3. TYPES OF HIGH-POWER LASERS Only high-power lasers are of interest for SPS application. Table 1 lists the various types of high-power lasers. These types are distinguished by their pumping mechanisms. The efficiency of gas dynamic lasers (GDLs) is too low (<1%) and, hence, their mass too great for them to be used in an SPS. The GDL will not be discussed further in this report. EDLs, direct and indirect SPLs, FELs, and closed-loop chemical lasers are described in the next sections. 3.1 Electric Discharge Laser In an electric discharge laser (EDL), an electric discharge pumps lasant molecules to the upper energy level of the lasing transition (e2 in Fig. 3). An electric field maintains the discharge and accelerates electrons to high speeds. The energy of the accelerated electrons is transferred by collisions to the lasant molecules. Waste heat can be removed by conduction through the walls of the laser tube, but the slowness of the conduction process severely limits power output. This limitation can be avoided by convecting the heated lasant gas out of the lasing cavity as shown in Fig. 4. The heat is removed from the hot gas by a heat exchanger and the cooled gas pumped back to the laser cavity. In that cavity, the gas flow is perpendicular to the laser beam. In addition to the problem of heat removal, nonuniformity of lasant excitation in the laser cavity has been a problem in the development of high-power EDLs (4). Uniform excitation is needed to produce an output beam of high quality. However, the tendency of the discharge in a high-power device is to become very nonuniform by collapsing to a small subvolume of the discharge region rather than filling up the entire region. This problem can be solved by using an electron beam sustainer device as shown in Fig. 4. Fig. 4. Closed-cycle, subsonic, CO2 electric discharge laser. Source: Adapted from Ref. 6.
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