4. Target protection system

the pinning force density Fpin can be found for the superconducting coils proposed for the sabot acceleration. For MgB2-cables of 1.18 mm in diameter, the critical current vs. the magnetic field at temperatures of 4.2, 9.8, 15, 20, and 25 K was measured in [30]. Using these data, we have made the calculations under the actual

External field (T) [30] 1 0.5 0.25 Critical current (A) [30] 2500 4000 5000 Sabot acceleration (g) 800 640 400 Acceleration length (m) 2.5 3.125 5.0

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

(see Section 2.2); (b) the HTSC-sabot is "open parallelepiped" to exclude a bend of the Gd123 tapes (see Section 3.2); and (c) the mass of the assembly "HTSC-sabot-

Thus, using the MgB2-driving body allows not only to accelerate the reactorscaled targets to the required injection velocities, but also to provide the system performance without exceeding the acceleration limits at 500 g. As one can see from the Table 3, the MgB2 coils (with parameters 2πRSC = 24 mm, B0 = 0.25 T, JC = 5000

Note that several important aspects related to a practical engineering are

• In our study, we have proposed the PMG configuration allowing in-space equilibrium position of the HTSC-sabot during its acceleration (it goes along a

whole magnetic track with the same levitation height and orientation).

• Taking into account that experimentally the HTSC-sabots keep their speed after acceleration pulse, they can be extra accelerated by using a multiple-stage

• Superconducting cables can be used not only in the driving body but also in the field coils. If these coils carry a current, which is less than the critical current,

• In our model, the MgB2-driving body represents a magnetic dipole (MD). The MD acceleration is carried out by a traveling magnetic wave or ARP at the consecutive switch of the field coils. From the view point of a relative positioning of the ARP and MD, the steady case is realized when the ARP pushes the MD but does not pull it for itself, that is, the area of a phase (longitudinal) stability is on a forward slope of the ARP [31]. In the

accelerating equipment, it is referred to as a principle of automatic phasing. This principle will be inherent for the MgB2-driving body, because, as a superconductor, it will be pushed out from the area of a stronger magnetic

Especially, note that the injection velocities Vinj ≥ 200 m/s are not a problem for the proposed noncontact schedule of the target delivery. It can successfully be used

in creation of a hybrid accelerator for future IFE power plants.

then large magnetic fields can be generated without heat generation.

operating conditions: (a) the target design is CHGT, and its mass is 5 mg

+ CHGT" is 0.5 g. The calculation results are presented in Table 3.

MgB2-driving body acceleration efficiency at TS = 20 K.

A) yield Vinj = 200 m/s at 400 g on the acceleration length of 5 m.

as follows:

Table 3.

accelerator.

field.

72

Target delivery into IFE power plant requires target acceleration (accompanied with mechanical and thermal loads) and repeatable injection into the reaction chamber (additional thermal loads). For this reason, the problem of using the cryogenic targets in the IFE experiments or in a future reactor includes not only an issue of fabricating the qualitative cryogenic fuel layer (nonuniformity <1%, roughness <1 μ), but also an issue of target delivery at the laser focus under conditions of the layer parameter survival. In our study, a number of protection techniques have been proposed and examined with the aim of risk minimization in the process of target acceleration and injection.

A promising direction for survivability of fuel layers is application of external target protective coatings, which reduce the risks of the fuel damage under the radiation exposure from the hot walls of the reaction chamber: cryogenic coatings (from the solid D2, H2, or Xe), metal coatings from Au, Pt, Pd, and their alloys, application of a double protection: "metal + cryogenic" (Figure 16a). Below, we demonstrate the practical possibilities of this direction.

To obtain the results in Figure 16b, an additional procedure is added in the FST formation cycle. First, the metal coating made from Pt/Pd is deposited on the CH-shell. Then, the shell is filled with the D2 gaseous fuel with 3% Ne as doping agents. The next step is D2 layer fabrication by the FST layering method.

The sabot is also a special element of the target protection system [12]. An important feature of its design is the shape of a target nest. Our study shows that a proper choice of the nest shape makes it possible to significantly increase the upper limit of the permissible overloads and to minimize the injector size. Being based on the discrete-continuous physical model of the shell stress, a simulation code SPHERA is developed that makes it possible to define the stress and deformation arising in the target during the acceleration. A shape analysis of the sabot bottom (in the target nest area) during the target acceleration is carried out for three sufficiently different cases: (1) flat bottom, (2) semispherical bottom with Rn > Rt (Rn and Rt are the nest and the target radii, respectively), and (3) conical bottom (Figure 7). Important conclusions followed from these calculations are listed below:


#### Figure 16.

Different protective coatings. In (a): double protection "Pd-coating of 150 Å thick and cryogenic O2-coating": 1 —1.2-mm CH-shell at 14.6 K before the experiment (liquid H2 inside as temperature indicator), 2—in the top part of the shell (from the outside), there is a solid deposit of oxygen (Ttp = 54.3 K), 3—after operation of the piezoelectric vibrator [22], the oxygen snow becomes redistributed onto the outer shell surface; in (b): single protection "Pt/Pd-coating of 200 Å thick": 1—1.5-mm CH-shell before the experiment, 2—cryogenic target at 5.0 K with a uniform D2 layer of 50 μm fabricated by the FST-layering method.

