Mercury-Vapor Lasers

The first experiments to study the possibility of developing metal-vapor NPLs were carried out in 1970 using the IIN water-pulsed reactor [110]. In these experiments, using the mixture 3He-Hg at the transition 7p2P3/2-7s2S1/2 of the ion Hg+ (X = 615.0 nm), light signals that exceeded the level of spontaneous radiation were registered. The pressure of the mixture 3He-Hg was equal to 0.46 atm. These investigations were developed further in the study [111], in which, during experi­ments with the SPR-II pulsed reactor, with excitation of the mixture He-Hg (Р = 0.8 atm) using nuclear reaction products 10B(n, a)7Li, lasing was achieved and studied at the 615.0-nm line. The output laser power was ~1 mW (n~ 10~6 %), and the laser threshold was reached at ФгА ~1016 cm-2 s-1. The authors of [111] note that attempts to obtain lasing for the conditions of [110], where the mercury — vapor pressure was three orders of magnitude greater, did not yield a positive result. Study [112], based on spectroscopic investigations of the mixture 3He-Hg, con­cluded that it is not possible to obtain a high efficiency at the 615.0-nm line. This conclusion is confirmed by the calculations of [113], which show that the maximal efficiency at this line is no more than 0.03-0.04 %.

Investigations [112, 114, 115] of luminescence spectra of 3He-Hg showed a high populating efficiency of level 73S1 of the Hg atom, which is the upper of the triplet transitions 73S1-63P0210 (A = 546.1; 435.8, and 404.7 nm). The levels 63P0210 of the Hg atom are metastable, so to obtain lasing in quasi-cw or cw modes, quenching of these levels is necessary using an additional impurity, for which in [112, 114] nitrogen was proposed. However, attempts to achieve lasing in the mixture 3He-Hg — N2 at A = 564.1 nm were unsuccessful. The authors of [114] attributed the absence of lasing to the insufficient quenching influence of the N2 impurities in the presence of large quantities of helium.

The proposals to use the molecules H2 and D2 to quench the levels 63P0210 were more successful. Studies [103, 115, 116] concluded that the most promising medium for NPLs is a mixture of (He)-Xe-Hg-H2, in which Xe is a buffer gas, and He is used to cool the plasma electrons. Even before the appearance of the first lasers, Fabrikant and Butayeva in 1959 conducted research [117] into selective “quenching” of low levels of the mercury triplet with molecules of H2 in a gas discharge.

The spectroscopic investigations cited above made it possible to pump laser on the mixture He-Xe-Hg-H2 (A = 546.1 nm) using uranium fission fragments in experiments on the EBR-L setup [33, 118, 119]. For the mixture He-Xe-Hg-H2 (35:35:2:10), at an initial pressure of 0.4 atm and optimal temperature of 480 K, output laser power of 20 W was obtained (n = 0.4 %). The laser threshold was reached at ФгА«2 x 1015 cm-2 s-1. We note that pumping of the Hg laser (A = 546.1 nm) was also carried out using excitation of the active medium by an electron beam [120].

Cadmium — and Zinc-Vapor Lasers

The majority of investigations of metal-vapor NPLs were dedicated to the cadmium-vapor laser. A scheme of energy levels of the cadmium atom and ion with laser transitions is shown in Fig. 3.5. The first successful pumping of a Cd-vapor laser by nuclear radiation was carried out by MIFI associates in 1979, in experiments with the BARS-1-pulsed reactor [121, 122]. When the mixture

Fig. 3.5 Diagram of energy levels of the cadmium atom and ion with laser transitions

3He-116Cd was used, lasing was achieved at transitions 4/2F05/2,7/2- 5d2D3/2,5/2 of the Cd+ ion (A = 533.7 and 537.8 nm). The first successful experiments to pump NPLs based on metal vapors (the mixture He-116Cd, A = 441.6; 533.7 and 537.8 nm) with uranium fission fragments were carried out in 1982 by associates of VNIITF and VNIIEF on the EBR-L reactor [31].

The basic characteristics of cadmium — and zinc-vapor NPLs, which have similar lasing mechanisms, are shown in Table 3.9. There is no information about the

Table 3.9 Results of experimental investigations of Cd — and Zn-vapor NPLs

Mixture

A, nm

P

1 opt

atm

Wout,

W

Пі %

Ф* x 10-14,

cm-2 s-1

Тopt,

°K

Reactor

(laboratory)

Works

cited

3He-116Cd

533.7;

537.8 (Cd+)

0.53

0.1

3

600

BARS-1

(MIFI)

[122]

3He-116Cd

441.6 (Cd+)

0.53

0.05

2.5

680

BARS-1

(MIFI)

[123]

3He-116Cd

441.6 (Cd+)

1

3

660

VIR-2M

(VNIIEF,

MIFI)

[124]

3He-Zn

747.9 (Zn+)

1.1

2

17

740

VIR-2M

(VNIIEF,

MIFI)

[125]

He-116Cd

441.6 (Cd+)

1.8

1,000

0.4

7

660

EBR-L

(VNIITF)

[32,

33]

He-116Cd

533.7;

537.8 (Cd+)

470

0.3

6

740

EBR-L

(VNIITF)

[32,

33]

He-116Cd

806.7;

853.1 (Cd+)

35

0.02

320

740

EBR-L

(VNIITF)

[32,

33]

He-116Cd

1,430; 1,650 (Cd)

1-2

100

650

EBR-L

(VNIITF)

[32,

33]

He-Zn

747.9 (Zn+)

60

0.05

100

770

EBR-L

(VNIITF)

[32,

33]

Note: Topt is the optimal heating temperature of the active medium; Popt is the optimal pressure of the medium at an initial temperature of 300 K

investigations of Cd and Zn vapor NPLs outside of Russia. Only the survey study [126], published in 1983, notes that such experiments were carried out on a pulsed reactor (probably the APRF) using the mixture 3He-Cd (Р = 0.8 atm). Intensive radiation was observed at the 533.7- and 537.8-nm lines, but reliable proofs of the presence of lasing were not obtained.

Information about high-pressure NPLs using Cd and Zn vapors may be supplemented with the results of investigations of these lasers when pumped by electron beams (see review [95] and the studies cited there). For Cd-vapor lasers, apart from the laser lines 441.6, 533.7, and 537.8 nm, registered during nuclear pumping, lasing was also obtained in the UV range of the spectrum at transitions of the Cd+ ion (A = 325.0 nm) and Cd atom (A = 361.0 nm). In the mixture He-Zn, lasing was obtained at the transition of the Zn+ ion (A = 610.2 nm), which was not observed under nuclear pumping conditions. The efficiency of Cd-vapor lasers was ~0.1 %. For electron beam pumping, lasing was also observed in the mixture He-Sr (Р = 3.5 atm) at transitions of the Sr+ ion (A = 416.5 and 430.5 nm) [127]. We note that the possibility of achieving lasing at the transition of the ion Sr+ (A = 430.5 nm) for the mixture He-Sr pumped by nuclear radiation was considered previously in the study [128].

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