For condensed laser media, pumping with the help of NOCs was more successful than direct pumping of these media with nuclear radiation, as was considered in the previous section. There are two experimental studies which show that neodymium lasers may be pumped using NOCs based on xenon plasma [41] and the phosphor CsI(Tl) [55]. This method was also used successfully in experiments on the TRIGA reactor, in which a photo-dissociation iodine laser was pumped with the lumines­cent emission of XeBr* molecules (see Chap. 3, Sect. 6).

Experiments that pumped solid-state lasers with NOC emission were carried out on pulsed reactors [40, 41, 55].

Fig. 11.5 The spectra of the luminescent crystal CsI (0.5 % Tl) at different points of the pulsed irradiation on the VIR-2 reactor at Dy = 3.5 x 103 Gr and Ф = 4 x 1014cm~2. The insert shows the time dependence of the dose rate PY (upper curve) and the luminescent power density IY (lower curve) and indicates the moment in time (1, 2, 3, 4, 5, 6) when the spectra were recorded [2]

A series of experiments [40] on the TRIGA reactor used gas NOCs based on 3He-Kr(Ar, Xe) mixtures with excitation by the products of the nuclear reaction 3He (n, p)3H. The spectral-luminescent characteristics of these mixtures are given in two works [40, 52]. The specific power deposition at the maximum of a 12-ms reactor pulse was about 20 W/cm3. A schematic of the experiment is shown in Fig. 11.6. The light radiation from the NOC, which was placed near the reactor core, was transmitted to the active laser element Y3Al5O12:Nd3+, 3 mm in diameter and 62.5­mm long, using a bundle of ~2,000 quartz light-guide fibers that were 5-m long. To pump the volume of the laser element uniformly, the end faces of the fibers were uniformly distributed about its surface (Fig. 11.7). Laser action was not observed in these experiments. In study [40] absence of laser action is explained by the low specific power deposition in the laser medium due to loses during transportation of the light radiation and poor correlation between the plasma luminescence spectrum and the absorption spectrum of the Y3Al5O12:Nd3+ crystal.

Experiments performed in VNIIEF on the VIR-2 M reactor were more success­ful. As part of these experiments, fiber neodymium lasers were pumped with the light radiation of xenon plasma [41]. Prior studies have shown that the spectrum of the plasma radiation that appears when xenon at 1 atm is excited with uranium fission fragments (the specific energy deposition is ~5 J/cm3) is close to the emission spectrum of an absolutely black body at temperatures <6,000 K [41].

In experiments on the VIR-2 M reactor, the laser active element was placed inside the NOC between two plates with uranium layers (Fig. 11.8). The 10 x 6 cm plates were placed parallel to each other 2 cm apart. Their internal surfaces have a deposited layer of uranium oxide-protoxide. The thickness of the metallic 235U layer was about 5 mg/cm2. A fiber laser 1-4-m long with a fiber diameter of 40-50 ^m was wound around a frame and placed symmetrically between the

uranium layers. Irradiation of the NOC was done at the bottom of the central reactor channel. This series of experiments obtained laser action when using fiber neodym­ium lasers based on silicate and phosphate glasses. The results of one experiment on a neodymium laser based on phosphate glass with the NOC filled with xenon at 0.5 atm (plasma temperature 4,700 K) are shown in Fig. 11.9. The maximum energy of the laser radiation over a single pulse was 2.5 J.

Besides using gas NOCs to pump neodymium lasers, in VNIIEF also used NOCs based on solid-state scintillator. A brief overview [55] of these experiments reported that lasing was obtained when pumping a laser element based on neodym­ium glass with radioluminescent radiation from the CsI(Tl) crystal. The laser element and CsI(Tl) crystal were irradiated in the central channel of the BR-1 pulsed reactor [16]. The maximum laser power was equal to 4 kW at a laser pulse of approximately 60 ^s duration.

In conclusion, a recently published study [56] should be noted where pumping a laser based on the crystal YAP:Tm3+ (A = 1.85-2.0 ^m) was fulfilled using gas NPLs at the transitions of Ar (A = 1.79 ^m) and Xe (A = 1.73 ^m) atoms as a NOC. The active region of the laser crystal was 5-mm long with a volume of about 4 mm3. From the oscilloscope traces in Fig. 11.10, it can be supposed that the experiments were performed on the EBR-L pulsed reactor [16]. When a xenon NPL (A = 1.73 ^m) was used as the source for pumping, the threshold specific power deposition was about 35 kW/cm3, and the conversion efficiency of the absorbed optical radiation into laser radiation was 4 %. Such low laser efficiency is rather

unexpected since prior executed studies [56] showed that the conversion efficiency of laser radiation on lines 1.73 and 1.79 ^m into luminescent radiation was equal to 54 and 76 %, respectively.