3. Target repeatable delivery

During the target delivery, it is necessary to maintain the fuel layer quality in the process of target acceleration and injection. For this reason, the target must be placed into a special target carrier (sabot). Using sabots, there occur some contact problems. Because of a tight seal between the sabot and the barrel, any damage of the barrel and the sabot surface will affect the injector performance and sabot reusing.

directions that leads to its simultaneous acceleration and levitation (simultaneous presence of the driving force along the vector B1 and of the levitation force along

Schematic of "EM-AC + PMG". In (a): 1—target, 2—conical support of the target nest, 3—sabot matrix (polymer), 4—HTSC-coils (driving body based on superconducting MgB2-cables), 5—HTSC-plates for providing a stable levitation along the magnetic track, 6—magnetic track, 7—field coils, 8—protective cover; in (b) and (c): parameters and a side view of the experimental setup with directions of the driving force B1 and

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

• To ensure the successful acceleration, the field coil and HTSC-sabot are fixed over the magnetic track of the PMG-system so that the horizontal axis of the

generated by the field coil, that is, by using a running gradient of the magnetic

• As the HTSCs are diamagnetic, the HTSC-sabot is pushed out from the area of

• The starting parameters of the coil pulse: the pulse duration is 1 ms, the current

• The temperature in the experiments was T = 80 K for the following reason. Our previous studies have demonstrated a high efficiency of "HTSC-PMG" interaction in a wide temperature range ΔT=5–80 K [4]. Consequently, it is possible to study the HTSC-sabot acceleration at temperatures close to 80 K, that is, under the nitrogen cryogenics. This is especially important because it

• The HTSC-sabot motion has been driven by the electromagnetic pulse

amplitude is 200 A, the maximum magnetic induction is 0.35 T.

HTSC-sabot friction-free acceleration by the driving electromagnetic pulse generated by the field coil (T = 80 K). In (a): HTSC-sabot before acceleration; in (b) HTSC-sabot starts motion, in (c) and (d): in-time development of the acceleration process (the gap between the HTSC-sabot and the magnetic track is keeping

The experiments were conducted under the following conditions.

coil and the HTSC-sabot coincide (Figure 8b).

the levitation force B1 (9—ferromagnetics, 10—permanent magnets).

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

induction (accelerating running pulse, or ARP).

a stronger magnetic field.

the vector B2).

Figure 7.

Figure 8.

63

unvarying with time).

Recently, we have started the investigation into magnetic levitation as an alternative technology of noncontact manipulation, positioning and transport of the finished cryogenic targets. From the moment of discovery of bulk high-temperature superconductors (HTSC), which can stably levitate above the permanent magnets, the magnetic levitation (maglev) transport systems are of great interest for their potential application. The transport process with levitation results from the direct use of the diamagnetic characteristics of the HTSC materials. Their unique features can be exploited in the process of levitation and guidance of a HTSC-sabot as well.

In IFE research, this approach attracts a significant interest due to maglev potential for almost frictionless motion. The challenging scientific and technological issues associated with this task are being addressed through a combination of material selections and material property measurements, mathematical and experimental modeling, demonstration of the HTSC-sabot acceleration in laboratory-scale tests.

#### 3.1 Noncontact acceleration system

A noncontact acceleration system proposed at the LPI is a combination of three basic elements: (1) electromagnetic acceleration system (EM-AC), which includes the field coils generating the traveling magnetic waves, (2) levitation system (permanent magnet guideway or PMG), which includes a magnetic rail (or magnetic track), and (3) sabot including several HTSC components. Figure 7 illustrates the operational principle of the system. During acceleration, the target is protected with a levitating HTSC-sabot, and the diameter of the barrel exceeds the sabot diameter. This is a small-scale prototype under construction of a hybrid accelerator "EM-AC- + PMG" at the LPI. The concept of "EM-AC + PMG" is completed and the proof-ofprinciple (POP) experiments have confirmed the benefits of this approach.

