One of the main problems that arise when creating NPLs and RLs is the develop­ment of uranium layers that are stable relative to different influencing factors. These layers serve a dual function in RLs: they serve as reactor fuel and provide fission fragments to excite the laser medium.

Experience with NPLs using different types of uranium layers makes it possible to define their main performance requirements.

1. The uranium layer must be deposited on substrate made of a material that absorbs neutrons poorly, for example, aluminum or zirconium. The optimum thickness of the layer is about half the path length of the fission fragments in the layer. The density, thickness, and 235U concentration of the layer should be as uniform as possible.

2. The layer should be mechanically durable and thermally stable and have high adhesive bond to the substrate. Localized bond decrease between the layer and substrate disrupts heat removal and leads to the mechanical deterioration of the layer. This effect may arise with long-term storage of the layer and as a result of thermocyclic tension.

3. The “substrate-layer” boundary should be resistant to the thermal and radiation — stimulated diffusion of the layer material into the substrate.

4. To increase the operational lifetime of the NPL and RL, layers must be devel­oped with a minimum sputtering factor of the layer surface during output of the fission fragments to the gas. The sputtering factor (the average number of dispersed atoms from the surface per one emergent fragment) in a complex manner depends on the microstructure of the layer surface and changes within a wide range 1-104 atoms/fragment [20, 21].

The first experiments at VNIIEF on reactor VIR-2 (see Chap. 2, Sect. 2.3) used uranium oxide-protoxide layers manufactured using chemical deposition from solutions of uranium salts onto aluminum substrates. These layers have a loosely coupled structure and poor durability. This led to contamination of the laser channel, including the cavity mirror, with uranium dust.

To study NPLs, VNIIEF associates used uranium layers produced using the following technologies:

• Electric precipitation of uranium from a solution onto an aluminum substrate (see, for example, [22]), resulting in the formation of a layer that is a mixture of uranium oxide (UO2) and uranium protoxide (U3O8).

• Deposition of uranium from gas-phase compound onto a heated aluminum substrate (see, for example, [23]) with the formation of UO2.

• Magnetron ion-beam sputtering in a vacuum [24] resulting in the formation of metallic layers (a-uranium) or intermetallic compounds (UAl3 and UAl4). To protect the metallic uranium from corrosion and reduce the sputtering factor, the surface of the metallic uranium layer may be covered by a thin aluminum film with a thickness of about 0.5 pm.

The main characteristics of layers produced using the above two technologies are given in Table 10.4. Table 10.5 shows the comparative efficiency of different layers with 235U thicknesses of about 4.5 mg/cm2 (the efficiency of the layer was determined as the ratio of the power output by the fragments into the gas to the power released in the layer).

Each technology has its advantages. For example, magnetron sputtering yields metallic layers (a-uranium) with maximum efficiency. However, to prevent corro­sion of these layers, layer protection, which reduces efficiency, is necessary. Magnetron sputtering allows layers to be deposited in the intermetallic phase of uranium compounds (UA13 and UAl4), which have exceptionally high strength properties, but lower efficiency.

The adhesive strength of the uranium oxide layers is somewhat lower than that of the metallic layers. However, not one destruction has been noted in almost 20 years. Over this period, the VIR-2M/LUNA-2M experimental setup (see Chap. 2, Sect. 2.4) generated about 2,000 pulses with a total neutron fluence of ~1016/cm2. Moreover, no deterioration in NPL characteristics was observed. Tests [25] of layers made of metallic a-uranium showed that the adhesion of the layers does not diminish when the layer is kept for 10 years and after 150 heating cycles from 20 to 300 °C with a 5 min cycle time.

Oxide layers have a grain texture that is coarser than metallic layers. This is an advantage from the point of view of the suppression effect of sputtering. Metallic layers were studied by VNIIEF associates on the VIR-2M pulse reactor using mass spectrometry, electron microscopy and a — and y-spectrometry. These investigations established the following: sputtering of the layer mainly occurs by inelastic energy transfer from the fragment to the target atoms [20] and, for oxide layers, the sputtering factor is 103 atoms per fragment [25]. This factor may be reduced by about tenfold using special thermal processing [26]. Deposition of thin films (for example, 0.5 pm thick Al) to the surface of the uranium layer reduces the sputtering factor to several units.

Investigations of radiation-induced diffusion have shown [25] that this effect arises at a neutron fluence of about 2 x 1018/cm2. In this case, the uranium layer “descends” into the substrate to a depth of about 0.5 pm. This reduces its efficiency by 7 %.

Life tests of the metallic layers, which were performed on the RPT-6 reactor (NIIAR, Dimitrovgrad) up to a neutron fluence of ~1020/cm2, have shown that the uranium layers on the whole retain their initial texture to a 2 x 1019/cm2 fluence of thermal neutrons [27]; the operational life of metal layers with a protective alumi­num layer exceeds 0.1 % with respect to burn-up and, for intermetallic layers, 0.4 %. With this, the main characteristics of the layer remained highly stable. Potential methods for increasing the operational life of layers up to 1-2 % with respect to burn-up have been examined in study [28].

The problems of operational life and durability of uranium layers, discussed in this section do not belong among the principal physical problems determining the potential for creating RLs. However, they broadly impact the appearance of the RL and its possible applications.