The next step is a shield (or cover) for application to protect injected target from a head wind of a residual gas. It has been considered since 1982 [32]. In [16], we have proposed a new design of a protective cover made from solid xenon or deuterium. At the current research stage, we have analyzed the cover and the target interaction with the reactor chamber environment using the direct simulation Monte Carlo (DSMC) approach as well as using the results of numerical studies of gas flow interaction with bodies. The following relations were considered:

• motion equation

$$L = \text{Ut} - F\_{\text{D}}t^2/2\text{M}\_{\text{b}}.\tag{5}$$

Basing on these results as well as on the results presented in Sections 2 and 3, we propose a multiple target protection system for the effective delivery of a cryogenic

Protective cover: (a) protective cover forms a wake area in the fill gas to protect a target from the head wind and to avoid the convective heating; (b) target thermal history in IFE chamber under exposure of the wall radiation and gas convection: 4-mm targets with D2 fuel (200 μm), reactor wall temperature is 1773 K, residual gas Хе 0.5 Тorr (1—target without reflecting layer, 2—target with reflecting layer, 3—injection at 17 K, 4—

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

1. Cryogenic layer formation with an isotropic ultrafine fuel structure (which can be referred to as layers with inherent survival features) to reduce the target

2. Use of friction-free acceleration of the "HTSC-sabot + target" assembly to reduce the heat flux on the target under development of a noncontact delivery

3. Use of conical supports for a target nest in the sabot to reduce the mechanical

4.Use of outer coatings (cryogenic, metal) in the target design to reduce risks of cryogenic layer damage as a result of target heating by thermal radiation of the

5. Coinjection of a target and a protective cover from frozen gases (D2, Xe) to reduce risks of cryogenic layer damage as a result of target heating by hot

The important remaining factors of the research include issues of complex

The purpose of this work was to study a repeatable target production and methods of their noncontact delivery in accordance with the scope of MM-IFE program. Various physics and technology problems accompanying IFE targetfueling development were considered, and approaches to their solution were proposed and experimentally tested partially. Our thermal, mechanical, and levitation modeling (theoretical and experimental) are important tools in planning future

loads during acceleration of the "HTSC-sabot + target" assembly.

sensitivity to the external thermal and mechanical loads.

system with linear or circular accelerators.

residual gases in the reaction chamber.

technology optimization and system integration.

experiments on MM-IFE and studying IFE reactor fueling.

target without its damage.

injection at 5 K, 5—18.5 K, point of destination).

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

Figure 17.

hot chamber walls.

5. Conclusion

75

• velocity in a laminar circular wake behind the cover [33]:

$$u(\varkappa, \mathfrak{y}) = U[(\pi \mathcal{C}\_{\mathcal{D}})/32] \cdot [(2\mathcal{R}/\varkappa)\mathcal{R}\mathcal{F}\_{\mathcal{D}}] \exp\left(-\eta^2\right),\tag{6}$$

• drag force equation for a sphere [34]:

$$\mathbf{F}\_D = -(\mathbf{3}/4)a^2 n u \left( m\_\mathbf{g} k\_B T / 2\pi \right)^{1/2} \mathbf{F}\_\mathbf{1} \mathbf{k}.\tag{7}$$

Here F1 = F1o + F1p, F1o = 8π(8 + π)/9, F1p = �21.28/ξ <sup>2</sup> (for two equal spheres), k is the unit vector in a target motion direction, L is the distance between the body (target or cover) and the burn area, R is the characteristic dimension (target or cover), t is the in-flight time, FD is the drag force, Mb is the mass (target or cover), U is the injection velocity, u(x, y) is current velocity, η and ν are the dynamic and kinematic viscosities of a residual gas, M is the Mach number, mg is the mass of residual gas, a is the sound speed in gas, kB is the Boltzmann constant, F1o and F1p are the dimensionless drag coefficients, ξ is the dimensionless distance measured in target radii, and CD is the coefficient of gas molecule accommodation by the target surface. The following parameters are used in our estimations: velocity is 250 m/s, residual gas is Xe at 0.5 Torr, reactor chamber radius is 5 m, cylindrical cover from solid Xe with a mass of 87 mg, and target mass is 5 mg (CHGT).

