All the calculations of the characteristics of the alteration of cavity stability, S(t), described below were performed using data from experiments for He-Xe and Ar-Xe mixtures (the LUNA-2 M setup with planar uranium layers and the VIR-2 M reactor), as well as Ar-Xe and Kr-Xe mixtures (a cell with a cylindrical uranium layer and the TIBR-1 M reactor). The laser cell characteristics are presented in

b

0. 8

^ (t) 0 6 0.4

0.2

Sect. 7.4 of Chap. 7. Optical inhomogeneity evolution was calculated using the procedure described in the second section of this chapter.

The calculations revealed that, for the cell designs examined under standard pumping conditions, the cavities of lasers with active media that consist of helium — based mixtures do not leave the stable state. For mixtures of this type over the course of an entire pulse, S(t) = 1. This result is in full agreement with the experimental data, because the pulse shape therein after the lasing threshold is exceeded virtually duplicates the shape of the pumping pulse. However, when mixtures are used in which the buffer gases have high atomic numbers, as exem­plified by Ar or Kr, loss of stability is observed, during which the stability loss

effect is more clearly manifested as the mixture pressure is increased. However, if the pressure remains unchanged, this effect is intensified with an increase in the atomic number of the buffer gas.

The experimental dependences of the output laser power of a mixture with a value of Ar:Xe = 2,000:1 at P0 = 0.5 atm in a laser with planar uranium layers (I) and a neutron pumping pulse (n) upon time, normalized to their maximum values, are shown in Fig. 8.19а. The calculated temporal Sy(t) and Sx(t) stability character­istics are shown in Fig. 8.19b. The only small trough on the Sy(t) temporal diagram is in agreement with the trough at the experimental laser pulse peak. The results of the Sx(t) calculations demonstrate that a stability decrease for the x-component

clearly occurs in the vicinity of the laser pulse peak. Under experimental condi­tions, this decrease apparently also leads to the earlier decrease of the laser pulse as compared to the neutron pulse, which creates the outward appearance of a lasing pulse shift relative to the pumping pulse. The slight nonsynchronism between the troughs at the laser peak and on the Sy(t) diagram, as well as between the start of laser pulse decrease and the start of the dip on the Sx(t) diagram, can be explained by the shortcomings of the calculation procedure used, in particular, an escalating error in the calculation of density distributions past the pumping pulse peak. These same shortcomings should apparently also explain the fact that Sx(t) instability develops just as consistently, but not as intensely as the experimentally obtained the behavior of trailing edge of laser pulse.

The similar dependences obtained for a mixture Ar:Xe = 600:1 at P0 = 1.5 atm when using the very same laser cell are presented in Fig. 8.20. Here, as in the previous example, a some correlation is observed between kinks on the laser pulse front and the Sy(t) characteristics. So, the first three laser pulse kinks can be compared to the first three steps on the Sy(t) curves and to the dual dip in the vicinity of the peak—two closely positioned dips at Sy(t) immediately past t = 8 ms. Starting at roughly this moment in time, instability develops for Sx(t) and the aggregate effects of both the instabilities lead to the cessation of lasing.

Calculations of Sy(t) and Sx(t) characteristics revealed that the development of a positive gas lens (8.38) in a laser with planar uranium layers leads to cavity stability fluctuations in time, whereas a negative gas lens (8.41), starting at a specific moment in time, produces a consistent stability decrease.

The calculation results for a laser with a cylindrical uranium layer are presented in Figs. 8.21 and 8.22. In this instance, only a positive gas lens originates and cavity stability is described by correlations (8.38)-(8.40), in which the y coordinate must be replaced with the radial coordinate r. A relatively good correlation was observed between the laser pulse and the S(t) characteristics for a mixture Kr:Xe = 200:3 at P0 = 1 atm. The worst correlation between the laser pulse and the calculated S(t) characteristics was obtained for a mixture Ar:Kr = 100:1 at P0 = 1 atm.