2. FABRICATION METHODS FOR SPHERICAL SHELL TARGETS

Fusion targets for spherical plasma heating and compression are complex multilayer miniature constructions—hollow microspheres—done with extreme accuracy. So the wall-thickness deviations for the spheres of 2-mm diameter of 100-μm shell wall thickness must not exceed 0.1 μm, and the outside surface roughness must be less than 0.03 μm. The symmetry requirements are derived from compression stability conditions and are known to the factor of 2 because of laborous mathematical modeling and codes. Target and fuel masses are dependent on driver energy and pulse duration.

The thermonuclear target laboratory of the Lebedev Physical Institute started spherical target production almost 30 years ago. During the past years the unique theoretical, experimental, and fabrication opportunities were obtained. Targets, equipment for target production, and monitoring are provided. Targets, relevant equipment, and scientific reports have been sent to many scientific centers of our country and to nine countries abroad (United Kingdom, United States, France, Italy, Germany, China, India, Japan, and Poland).

Realizing the long-term program, a number of original methods for shell formation were invented. Thus the technique of rapid heating and cooling was used for target production during short-period weightlessness. For example, a ballistic furnace for making spherical shell targets for laser energies up to 2 MJ was developed and built. Besides, the new and nonexpensive installations for DT cryogenic fuel formation on the inner surface of the shell were created together with the system for cryogenic target delivery into the laser focus.

Experiments with megajoule drivers are expected to demonstrate the ignition. For that, the large targets of 1.5–4 mm should have optimized parameters such as mass, size, and thickness.

To manufacture the large shells of a fixed mass, we developed a new realization of drop tower method. Large drops (diameter > 0.3 mm) need a long drying time of more than 3 s, after which the shell is formed. This could hardly be achieved by drying in a free fall. The natural idea of freeze-drying the frozen collected initial particles emerged. The following three-step technique is proposed.

On the one hand, the developed installation is fitted for subsequent freeze-drying; on the other hand, it helps the evaporation process due to the hot dispersion of concentrated alkali silicates in water (see Fig. 2). In the first step, the helium atmosphere inside the furnace causes a five-fold drying rate increase. Partly dried particles are collected in the liquid nitrogen and are then subjected to freeze-drying at low temperatures to preserve particles' sticking (second step). The third step is to form large (1.2–1.8 mm) glass microballoons (Akunets et al., 1996), from identical initial particles obtained in the two previous steps.

Dispenser scheme. Drying zone operates with helium atmosphere and ends by collector with liquid nitrogen. 1: droplet generator; 2: gas control unit; 3: heating tube for drop drying; 4: cooling tube; 5: collection vessel; 6: support farm; 7: electronic control unit; 8: strobe light; 9: microscope; 10: balcony barrier.

2.1. We propose applying alternative fuels for large shell manufacturing

The light elements and hydrogen-containing materials especially are advantageous for higher plasma stability, hot plasma burning, and decreasing the influence of shell and fuel mixing (see Table 1). The main drawback of light elements' hydrides in comparison with beryllium is lower initial ion concentration preventing the highest compression degree (and thus DT maximum density). The initial better compressibility may counter this drawback.

Physical properties of alternative fuel as material for laser targets

It is desirable to use the additional burning of shell material in the large targets, so the shell should be made of alternative fuels such as BeD₂, LiBeD₃, ND₃BD₃, or their mixtures. About 50% of deuteride atoms should be substituted by tritium. The technique is being developed in the Bochvar Institute for Non-Organic Materials (Russian abbreviation: VNIINM). BeD₂ shells up to diameters of 0.4 mm have been obtained by now.

3. LOW-DENSITY FOAMS FOR FUSION TARGETS, THEIR PROPERTIES, AND FABRICATION METHODS

In the past decade much attention was paid to high-density energy interaction with low-density (structured) materials in order to uniformly distribute the heating energy. Mainly the polymers and silica aerogels were utilized (Borisenko & Merkul'ev, 1996). Only a few publications were devoted to laser interaction with low-density snowlike heavy metals; their production is discussed in Borisenko et al. (1994).

The future targets are believed to be cryogenic. It is well known that the uniform layer of solid DT could be only achieved by its quick cooling, which is impossible if the shell wall is thermally insulated. In addition there exists a thermal flux from tritium decay also resisting the rapid quenching of a fuel. The problem could be overcome by using metal foams instead of conventional ones, because cooling down to liquid helium temperatures increases rapidly the heat conductivity for metal foams unlike polymer ones (see Table 2).

Physical properties of low density materials

Now we discuss the following converter—a low-density beryllium with a high concentration of a heavy element inside.

It is necessary to remember that the only metal considered as the ablator for the outer layer of an ICF target or for the whole shell is beryllium. And there is a technology route developed at VNIINM on how to obtain foam beryllium (Gorlevsky et al., 1998) from beryllium hydride. Simultaneously the essential amount of the heavy element could be introduced into the structure of the foam (Dorogotovtsev et al., 2001).

The rapid viscosity decrease and rather small hydrogen diffusion coefficient at glass transition temperature result in “foaming” of beryllium hydride in the course of thermolysis. The microcell structure is then obtained, hydrogen being the foaming agent. Pyrolysis in the system of BeH₂-Be fixes the structure due to the growing viscosity. The pores formed are fully open.

The Be structure of the density 0.45 g/cm³ is presented in Figure 3. Cell walls are seen to have a thickness of ∼1 μm and to form round grains of ∼30–50 nm.

SEM picture of microcellular nanoberyllium structure. Density equals to 0.45 g/cm3.

For intense laser radiation conversion into X rays a simultaneous codeposition of beryllium deuteride and of copper on the quartz substrate was done (see Fig. 4). The necessary profiles are possible. The obtained BeD₂-Cu layers were also foamed.

Copper concentration (upper curve 1 unit: 10−2%) via depth of the layer (unit: 0.25 μm) of beryllium deuteride. The lower curve: beryllium registration.

3.1. About heavy-ion energy conversion into X rays

We stressed already that the converter is one of the important elements of the HIIF target. Absorbing heavy ions, the converter material must be heated to the temperatures of which soft X rays (0.1–0.3 keV) are emitted. But the stopping range for ions grows with heating. The optimum converter should stop and extract most of the energy from the ions in the same place on the boundary of converter and the target both in the end and in the beginning of the pulse.

To solve this dilemma, the microheterogeneous converter may be used with the high-Z clusters distributed inside the light part (Dorogotovtsev et al., 2001). The cluster size and concentration is to be adjusted so that diffusion of the heavy elements at heating provides compensation of the range growth because of the effective growing concentration of heavy element in the ion path (see Fig. 5).

Microheterogeneous material: glass seeded with Cu 0.1 μm (Borisenko & Merkul'ev, 1996; Borisenko et al., 1994). Scanning electron microscope image.

The energy losses in such materials might be less than in uniform ones, but the idea is still to be proved experimentally and theoretically. Microheterogeneity can also be realized inside the foam, but then hierarchy of the concurrent processes is still more complicated and not quite clear.