NewTrans1.txt[9/15/2024 8:15:35 PM] LASERS AND ENERGY TRANSMISSION A. ORSZAG Applied Optics Laboratory Ecole Polytechnique — Ecole Nationale des Techniques Avancées 91400 Palaiseau, France The concept of using lasers for energy transmission in general, and from space in particular, has already been the subject of numerous studies (1). First of all, it should be noted that the evaluation of the prospects offered by lasers drawing their energy from the sun, then radiating from space to terrestrial collectors, involves several processes that have in common the fact that they have low efficiencies and are located in rapidly evolving scientific or technological fields. The final efficiency of the complete system, the product of the efficiencies of the various previous processes, therefore depends primarily on the state of the art, and even, too often, on the optimism shown by the authors in evaluating the current state, and even more so on the progress to be expected, in the various fields concerned. One of the most crucial is, of course, that of lasers, and I therefore suggest that we examine more closely the various paths at the end of which the primary energy — coming from the ground — can end up in the form of a beam of coherent light. 1. PUMPING To evaluate the possibilities more precisely, it is necessary to recall the “energy” properties of the laser which are essentially defined by its amplifying medium. The atoms (2) of the latter are generally characterized by energy levels arranged as shown in Fig. 1. During the operation of the laser, an appropriate energy source carries a continuous flow of atoms from a minimum energy starting state Eo to an excited state E2. From this state the atoms spontaneously de-excite, by any mechanism (radiative, thermal or vibrational), but which must be rapid, to the level E} which will be the starting point of the “laser” emission. This one draws its energy from the transitions of Et towards Eo (or possibly towards one or more intermediate levels E3, E4 . . from where they finally return towards Eo) to then start their cycle again. The “fall” of each atom from E, towards Eo (or E4) is accompanied by the emission of a photon whose energy hv carries the corresponding energy. In the best case scenario, a limit thus appears to the efficiency of any laser: it is the “intrinsic” efficiency: Rt = (E, - E^I(E2 - Eo), ratio of the energy of the emitted photon to that which had to be supplied to the emitting atom to “climb” from Eo to E2 (a process called “pumping”). The second factor which limits the efficiency of lasers is, precisely, the difficulty of “pumping” the E2 level efficiently, that is to say, of making the Eo-^ E2 transition absorb the energy from an external source. In our case, the first source to consider is the solar flux itself, which we can roughly consider as a black body around 5900 °K. 2. SOLAR PUMPING We are then led to consider two types of lasers: those whose amplifiers are gaseous media, and those whose amplifying medium is dense (liquids or solids).
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