As the results of experiments and calculations showed (see Chap. 8), it is necessary to fulfill two basic conditions to obtain lasing in stationary mode: provide the necessary specific power deposition of the laser medium, and organize flowing of the gas mixture in the laser cell so as to eliminate its overheating and reduce the optical non-conformities. Taking into account these conditions, VNIIEF designed and built laser module LM-4 [1—6] in 1994 for joint operation with the powerful BIGR pulsed reactor [7], which can, in one of its operating modes, generate neutron pulses with a duration of >1 s. The experimental complex was supposed to demonstrate the possibility of creating reactor laser-type units with a stationary operating mode.

The LM-4 laser module [1—6] is a stationary-operating gas NPL functioning in gas-flowing mode (Figs. 6.1 and 6.2). The module and device for circulation of the gas were placed on a cart (see Fig. 6.1) and moved to the reactor core on it. The LM-4 module consists of four identical laser channels (Fig. 6.3). Over the length of the uranium layers, the average thermal neutron fluence reached values of (5-7) x 1014 cm~2 with a duration of 0.5-1.7 s.

Laser channels of the module are included in a single gas loop and are separated from one another by heat exchangers (radiators), which cool the gas when it is passed through. The radiators are packages 100-cm long, comprised of thin

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

DOI 10.1007/978-3-319-08882-2_6

aluminum fins around 0.3-mm thick, and the same gaps between them. To equalize the gas dynamic perturbations of the gas flow, the edges of the fins were sharpened. The radiators do not have forced cooling, so their functionality is limited by their own heat capacity.

The active length of each channel, determined by the length of the uranium layers along the optical axis, is equal to 100 cm. The dimension of the channel in the direction perpendicular to the gas flow is 2 cm; in the direction parallel to the flow it is 6 cm. Layers of metallic 235U about 5 mg/cm2-thick are applied to 100 x 6 cm2 plates. At their ends, the channels are closed by windows arranged at a Brewster angle. The experiments primarily used stable optical cavities consisting of a 100 % spherical mirror with a radius of curvature of 20 m and a flat semitransparent dielectric mirror. The distance between the mirrors is about 170 cm.

The gas-flowing system provides flow of the gas mixture sequentially through all the laser channels in the transverse direction relative to the channel axis at rates of 4-10 m/s. At a gas flow rate of 10 m/s, the duration of the stationary flow is 3-5 s. The gas-flowing device is a circular, single-blade force-pump capable of creating an overpressure of 80 Torr. The internal diameter of this unit is 80 cm, the blade length is 109 cm, and the angle of the full rotation of the blade is 270°.

To ensure the functionality of the LM-4, the following devices were developed: a transport system, which accomplishes and controls the module movement from the measurement area to the reactor hall, and fixes it close to the reactor core [1]; a system for degassing, filling, and flowing of the gas [1, 4]; the systems for remote adjustment of the cavity mirrors and withdrawal of laser radiation from the reactor hall to the measurement area [6, 8, 9]; an automated system for registration of parameters of the laser radiation, neutron flux, and gas mixture rate [1, 6,10]; and a technological monitoring system for communications between the BIGR reactor and the module [1, 6].

The optical circuit scheme of the experiments is shown in Fig. 6.4. The route for withdrawal of laser radiation from the reactor hall includes aluminum 100 %

mirrors (parts 2-6) and a beam splitter (7). A lens system (10) was used to reduce the transverse cross section of the beam and construct a reduced image of the output mirrors of the laser channels at the input of the registration unit. Magnetostrictive motors with a feed pitch of around 1 ^m were used to rotate some of the mirrors.

Depending on the experimental tasks, the registration unit included various instruments and methods: the photoresistors FSA-G1 and avalanche photodiodes LFD-2 for registration of the lasing pulse shape, laser pulse energy meters (IMO-2N and J50-710 of the Molectron Company), methods based on the Ragulsky wedge to measure radiation divergence, a lateral-shift interferometer with a Pulnix camera by the Spiricon Company to study the optical non-uniformities of the laser media, etc.

