Already in the first experiments performed with the VIR-2 reactor in 1972, the output power of the xenon laser with an optimal pressure and composition of the He-Xe mixture was 25 W with nl~0.5 % [1]. In subsequent experiments using the TIBR-1M, VIR-2, and VIR-2M reactors, lasing was obtained in a spectral range of 1.15-3.65 pm at 24 transitions of the Xe, Kr, and Ar atoms, with excitation of active

© Springer Science+Business Media New York 2015 53

S. P. Melnikov et al., Lasers with Nuclear Pumping,

DOI 10.1007/978-3-319-08882-2_3

.М3/: I

5*45/2],

5*45/2];

Fig. 3.1 (continued)

b
eV 14
5 pdfs’ p’ d f’
6 5
13
12
11
10
5
Fig. 3.1 (continued)

Fig. 3.1 Diagrams of excited levels of atoms Xe (a), Kr (b), and Ar (c) with laser transitions

media by uranium fission fragments. Their spectral, energy, and threshold charac­teristics were studied in detail. In part these results are cited in the reviews [1—4]. Table 3.1 shows the basic data for all the lasers studied at VNIIEF operating on transitions of Xe, Kr, and Ar atoms with references to the original studies. Experimental units and methods are examined in Chap. 2, Sects. 2.3 and 2.4.

The majority of laser lines belong to the transitions nd-(n + 1)p of Xe, Kr, and Ar atoms (see Fig. 3.1). Apart from that, lasing was observed at 3.65 pm (transition 6p — 6s XeI); 3.07 and 2.86 pm (transitions 5p-5s KrI); 1.87 pm (transition 4d’-5p’ KrI); 2.87 pm (transition 5p’-5s’ ArI); 1.27 pm (transition 3d’-4p’ ArI) and 1.15 pm (transition 4p-4s ArI). We note that nearly all of the laser transitions shown in Table 3.1 were observed early in low-pressure gas discharge lasers [18] except for the 2.81 pm line of the Хе atom and the 1.87 pm line of the Kr atom. The mechanism of creating the population inversion on transitions of Xe, Kr, and Ar atoms is reviewed in Sect. 5.2 of Chap. 5.

The laser operating on transitions of the Xe atom has the maximal energy parameters. The active medium of this laser is mixtures of He-Xe, Ar-Xe, Kr-Xe, He-Ar-Xe, Ne-Ar-Xe and pure Xe at pressures <6 atm. Helium, neon, argon, and krypton are buffer gases, while the concentration of xenon is 1-10 %. Depending on the conditions of the experiment (pressure and composition of mixtures, power deposition, properties of the cavity mirrors), lasing was observed at different lines in a spectral range of 1.7-3.5 pm.

The maximal energy parameters (output power Wout < 2 kW, ni < 2.5 %) were registered at the 1.73; 2.03 and 2.65 pm lines of the Xe atom, which originate from the level 5d[3/2]10. For these same lines, the minimal laser thresholds Ф^ < 1013 cm-2 s-1 were observed. The lowest laser threshold, at Ф^ = 1.5 x 1012 cm-2 s-1 (specific power deposition at laser threshold qth~ 0.1 W/cm3) was registered for mixtures of Ar-Xe (A = 2.03 pm) [6, 7]. Such low thresholds make it possible to use not only neutron radiation of stationary nuclear reactors to pump an Ar-Xe laser, but also radioisotope sources [7].

NPLs operating on transitions of Kr and Ar atoms have lower energy parame­ters—in pumping mixtures of He(Ne)-Ar and He-(Ne)-Kr at atmospheric pressure with uranium fission fragments, щ < 1 % was obtained. Among NPLs using Ar atom transitions, one should note the He-Ar laser (A = 1.15 pm) [8, 16], in which quasi-CW lasing occurs at transition 4p[1l/2]1-4s'[1/2]1° as a result of collision “quenching” of the lower metastable level 4s'[1/2]1° by atoms of the buffer gas, helium.

