2. Target mass production

modern requirements are asking for development of structure-sensitive methods aimed at new layering techniques meeting the IFE needs. This is due to the fact that the progress in plasma implosion up to intensive fusion reactions lies in formation of a given fuel structure that must be isotropic for reaching fusion conditions. At the LPI, the concept of a mechanical mockup of IFE reactor has been proposed [3] to develop reactor-scale technologies applicable to mass production of IFE targets at significant rates (Figure 1). The LPI program also includes extensive development work on creation of different designs of the hybrid accelerators for

Nuclear Fusion - One Noble Goal and a Variety of Scientific and Technological Challenges

The MM-IFE is a modular facility representing in essence a free-standing target (FST) transmission line (integral part of any fusion reactor) designed to produce IFE targets and to provide their noncontact delivery at the laser focus and synchronous irradiation by a laser (1–10 Hz). It consists of 3 main blocks: (1) cryogenic target factory (CTF) operating with isotropic fuel layers of 200–300 μm thick (Figure 1a) [1]; (2) cryogenic IFE-target injector operating at accelerations <500 g and injection velocities Vinj ≥ 200 m/s [4–8]; and (3) tracking systems for online characterization and control of the injected targets [9–12]. Replacement of the FSTlayering module, being the main part of CTF, on the extruder of the solid fuel pellets allows developing the technologies for continuous fuel supply into magnetic

Basic elements of the MM-IFE have been tested at LPI as prototypes for risk minimization at the stages of MM-IFE construction and startup. We especially highlight that moving targets are the necessary condition for realizing the repeatable target production at required rates, their mass manufacturing and noncontact

In this chapter, we discuss some challenging scientific and technological issues associated with IFE targets, the current status of the R&D program on MM-IFE, and further trends in developing the advanced IFE technologies for high-repetition rate

Repeatable and mass production of the fuel targets/pellets for IFE (a) and for MFE (b) reactors. In (a): 1—fuel-filled polymer shell, 2—FST-layering module, 3—cryogenic targets batch, 4—shuttle, 5—pusher, 6—target carrier (sabot), 7—drum of a revolver type, 8—extruder for protective cover production, 9—coin for the protective cover formation and delivery, 10—field coil for "sabot + target + protective cover" pull out and delivery to the start point of injector. In (b): 1—extruder of solid fuel pellets, 2—pellet carrier (sabot), made from superconductor or ferromagnetic, 3—module for sabot-repeatable delivery to the rotating drum, 4—rotating drum for rep-rate assembly of the units "sabot + pellet," 5—heater for pellets production,

6—pusher, 7—linear electromagnetic accelerator (injector), 8—sabot brake.

IFE target transport with levitation (noncontact acceleration systems).

fusion energy (MFE) facilities (Figure 1b).

delivery.

Figure 1.

54

laser facilities.

The fuel structure is very important for the progress toward ignition. Considering the issue of high-quality cryogenic layer fabrication, we have to rely, first of all, on structural properties of hydrogen isotopes and their mixtures. Survivability of the fuel layers subjected to the environmental effects during target delivery may depend on the layer structure as well.

### 2.1 Structure-sensitive methods

Many important properties of materials are structure-sensitive, and often a relatively small number of defects have a disproportionately large effect on the material properties. Material structuring is very promising for creation of fusion materials with required properties. The role of structure-sensitive methods when developing new functional materials is especially underlined in [13]: "All materials have different chemical composition, aggregate states (solid body, liquid, and gas), allotropic modifications (graphite-diamond), or can be a mix of several substances (clay) that can be written as definition (1):

```
MATERIAL ¼
½Chemical composition Að Þþ Aggregate state Að Þþ Allotropic modification Að Þ�þ
½Chemical composition Bð Þþ Aggregate state Bð Þþ Allotropic modification Bð Þ� þ …"
                                                                               (1)
```
However, this is obviously an inexact characteristic. The natural or introduced imperfection in the material is more important. Defects or, more generally, microstructure, define many major structure-sensitive properties of the materials. Critical for material property parameters are the type of available defects, their spatially organized packing, and interaction at multiple hierarchy levels. Chemical composition, aggregate state, and allotropic modification as a cooperative characteristic can be replaced with a more common concept "Phase." Then, the previous definition (1) can be transformed in (2):

$$\text{Material} = \text{Phase}(\text{s}) + \text{Microstructure} \tag{2}$$

The defect structure plays an important role in determining many material properties. From technological applications, a particular interest is the mechanical and thermal characteristics. In the IFE, a practical tool for correlating the structure and properties of the hydrogen fuel is the thermal target environment; cooling rates, fuel doping, periodic mechanical influence on the target under the cryogenic layer freezing, etc. [1]. For example, depending on the cooling rates, the solid fuel layer can be in the state with a different microstructural length or grain size: isotropic ultrafine layers and anisotropic molecular crystals (real single crystals, coarse-grained crystalline). This becomes particularly important if one takes into account the properties of the hydrogen isotopes.

