 9 the atmospheric transmission as a function of altitude. We can immediately see the advantage of the CO laser (k — 5 /xm) compared to the CO2 laser, as well as the advantage of placing the receivers at a relatively high site (h > 2 km). More in-depth studies also show that the absorption of the beam increases at very high energy densities (>103 W/cm2), but it is unlikely, for other reasons, that such a density will be achieved. Indeed, the ground diameter d achievable with a mirror of diameter D would be at best (if the laser were optically “perfect”) of the order of d = 1.4 h\ID, if h is the altitude of the satellite. However, the need to remain in sight of the receiving stations for as long as possible encourages the choice
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