standards accept. Continuous wave laser beam of 2 pm is not capable of hole boring unless superimposed with a train of short duration pulses of about 100 kW/cm2. [Beverly, 1980e] Many studies have been made for aerosol scattering, absorption and excitation coefficients for various types of clouds and rain and snow distributions. Furthermore, seasonal and annual power availabilities and transmission frequencies have been studied on different locations over USA. [Beverly, 1980e] The total efficiency of power transmission from space to earth is highly affected by the availability of the beam and attenuation in atmosphere. To avoid wasting of available power there has to be many power receiving sites in view of the power satellite. If possible these receiving sites should have different kinds of weather probabilities. When the received power in receiving site reduces below some threshold level the power beam has to be refocused to another receiving site. The selection of the threshold level effects greatly to the total effectiveness. The threshold level is not only depending on the transmission efficiency but also on the time the transmission effectiveness exceeds some useful value. [Beverly, 1980d] Laser Receiver Laser energy receiver and converter may be either energy to electricity converter as photovoltaic cell or it may transfer the laser energy to heat. Heat can then be converted to mechanical energy and further to electricity. This section discusses first conversion to heat and on the second subsection conversion to electricity or mechanical energy. Receptors Heat exchanging devices need very highly concentrated laser radiation. Concentrating optics is not desirable since the environmental effect will decrease the reflectivity and large high power density reflectors are very expensive. To avoid optical concentrator, the laser spot on receiver site should be reduced as small as is possible considering the effect of diffraction, turbulence, thermal blooming, pointing accuracy and jitter. However reduced spot requires very dense power beam that in turn may cause other problems. Figure 7.37 Absorbing Sphere Concept
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