In the equilibrium state, the solid hydrogen isotopes consist of anisotropic molecular crystals. In our analysis, we are guided by the fact that the angular dependence of the sound velocity, V, is known for a number of substances crystallizing in the hexagonal close-packed (hcp) phase. As found in [14], the sound velocity anisotropy is inherent to hcp-H2 and hcp-D2 as well, and makes nearly 20% (longitudinal sound) and 33% (transverse sound). In accordance with the Debye theory, the lattice thermal conductivity is directly proportional to the value of V.

Then, even in the case of a uniform thermal environment on the target surface of the anisotropic single-crystal layers, there is a difference in the radial target temperature in time. Therefore, under uniform target heating or cooling, the normal temperature gradients onto the inner surface of such layers become different. This initiates the spherically asymmetrical sublimation (or condensation) of fuel in the target cavity and results in the layer degradation with respect to roughness and thickness.

and transport through the reaction chamber. During the FST layering, two processes are mostly responsible for maintaining a uniform layer formation:

Mechanical Mockup of IFE Reactor Intended for the Development of Cryogenic Target Mass…

• first, during the target rolling along the spiral layering channel, the forced

• second, the heat transport outside the target via conduction through a small contact area between the shell wall and the wall of the layering channel (metal

hollow tube helium cooled outside) results in a liquid layer freezing.

Figures 3 and 4 show the operational scenario of the FST layering module

• it works with a target batch at one time at cooling rates of q = 1–50 K/s;

• FST layering does not require the target surface to be near to isothermal;

• transport process is the target injection between the basic units: shell container

• SC holds the fuel-filled shells until the beginning of the FST layering process;

FST-LM: (a) overview, (b) single spiral LC in assembly, (c) schematic of the FST-LM maintaining repeatable operation: 1—gravitation loading of the target with liquid fuel from SC to LC, 2—LC, 3—vertical target

• targets remain unmounted (or free-standing) in each production step;

(SC), layering channel (LC), test chamber (TC);

target rotation results in a liquid layer symmetrization;

DOI: http://dx.doi.org/10.5772/intechopen.81518

(FST-LM):

Figure 3.

57

collector, 4—horizontal target collector.

A conventional approach to solid layering (known as beta-layering method [2]) involves crystallization from a single-seed crystal in the fixed target under extremely slow cooling (q <sup>3</sup> <sup>10</sup><sup>5</sup> K/s) and precise cryogenic temperature control (<100 μK) for obtaining the layers like a single crystal. In a uniform thermal environment, the beta-layering method can form a spherical fuel layer, but it is not efficient in preventing the local defects. The target lifetime (layer roughness is less than 1-μm rms) is of a few seconds after reaching the desired temperature [2]. This is implication of the fact that D-T layers formed by beta-layering are obtained as a result of almost equilibrium process of the crystal growth, and all the features of the equilibrium crystalline state will be inherent in such layers, including the temperature-dependent behavior of the local defects on the inner D-T surface. Besides, the total layering time is 24 h or even more [2]. Thus, the beta-layering method is not efficient for mass target fabrication for IFE. It is of one-of-a-kind technique, and this is very expensive [2].

In [1], it is shown that the fuel structure dominates among the important remaining risk factors because the progress in plasma implosion up to intensive fusion reactions lies in formation of the fuel structures which must be isotropic for reaching the fusion conditions. Figure 2 illustrates the cooling rates required to obtain isotropic fuel layers to withstand the thermal and mechanical environment during target fabrication and delivery.

