The efficiency of transformation of deposited power (energy) into laser radiation is the most important parameter for all lasers. In the case of NPLs, the maximal efficiency of transformation of deposited nuclear energy into laser radiation belongs to the lasers operating on transitions of rare gas atoms, and the highest n > 1 % were registered for lasers operating on transitions of the Xe atom (1.73, 2.03, and 2.65 pm) and the Ar atom (1.27 and 1.79 pm). Table 3.6 compares the results obtained by various laboratories, and also provides the maximal efficiencies (nmax) constituting the ratio of the laser photon energy to the energy consumption for forming one primary active particle of plasma (ion or excited atom). This question is discussed in greater detail in the first section of Chap. 5. For comparison, Table 3.6 also includes information on efficiencies obtained from excitation of gas media by electron beams.

It follows from Table 3.6 that in some cases the real efficiency (n) approaches maximal, which testifies to minor energy losses in plasmochemical reactions of energy transfer and high selectivity of the final process of populating of the upper laser levels. Possibly the high efficiency of NPLs is explained by the noticeable

contribution to populating the upper laser levels from the processes of associative ionization with the participation of metastable states 6s of the Xe atom or 4s of the Ar atom [65].

For quasi-CW NPLs with a laser duration of >100 ps, which is substantially greater than the characteristic times of plasma processes, the efficiency usually is defined as the ratio of the output power to the instantaneous power absorbed in the laser’s active volume. As a rule the active volume is understood to be the entire excited volume of the laser cell. In particular, this method was used to determine the efficiency in the studies by VNIIEF and VNIITF. However, sometimes qI was determined with respect to the deposition of power to the mode (radiating) volume, which was less than the total cell volume. Naturally, in this case the value of nI will be greater. For example, in study [40], the efficiencies for NPLs using the mixture 3He-Ar (A = 1.79 pm), calculated by two methods, differ by roughly a factor of 100.

Another reason for the discrepancy in the efficiency data obtained in the various laboratories is the use of two different methods to determine the energy deposition to the laser medium: from the results of measurement of the pressure jump in the gas medium, or by calculation. The results of investigations in different laboratories showed that the energy deposition measured in experiments is less than the com­puted value by roughly a factor of 1.5-2. The reasons for the discrepancy between the computed and experimental data are considered in Sect. 7.4 of Chap. 7. It cannot be excluded that the values of nI obtained in the Sandia experiments, which exceed maximal values, may be explained by errors in determining the energy deposition.