The prove-of-principle (POP) experiments (Figure 8) on magnetic acceleration of the levitating HTSC-sabot are made in the mutually normal magnetic fields: the first is B1 (from the field coil to move the HTSC-sabot) directed along the acceleration length and the second is B2 (from the permanent magnets to counteract the gravity) directed normally to the acceleration length (Figure 7c). The Meissner effect [25] dictates that both magnetic fields generate the surface currents around the superconductor (in our case, it is the HTSC-sabot) in corresponding

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

#### Figure 7.

These measurements show that 4-mm targets can be manufactured by the FST layering method using n-fold-spiral LCs at n = 2, 3, because they maintain the gain in time of the target residence in the LC and in fuel layer symmetrization during target rolling. Note that currently only curved LCs in a specialized geometry and moving targets are successful for developing the FST-LM of repeatable operation,

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

Our latest effort underlies the future research on creation of the FST-LM as a

During the target delivery, it is necessary to maintain the fuel layer quality in the

Recently, we have started the investigation into magnetic levitation as an alternative technology of noncontact manipulation, positioning and transport of the finished cryogenic targets. From the moment of discovery of bulk high-temperature superconductors (HTSC), which can stably levitate above the permanent magnets, the magnetic levitation (maglev) transport systems are of great interest for their potential application. The transport process with levitation results from the direct use of the diamagnetic characteristics of the HTSC materials. Their unique features can be exploited in the process of levitation and guidance of a HTSC-sabot as well. In IFE research, this approach attracts a significant interest due to maglev potential for almost frictionless motion. The challenging scientific and technological issues associated with this task are being addressed through a combination of material selections and material property measurements, mathematical and experimental modeling, demonstration of the HTSC-sabot acceleration in laboratory-scale

A noncontact acceleration system proposed at the LPI is a combination of three basic elements: (1) electromagnetic acceleration system (EM-AC), which includes the field coils generating the traveling magnetic waves, (2) levitation system (permanent magnet guideway or PMG), which includes a magnetic rail (or magnetic track), and (3) sabot including several HTSC components. Figure 7 illustrates the operational principle of the system. During acceleration, the target is protected with a levitating HTSC-sabot, and the diameter of the barrel exceeds the sabot diameter. This is a small-scale prototype under construction of a hybrid accelerator "EM-AC- + PMG" at the LPI. The concept of "EM-AC + PMG" is completed and the proof-of-

The prove-of-principle (POP) experiments (Figure 8) on magnetic acceleration

principle (POP) experiments have confirmed the benefits of this approach.

of the levitating HTSC-sabot are made in the mutually normal magnetic fields: the first is B1 (from the field coil to move the HTSC-sabot) directed along the acceleration length and the second is B2 (from the permanent magnets to counteract the gravity) directed normally to the acceleration length (Figure 7c). The Meissner effect [25] dictates that both magnetic fields generate the surface currents around the superconductor (in our case, it is the HTSC-sabot) in corresponding

process of target acceleration and injection. For this reason, the target must be placed into a special target carrier (sabot). Using sabots, there occur some contact problems. Because of a tight seal between the sabot and the barrel, any damage of the barrel and the sabot surface will affect the injector performance and sabot

means of a steady-state target-producing device, which is compatible with a

noncontact schedule of the target delivery to the reaction chamber.

which works with a target batch rolling along the LC.

3. Target repeatable delivery

3.1 Noncontact acceleration system

reusing.

tests.

62

Schematic of "EM-AC + PMG". In (a): 1—target, 2—conical support of the target nest, 3—sabot matrix (polymer), 4—HTSC-coils (driving body based on superconducting MgB2-cables), 5—HTSC-plates for providing a stable levitation along the magnetic track, 6—magnetic track, 7—field coils, 8—protective cover; in (b) and (c): parameters and a side view of the experimental setup with directions of the driving force B1 and the levitation force B1 (9—ferromagnetics, 10—permanent magnets).

directions that leads to its simultaneous acceleration and levitation (simultaneous presence of the driving force along the vector B1 and of the levitation force along the vector B2).

The experiments were conducted under the following conditions.


#### Figure 8.

HTSC-sabot friction-free acceleration by the driving electromagnetic pulse generated by the field coil (T = 80 K). In (a): HTSC-sabot before acceleration; in (b) HTSC-sabot starts motion, in (c) and (d): in-time development of the acceleration process (the gap between the HTSC-sabot and the magnetic track is keeping unvarying with time).

allows the acceleration experiments to be feasible at T80 K taking into account that such experiments at T18 K are unpractical inside a small test chamber of the cryostat.

Different HTSC materials can have different microstructures, from so-called "superconducting glass" (superconducting ceramics) to microstructures like a type of "mosaic" with macro-, meso-, and microlevels of material ordering. This creates favorable conditions for obtaining an optimal HTSC microstructure just for taking into account the design specifics of a noncontact delivery system intended for the

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

quently the energy dissipation. A vortex state looks like a "frozen" in the

is Tc 90 K, which is nearing the boiling point of nitrogen (Table 2).

The HTSC-sabot designs were in the form of an "open parallelepiped" (its cross section forms a trough) or in the form of a "hollow parallelepiped" (its cross section

Y123 [4, 6] 4. 33 >45 91 Gd123 [5, 26] 3.25 > 45 92

) ВС at 0 K (T) ТС (K)

superconducting material, and any spatial motion of the superconductor will cause the flux tubes to move. In order to prevent that, the superconductor remains "trapped" in its original state (be it levitating at the fixed point or under motion

Thus, the bulk of type-II, HTSC materials breaks down into two regions: superconductive—from which the external field is completely expelled, and normal—through which the external field penetrates. The diamagnetic characteristics of the material are more or less pronounced depending on its "degree of superconductivity." These features can be exploited in the process of HTSC-sabot levitation and guidance. Therefore, the superconducting material science and technology is of critical importance. Currently, there are many structural efforts to enhance the pinning properties in HTSCs by creating structural defects in them using different techniques. These structural defects can be in the form of periodic pinning arrays or random pinning distributions (i.e., with different ordering states of a vortex lattice) to improve the maglev properties of the HTSCs. In our experiments, the HTSCs are superconducting ceramics based on YBa2Cu3O7<sup>x</sup> (or Y123; production of LPI) and superconducting tapes of second generation (2G HTSC) based on GdBa2Cu3O7<sup>x</sup> (or Gd123; production of SuperOx, Ltd.). The obtained results have shown that these HTSCs can be successfully used to maintain a friction-free motion of the HTSC-sabots, and also to provide a required stability of the levitation height over the whole acceleration length due to the pinning effect. This becomes more viable because the critical temperature of Gd123 and Y123

At the same time, it is necessary to note one more important feature related to type-II superconductors. It is a flux (or vortex) pinning. Vortex pinning results from spatial imperfections (or defects) in the material that produces local reductions of the free energy of a flux line [25], thus attracting and holding vortices to these locations. In many respects, the basic magneto-mechanical phenomenon responsible for levitation is a result of the magnetic flux pinning inherent in the interaction between a magnet and a type-II superconductor. In the mixed state, the flux lines interact with different defects and may become pinned to them (frozen in the bulk superconductor). Such defects (e.g., crystal lattice defects, grain boundaries, twin planes, stacking faults, etc.) always exist in real superconducting materials. They could work as pinning centers (including pinning by surface roughness or at a step-like surface relief), avoiding the vortex motion and conse-

IFE experiments.

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

along a magnetic track).

forms a square).

Table 2.

65

HTSCs Density (ρ, g/cm3

Parameters of the HTSC materials used in the experiments.

Figure 8 shows the results of the demonstration tests: a set of freeze-frame shots of the acceleration process of the levitating HTSC-sabot at T = 80 K using the linear PMG-system.

The HTSC-sabot is trapped (Figure 8a) and accelerated in front of a magnetic traveling wave (Figure 8b–d). It reaches a velocity of 1 m/s and keeps this velocity on all sabot-track length of 22.5 cm (motion time is t = 0.22 s). The levitating drift is not observed. Technologically, this allows a convenient spacing of the multiple coils (also called a multiple-stage accelerator, Figure 7a), and leads to realizing very high velocities of the HTSC-sabot. It is necessary to highlight that in the experiments, we did not use a special driving body, and the obtained result of 1 m/s is due to the surface currents in the HTSC material itself arising from the magnetic field B1 generated by the field coil.