#### 2.2 FST layering method for high-repetition rate facilities

The LPI program on the target science and technology has recently been focused on the ability to inexpensively fabricate large quantities of targets by developing a specialized layering module of repeatable operation. The targets must be freestanding, or unmounted. At the LPI, the experience gained in the technology development based on rapid fuel layering inside moving free-standing targets (which refers to as FST layering method) can be used for creation of a nextgeneration FST layering module for high-repetition rate facilities. For typical shell sizes (1–4 mm), the FST-layering time is very small (<30 s, see Section 2.2) in comparison with the beta-layering method usually applied in the compression experiments (≥24 h [2]).

FST layering is a structure-sensitive method to govern the fuel layer microstructure. Such approach has been developed at LPI [1] to form an isotropic ultrafine solid fuel (submicron crystalline called "fine-grained" crystalline and nanocrystalline) which supports the fuel layer survivability under target injection

Figure 2.

The fuel structure of the cryogenic layers depends on the cooling rates, and so on the layering method.

Mechanical Mockup of IFE Reactor Intended for the Development of Cryogenic Target Mass… DOI: http://dx.doi.org/10.5772/intechopen.81518

and transport through the reaction chamber. During the FST layering, two processes are mostly responsible for maintaining a uniform layer formation:


Figures 3 and 4 show the operational scenario of the FST layering module (FST-LM):


#### Figure 3.

FST-LM: (a) overview, (b) single spiral LC in assembly, (c) schematic of the FST-LM maintaining repeatable operation: 1—gravitation loading of the target with liquid fuel from SC to LC, 2—LC, 3—vertical target collector, 4—horizontal target collector.

Then, even in the case of a uniform thermal environment on the target surface of the anisotropic single-crystal layers, there is a difference in the radial target temperature in time. Therefore, under uniform target heating or cooling, the normal temperature gradients onto the inner surface of such layers become different. This initiates the spherically asymmetrical sublimation (or condensation) of fuel in the target cavity and results in the layer degradation with respect to roughness and

Nuclear Fusion - One Noble Goal and a Variety of Scientific and Technological Challenges

A conventional approach to solid layering (known as beta-layering method [2])

involves crystallization from a single-seed crystal in the fixed target under extremely slow cooling (q <sup>3</sup> <sup>10</sup><sup>5</sup> K/s) and precise cryogenic temperature control (<100 μK) for obtaining the layers like a single crystal. In a uniform thermal environment, the beta-layering method can form a spherical fuel layer, but it is not efficient in preventing the local defects. The target lifetime (layer roughness is less than 1-μm rms) is of a few seconds after reaching the desired temperature [2]. This is implication of the fact that D-T layers formed by beta-layering are obtained as a result of almost equilibrium process of the crystal growth, and all the features of the equilibrium crystalline state will be inherent in such layers, including the temperature-dependent behavior of the local defects on the inner D-T surface. Besides, the total layering time is 24 h or even more [2]. Thus, the beta-layering method is not efficient for mass target fabrication for IFE. It is of one-of-a-kind

In [1], it is shown that the fuel structure dominates among the important remaining risk factors because the progress in plasma implosion up to intensive fusion reactions lies in formation of the fuel structures which must be isotropic for reaching the fusion conditions. Figure 2 illustrates the cooling rates required to obtain isotropic fuel layers to withstand the thermal and mechanical environment

The LPI program on the target science and technology has recently been focused on the ability to inexpensively fabricate large quantities of targets by developing a specialized layering module of repeatable operation. The targets must be freestanding, or unmounted. At the LPI, the experience gained in the technology development based on rapid fuel layering inside moving free-standing targets (which refers to as FST layering method) can be used for creation of a nextgeneration FST layering module for high-repetition rate facilities. For typical shell sizes (1–4 mm), the FST-layering time is very small (<30 s, see Section 2.2) in comparison with the beta-layering method usually applied in the compression

FST layering is a structure-sensitive method to govern the fuel layer microstructure. Such approach has been developed at LPI [1] to form an isotropic ultrafine solid fuel (submicron crystalline called "fine-grained" crystalline and nanocrystalline) which supports the fuel layer survivability under target injection

The fuel structure of the cryogenic layers depends on the cooling rates, and so on the layering method.

technique, and this is very expensive [2].

during target fabrication and delivery.

experiments (≥24 h [2]).

Figure 2.

56

2.2 FST layering method for high-repetition rate facilities

thickness.