In the drag force estimations, two cases were considered: solitary and joint flight of target and cover. Correction for the solitary case (effect of wake) is about 30%. The estimations show that due to the drag force action, the distance between the target and the cover rises from the initial 1 mm at the moment of injection up to 15 mm at the center of reaction chamber. Thus, the drag force provides necessary separation of the cover and target inside the reaction chamber. The protective cover forms a wake region (Figure 17a) with reduced flow velocity and temperature and effectively reduces the gas heat flow by a factor of 4÷5, which is in a good agreement with calculations in [35]. Thus, the concept of protecting the direct drive target in the reactor chamber by a cover moving ahead can be considered as a possible way of solving the target delivery problem.

Note that the problem of target survival is the more difficult the higher the target temperature at the moment of injection. Estimations show that radiation heat flow from the chamber wall is an order of magnitude higher than the gas heat transfer (Figure 17b). Therefore, target injection at 5 K is more preferable than at 17 K. This can be realized only with an ultrafine fuel structure [1] because the single crystalline fuel reveals undesirable roughness on target cooling [2].

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

Figure 17.

The next step is a shield (or cover) for application to protect injected target from a head wind of a residual gas. It has been considered since 1982 [32]. In [16], we have proposed a new design of a protective cover made from solid xenon or deuterium. At the current research stage, we have analyzed the cover and the target interaction with the reactor chamber environment using the direct simulation Monte Carlo (DSMC) approach as well as using the results of numerical studies of

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

2

u xð Þ¼ ; <sup>y</sup> <sup>U</sup>½ �� ð Þ <sup>π</sup>CD <sup>=</sup><sup>32</sup> ½ � ð Þ <sup>2</sup>R=<sup>x</sup> RFD exp �η<sup>2</sup> , (6)

nu mgkBT=2π <sup>1</sup>=<sup>2</sup>

k is the unit vector in a target motion direction, L is the distance between the body (target or cover) and the burn area, R is the characteristic dimension (target or cover), t is the in-flight time, FD is the drag force, Mb is the mass (target or cover), U is the injection velocity, u(x, y) is current velocity, η and ν are the dynamic and kinematic viscosities of a residual gas, M is the Mach number, mg is the mass of residual gas, a is the sound speed in gas, kB is the Boltzmann constant, F1o and F1p are the dimensionless drag coefficients, ξ is the dimensionless distance measured in target radii, and CD is the coefficient of gas molecule accommodation by the target surface. The following parameters are used in our estimations: velocity is 250 m/s, residual gas is Xe at 0.5 Torr, reactor chamber radius is 5 m, cylindrical cover from

In the drag force estimations, two cases were considered: solitary and joint flight of target and cover. Correction for the solitary case (effect of wake) is about 30%. The estimations show that due to the drag force action, the distance between the target and the cover rises from the initial 1 mm at the moment of injection up to 15 mm at the center of reaction chamber. Thus, the drag force provides necessary separation of the cover and target inside the reaction chamber. The protective cover forms a wake region (Figure 17a) with reduced flow velocity and temperature and effectively reduces the gas heat flow by a factor of 4÷5, which is in a good agreement with calculations in [35]. Thus, the concept of protecting the direct drive target in the reactor chamber by a cover moving ahead can be considered as a

Note that the problem of target survival is the more difficult the higher the target temperature at the moment of injection. Estimations show that radiation heat flow from the chamber wall is an order of magnitude higher than the gas heat transfer (Figure 17b). Therefore, target injection at 5 K is more preferable than at 17 K. This can be realized only with an ultrafine fuel structure [1] because the single

crystalline fuel reveals undesirable roughness on target cooling [2].

=2Mb, (5)

F1k: (7)

<sup>2</sup> (for two equal spheres),

gas flow interaction with bodies. The following relations were considered:

L ¼ Ut � FDt

• velocity in a laminar circular wake behind the cover [33]:

<sup>F</sup><sup>D</sup> ¼ �ð Þ <sup>3</sup>=<sup>4</sup> <sup>a</sup><sup>2</sup>

Here F1 = F1o + F1p, F1o = 8π(8 + π)/9, F1p = �21.28/ξ

solid Xe with a mass of 87 mg, and target mass is 5 mg (CHGT).

possible way of solving the target delivery problem.

74

• drag force equation for a sphere [34]:

• motion equation

Protective cover: (a) protective cover forms a wake area in the fill gas to protect a target from the head wind and to avoid the convective heating; (b) target thermal history in IFE chamber under exposure of the wall radiation and gas convection: 4-mm targets with D2 fuel (200 μm), reactor wall temperature is 1773 K, residual gas Хе 0.5 Тorr (1—target without reflecting layer, 2—target with reflecting layer, 3—injection at 17 K, 4 injection at 5 K, 5—18.5 K, point of destination).

Basing on these results as well as on the results presented in Sections 2 and 3, we propose a multiple target protection system for the effective delivery of a cryogenic target without its damage.


The important remaining factors of the research include issues of complex technology optimization and system integration.