The first series of experiments from 1994 to 1995 demonstrated the possibility of NPL operation in stationary mode with a lasing duration of up to 1.5 s [2—5]. The choice of composition and pressure of the mixtures (He, Ne)-Ar-Xe (A = 1.73, 2.03 ^m) was made on the basis of experimental data obtained using the LUNA — 2M setup (see Chap. 3, Sect. 3.1). Oscillograms of one of the experiments are shown in Fig. 6.5. Gas flowing was started roughly 0.5 s before the start of the neutron pulse. In the absence of gas flowing through the gas channels, there was no

lasing. The lasing threshold for different mixtures was achieved with thermal neutron flux densities of (1-5) x 1014 cm-2 s-1. When the lasing threshold was exceeded by a factor of 2-3, the lasing and pumping (neutron pulse) pulse shapes were roughly the same. The maximal lasing power was 15-20 W for each channel.

Before the second series of experiments (2001), the systems for module control, registration of radiation, and calibration of the optical scheme were improved. Experiments were aimed at studying the possibility of serial addition of laser channels (see Chap. 10, Sect. 10.5) and development of methods based on the lateral-shift interferometer to determine the optical non-uniformities of the laser medium [5, 6, 11, 12]. As a result of the experiments, the possibility of a serial scheme for adding laser channels was demonstrated. For example, for an Ar-Xe (100:1) mixture at a pressure of 0.5 atm, the output power at the 1.73 pm line when two channels were added proved to be 2.4 times more than for a single channel. The lasing duration at this time increased from 0.65 to 1.2 s.

Interferometric investigations of optical non-uniformities showed the correct­ness of conception about gas-dynamic and thermophysical processes in NPLs with transverse gas flowing (see Chap. 9, Sect. 9.3). Thus it follows from the interfero — grams obtained that an optical wedge forms in the longitudinal direction with respect to the gas flow, while in the transverse direction there is a positive distrib­uted lens. The data of interferometric measurements can also be used to determine the energy deposition to the gas medium [13]. A comparison of the specific power depositions determined by the interferometric method and as a result of calculations using the measured number of fissions in uranium layers showed their approximate agreement, with a precision of 10-20 %.

Possible causes of insufficiently high energy parameters obtained using the LM-4 module include a number of design defects, in particular the presence of windows arranged at the Brewster angle in the laser channels, as well as the

Fig. 6.6 Transverse section of LM-8 laser module: (1) cover of housing; (2) gas pipeline; (3) body of module’s work zone; (4) shielding made of borated polypropylene; (5) radiator; (6) laser channel; (7) supplying gas pipeline; (8) graphite neutron reflector; (9) neutron moderator; (10) assemblage with four laser channels; (11) deferent gas pipeline; (12) force-pump housing; (13) force-pump blade

placement of the Plexiglas fast neutron moderator inside the sealed space to be evacuated. The latter circumstance can lead to a reduction in the purity of the gas mixture owing to its contamination by impurities adsorbed by the moderator. To eliminate these defects, the LM-8 eight-channel laser module [5, 14] was devel­oped, in which the polyethylene moderator and graphite reflector are arranged outside the active laser volume. A diagram of the transverse section of the LM-8 module is shown in Fig. 6.6.

An optical scheme of the first series of experiments with the LM-8 laser module is shown in Fig. 6.7. The module allows some experiments with serial addition of two and four laser channels to be carried out. Channels 1,8, and 3-6 are combined by optical cavities using reflecting mirrors. The numbering of the channels follows the direction of gas mixture flowing. The individual channels 2 and 7 are “control” channels.

In the first experiments [14] with the LM-8 module on the BIGR reactor, it was possible to serially add the radiation from four laser channels, and to obtain a relatively high efficiency of laser conversion (n ~ 0.7 %) at low neutron fluences of ~1013 cm~2, which corresponds to a power deposition of just 0.4-0.6 W/cm3.

All of the examined NPL designs and LM-4 and LM-8 modules are intended basically for operation close to the reactor core. The development of a laser module that is an elementary cell of the reactor core is the next phase in the path of development of the reactor laser. One such version, examined in study [15], was the 16-channel LM-16 module, surrounded by a graphite neutron moderator. Laser channels used in the LM-4 and LM-8 devices formed the basis of the LM-16 module. When such a module is used in the reactor core of a continuous stationary reactor, it is possible to resolve problems associated with the compatibility of the laser cell with the reactor core, and certain “laser” problems, for example addition of radiation of the laser channels and its withdrawal from the reactor core [15].