A promising way for online measurement of the actual position and quality of the flying target in the reaction chamber is automatic target tracking algorithm based

Mechanical Mockup of IFE Reactor Intended for the Development of Cryogenic Target Mass…

1. A fundamental difference of the FST layering from the generally accepted approaches is that it works with free-standing and line-moving targets that allow starting developments on the FST transmission line of repeatable

2. High cooling rates combined with high-melting additives to fuel content (Figure 4a) result in creation of a stable ultimate-disordered structure with a

3. Additives work as stabilizing agents keeping the grain size stable and, as a consequence, keeping the thermal and mechanical stability of the ultrafine

4.For D-Т mixture (having the molecular composition: 25% of D2, 50% of DT molecules, and 25% of T2), just T2 is considered as a high-melting additive with

5. Isotropic ultrafine layers have an adequate thermal and mechanical stability for the fuel layer survivability under target injection and transport through the

6.An important parameter is the target lifetime within a temperature interval ΔТ, in which a stable ultrafine fuel structure can exist. Our study shows that the fuel doping in the range of η = 0.5–20% (neon, argon, tritium) makes this interval greatest possible, from 4.2 K right up to the temperature of fuel

7. Vibrations during FST layering are additional and effective means to meet the demands on the fuel layer formation with inherent survival features. Periodic mechanical impact on the fuel is one more option to a fuel structurization. Therefore, we plan experiments using a classical FST-LM combined with a special vibrator for launching the high-frequency waves in the top part of the LC which in turn will work as a waveguide, maintaining a vibration loading on

In [15], we proposed a model for rapid fuel layering inside moving, freestanding targets. It is based on solving the Stephen's problem for moving boundaries between the fuel phases (gas, liquid, and solid) and for nonlinear boundary condition onto the outer shell surface. The heat transport through the target is conduction through a small contact area. The computational tools allow one to model the layering time as a function of the target and LC parameters and other experimental

the moving targets during their layering [1, 22].

2.3 FST layering time for direct-drive high-gain targets

high defect density or isotropic medium (ultrafine fuel layers).

the TCs (vertical and horizontal) with a rate of 0.1 Hz at T = 5 K is shown in

Peculiarities of the FST layering process consist in the following:

The targets' gravitational loading (one-by-one target injection) from the LM to

on the Fourier holography [11, 12].

DOI: http://dx.doi.org/10.5772/intechopen.81518

Figure 4.

operation.

cryogenic layers.

melting [1, 19].

59

respect to D2 and DT (Figure 4).

reaction chamber [1, 15–19].

Figure 4.

The FST-LM operation with a rate of 0.1 Hz at T = 5.0 K. In (a) FST-layering results: 1—40-μm-thick layer (D2 + 20% Ne) in the CH shell of 1.23-mm diameter (20% Ne-additives are used for modeling T2 in the D-Т fuel), 2—44-μm-thick H2 layer with (H2 + 5% HD) in the CH shell of 1.2-mm diameter; in (b) horizontal TC: 1—1 target (0 s), 2—10 targets (100 s); in (c) vertical TC: 12 targets in 100 s.


The goal of the target characterization program is to provide reliable data in an available time. In this respect, two technologies are important: (1) fuel layering technique development (detailed information about the spatial modes, which break the target symmetry) and (2) reaction chamber fueling of a commercial power plant (targets must be injected at significant rates (10 Hz) which indicates evidence of a threshold behavior of the characterization process).

The first technology requires enhancing the obtained data, and the second needs shortening the characterization time. This indicates that the process details for the characterization technology dealing with the operational times and information content are of critical importance.

A hard development work is needed to take into account the specifics of both technologies for developing a reliable characterization system to control the IFE target parameters. In the FST-LM, the reconstruction algorithms for tomographic data processing of the target layering stage are used in a hundred-projection microtomograph with a spatial resolution of 1 μm [9, 10]. Moving target tracking is a challenge task and it is becoming increasingly important for IFE applications.

Mechanical Mockup of IFE Reactor Intended for the Development of Cryogenic Target Mass… DOI: http://dx.doi.org/10.5772/intechopen.81518

A promising way for online measurement of the actual position and quality of the flying target in the reaction chamber is automatic target tracking algorithm based on the Fourier holography [11, 12].

The targets' gravitational loading (one-by-one target injection) from the LM to the TCs (vertical and horizontal) with a rate of 0.1 Hz at T = 5 K is shown in Figure 4.

Peculiarities of the FST layering process consist in the following:

