Laser output intensity is linked to the size of the working gas’s active region that takes part in lasing: reducing the size of the passive zone in a gas mixture can increase laser emission intensity.

The typical temperature equalization time in a gas mixture with a transverse dimension of r1 is

tt ~ rja. (8.33)

If a laser cell is irradiated by a time-varying neutron flux, the typical rise time of which is t < tt, it can then be anticipated that a similar procedure would make it possible to slow passive zone development. The time dependence of this type that is the simplest one actually feasible, and that is the most convenient for numerical and theoretical investigations, consists of an exponentially rising neutron flux density

Ф() = Ф0^=т. (8.34)

The capabilities of this method were investigated based on the example of a helium-filled cylindrical cell with an internal radius of r1 = 1 cm and a wall thickness of Sw = 1 mm (the wall material during the calculations was zirconium). During all the calculations, it was assumed that Ф0 = 1013 cm-2 s-1. The reduced transverse dimension values of the cell’s metallic uranium layer and the tube were selected as D1 = 0.5 и D0 = 0.5 (which corresponds to 5U = 2.775 x 10~4 cm and P0 = 2.88 atm) at an initial temperature of T0 = 300 K. The calculated temperature profiles in the laser cell for several successive moments in time at a power deposition rise time of t = 10 ms are represented by the solid lines in Fig. 8.13. The passive zone involves the region between the cylindrical surface, r = rA, on which the maximum temperature is reached and the cell’s inner surface, r = r1.

The dependences of the radial coordinate of the active region’s outer boundary, rA, upon the thermal neutron flux rise time, t, and fluence, ф, which equals the

Fig. 8.14 Dependence of the radial coordinate of an active region’s outer boundary upon the thermal neutron flux rise time, t, and its fluence, ф, at D0 = 0.5: Curves 1-6 correspond to the values ф = (0.5, 1, 2,

3, 4, and 5) x 1013 cm~2 integral of expression (8.34), that were obtained as a result of calculations are shown in Fig. 8.14. Here, by way of comparison, it seems appropriate to recall that the fluence in the center of the laser cell during experiments performed on a VIR-2 M reactor reached 1.4 x 1013 cm~2 [2, 5—7,14]. For the sake of convenience in comparing the results obtained, the relationship between the pumping power rise time, t, thermal neutron fluence, ф, and the moment in time, t, by which this fluence is reached is reflected in the time scales in Table 8.4.

The results obtained make it possible to reach a conclusion: in order for at least half of the gas mixture volume in a cell with the parameters selected (which correspond to rA « 0.7 cm) to be an active region, the neutron flux rise time must be no higher than t = 7.5 ms. Here, the time that an active region with this volume exists does not exceed 0.1 s.