In experiments using an Ar-Xe mixture carried out on the LUNA-2M setup in 1985, a competition effect was observed among the laser lines 1.73; 2.03 and 2.65 pm of the Xe atom, which have a common upper laser level. Figure 3.2 shows oscillograms of pulses of neutron and laser radiation for the mixtures He-Xe, Ar-Xe, and Ar-Xe-He [6]. In the He-Xe mixture, lasing occurs only at A = 2.03 pm (Fig. 3.2а). In the mixture Ar-Xe, lasing first occurs at A = 2.03 pm. With an increase in the pumping power, the line 1.73 pm appears, and there is stopping of lasing at A = 2.03 pm. The 2.03-pm line arises again at the end of the pumping pulse after cessation of lasing at A = 1.73 pm. Small additions of helium

1.78	He-Kr	2	50	0.3	0.6	1.1	VIR-2	[14]
2.52	He-Kr	2	110	0.6	2.6	1.1	VIR-2	[14]
3.07	He-Kr	2	40	0.2	1.7	1.1	VIR-2	[14]
1.78 1.87 2.19	He-Kr	2	120	0.2	0.29	2.5	VIR-2M	[15]
1.78 1.87 2.19	He-Ne-Kr	1	140	0.2	0.19	2.5	VIR-2M	[15]
2.52 3.07	He-Kr	2	420	0.6	3.2	2.5	VIR-2M	[15]
2.19 2.86 3.07	Ne-Kr	1	100	0.1	0.68	2.5	VIR-2M	[15]
2.19 2.52 2.86 3.07	He-Ne-Kr	1	460	0.6	1.2	2.5	VIR-2M	[15]
2.63	Kr	0.25	2	—	0.2	2.5	VIR-2M	[15]

(continued)

3.1 IR Lasers Operating on Transitions of the Xe, Kr, and Ar Atoms

LA

VO

Atom	Я, pm	Mixture	P, atm	wOM, w	>1b %	Ф„, x 10~14, cnT2 s_1	Ф„мл x 10 1S, cm 2 s 1	Reactor	Work cited
Ar	1.15 1.19	He-Ar	2	250	0.1	100	26	TIBR-1M	[8, 16]
1.27	He-Ar	1	10	—	5.0	2.1	VIR-2M	[17]
1.69 1.79	He-Ar	2	390	0.6	3.2	2.1	VIR-2M	[17]
1.69 1.79	Ne-Ar	1	140	0.2	0.9	2.1	VIR-2M	[17]
2.10	He-Ar	2	220	0.3	6.3	2.5	VIR-2M	[15]
2.06 2.10 2.21	Ne-Ar	1	110	0.15	1.2	2.5	VIR-2M	[15]
2.40	He-Ar	1	—	—	—	26	TIBR-1M	[8]
2.40 2.87	He-Ar	2	190	0.25	8.4	2.5	VIR-2M	[15]
2.21 2.31 2.40	Ne-Ar	1	60	0.1	1.3	2.5	VIR-2M	[15]
2.31 2.40 2.87	He-Ne-Ar	1.3	140	0.15	2.0	2.5	VIR-2M	[15]

Note: P pressure of active medium, W power of laser radiation (output power),power efficiency (ratio of output power to power deposition), thermal — neutron flux density averaged over active medium length, at which the laser threshold is achieved (threshold thermal-neutron flux density), Ф„ил maximum thermal-neutron flux density averaged over active medium length, at the reactor pulse maximum

Fig. 3.2 Oscillograms of thermal neutron pulse (a) and of laser pulses: (b) mixture of He-Xe (1,000:1), Р = 2 atm; (c) mixture of Ar-Xe (100:1), Р = 0.5 atm; (d) mixture of Ar-Xe-He (100:1:100), Р = 1 atm [6]

(0.25-0.5 atm), which insignificantly affect the power deposition, lead to elimina­tion of the 1.73-qm line (Fig. 3.2d).

Competition of laser lines having the common upper laser level 5d[3/2]10 may be explained by the differences in probabilities of radiative decay and the rate con­stants of the processes of collision “quenching” of the lower laser levels by atoms of the buffer gas. An analogous effect was observed in the Ar-Xe mixture for two other lines of the Xe atom (2.65 and 1.73 qm; 2.03 and 2.65 qm) [6, 7], in the Kr-Xe mixture for the 2.63 and 2.81 qm lines of the Xe atom [9], as well as in the mixture Ne-Ar for the 2.31 and 2.21 qm lines of the Ar atom [15]. Competition of lines was also observed in excitation of the mixture Ar-Xe by an electron beam [19, 20]. The most complete analysis of the different variants of competition of laser lines having a common upper or lower laser level was done in studies [21—23].

For a more detailed study of xenon NPLs using mixtures of He-Xe (1.73; 2.03 and 2.65 qm), Ar-Xe (1.73 qm), and He-Ar-Xe (2.03 and 2.65 qm), a series of

Table 3.2 Small-signal gains (a0) and saturation intensities (Is) for NPLs transitions of the Xe atom at optimal xenon pressure Mixture He-Xe He-Xe He-Xe Ar-Xe He-Ar-Xe He-Ar-Xe Parameters (1.73 pm) (2.03 pm) (2.65 pm) (1.73 pm) (2.03 pm) (2.65 pm) Buffer gas He He He Ar He-Ar(1:1) He-Ar(1:1) Pressure, atm 2 2 2 0.5 1 1 Pxe, Torr 3.8 0.95 0.95 0.95 0.95 0.95 a0, cm-1 6.8 x 10-4 1.7 x 10-3 3.6 x 10-3 3.6 x 10-3 1.6 x 10~2 2.4 x 10~2 Is, W/cm2 120 80 72 110 20 7.3 Works cited [24] [24] [12] [12, 13] [12] [12, 13]

experiments was carried out on the LUNA-2M setup to measure the energy and threshold characteristics as a function of the partial pressure of the xenon with various transmission coefficients of the cavity mirrors [12, 13, 24]. Narrow-band dielectric mirrors were used in the experiments, which made it possible to preclude simultaneous lasing at several lines. The basic goal of the experiments was to obtain information about the parameters of the active media of xenon NPLs (small-signal gain a0, saturation intensity Is, and coefficient of distributed losses P) as a function of the partial pressure of the xenon. This information, obtained as a result of processing of experimental data using an approximation by Rigrod’s formula [25], is shown in Table 3.2 for q = 40 W/cm3 (maximum of pumping pulse) and optimal partial pressures of xenon.

To study the parameters of the active medium of a xenon NPL, a direct method of measurement by the “oscillator-amplifier” scheme was also used [26]. Experi­ments were carried out on the LUNA-2M setup for the mixture He-Ar-Xe (380:380:1) at a pressure of 1 atm at the 2.03 pm line. The small-signal gain increases approximately linearly with an increase in the specific power deposition, and at a maximal value of q = 40 W/cm3 reaches 1.2 x 10~2, while the saturation intensity is virtually independent of q and is 70-90 W/cm2. The output laser power at the pumping pulse maximum at the outlet from the amplification channel (790 W) proved to be virtually twice as high as at the input (400 W).

In the experiments considered above, simple, stable two-mirror cavities were used. The presence of two identical laser channels in the LUNA-2M setup makes it possible to study more complex schemes for combining the NPL channels and the radiation withdrawal, which is of interest for forming output radiation of multichannel laser facilities, particularly to reduce the number of light beams (see Chap. 10). VNIIEF proposed two methods of combining the NPL channels, which by analogy with the parallel and serial connection of elements in electrical circuits, can be called “serial” and “parallel” combining of laser channels (see Fig. 10.7). The results of computational and experimental investigations in this area are shown in studies [27—30].

The basic results of investigations on combining the radiation of laser channels on the LUNA-2M setup are shown in Table 3.3. The active laser media used were mixtures of He-Ar-Xe (380:380:1) at a pressure of 1 atm (A = 2.03 pm) and Ar-Xe

Table 3.3 Maximal energy parameters with serial and parallel combining of two laser channels of the LUNA-2M setup [28—30] A, qm Mixture Pressure Type of laser channel ropt, % E, J Wout, kW 2.03 He-Ar-Xe (380:380:1) 1 atm A 71 1.5 0.54 B 46 1.8 1.0 C No output coupler 1.9 0.87 1.73 Ar-Xe (380:1) 0.5 atm A 68 0.96 0.52 B 47 1.1 0.62 C No output coupler 1.7 0.98 Note: A is one standard channel (two-mirror cavity with radiation output in the longitudinal direction relative to the optical axis); B is two serial combined laser channels (see Fig. 10.7a); C is two parallel combined laser channels (see Fig. 10.7b); ropt is the optimal reflectivity of the output mirror; E is the output energy of laser radiation per pulse; Wout is the maximal output power of laser radiation in the process of the pumping pulse

(380:1) at a pressure of 0.5 atm (A = 1.73 qm). The active volumes of the laser channels were identical and equal to 1.9 l. It is clear from Table 3.3 that the output power of two serially combined channels (A = 2.03 qm) and parallel combined channels (A = 1.73 qm) is virtually twice the output power of a single channel. It should be noted that optical non-uniformities arising in the laser medium in the process of the pumping pulse have a significant effect on the laser parameters of the double laser channel with serial combining. This is particularly distinctly demon­strated for the mixture Ar-Xe. In the case of the parallel combining, the shape of the laser pulse of several combined channels does not depend on the number of channels, but the output power and energy are proportional to the number of combined channels. This is explained by the fact that with parallel combining (in contrast to serial), there is no accumulation of optical non-uniformities. The parallel scheme is therefore very promising for use in multichannel NPLs.