Thus, a friction-free HTSC-sabot transport can be realized with the levitation devices using superconductors and permanent magnets. The continuous space range of the stable position of the levitated HTSC-sabot has been demonstrated by the experimental results. Features of the device concepts and their future applications in the noncontact delivery system are discussed below.

#### 3.2 HTSC materials

Generally, superconducting material selection for sabot manufacturing is defined, first of all, by the temperature requirement for IFE targets which must be at T = 18.3 K [1–3]. Superconductors are classified into two types [25], called type-I and type-II, based on their diamagnetic properties (magnetic susceptibility χ < 0). Type-I superconductors (low-temperature superconductors) are in the state which is called "perfect diamagnetism" (the Meissner effect at which the magnetic lines bend around the superconductors). As the applied magnetic field increases, so does the opposing magnetization until the field reaches the critical field BC, whereupon the superconductivity disappears. Since the type-I superconductors have the critical temperature TC < 10 K (i.e., their heating above 10 K destroys their superconductivity), they cannot be considered as candidates when developing a maglev transport system for application to IFE. Besides, type-I superconductors typically have the critical field values too low for practical applications.

In the study, we use the samples from HTSCs, which are known to be type-II superconductors. They have two values of the critical magnetic field BC (called first BC1 and second BC2). Below BC1, type-II behaves similar to the type-I. When the applied magnetic field is between BC1 and BC2, the magnetic field penetrates the type-II superconductors in the form of quantized magnetic flux lines (either tube or vortices), and they become a mixture of the normal and superconducting states. Emphasize that inside each magnetic flux tube, superconductivity is locally destroyed. Such materials can be subjected to much higher external magnetic fields and remain superconducting. This property is used for obtaining strong magnetic fields under the conditions of no thermal losses when the high currents are passing through HTSCs. In addition, the HTSC materials with critical temperatures in the range of 90–120 K have received a great deal of attention because they can be maintained in the superconducting state with liquid nitrogen (77 K).

The second issue under superconducting material selection is structural characteristics of HTSCs, which influence on the potentialities of their levitation properties. The authors of [13] revealed many interesting features related to this problem.

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

Different HTSC materials can have different microstructures, from so-called "superconducting glass" (superconducting ceramics) to microstructures like a type of "mosaic" with macro-, meso-, and microlevels of material ordering. This creates favorable conditions for obtaining an optimal HTSC microstructure just for taking into account the design specifics of a noncontact delivery system intended for the IFE experiments.

At the same time, it is necessary to note one more important feature related to type-II superconductors. It is a flux (or vortex) pinning. Vortex pinning results from spatial imperfections (or defects) in the material that produces local reductions of the free energy of a flux line [25], thus attracting and holding vortices to these locations. In many respects, the basic magneto-mechanical phenomenon responsible for levitation is a result of the magnetic flux pinning inherent in the interaction between a magnet and a type-II superconductor. In the mixed state, the flux lines interact with different defects and may become pinned to them (frozen in the bulk superconductor). Such defects (e.g., crystal lattice defects, grain boundaries, twin planes, stacking faults, etc.) always exist in real superconducting materials. They could work as pinning centers (including pinning by surface roughness or at a step-like surface relief), avoiding the vortex motion and consequently the energy dissipation. A vortex state looks like a "frozen" in the superconducting material, and any spatial motion of the superconductor will cause the flux tubes to move. In order to prevent that, the superconductor remains "trapped" in its original state (be it levitating at the fixed point or under motion along a magnetic track).

Thus, the bulk of type-II, HTSC materials breaks down into two regions: superconductive—from which the external field is completely expelled, and normal—through which the external field penetrates. The diamagnetic characteristics of the material are more or less pronounced depending on its "degree of superconductivity." These features can be exploited in the process of HTSC-sabot levitation and guidance. Therefore, the superconducting material science and technology is of critical importance. Currently, there are many structural efforts to enhance the pinning properties in HTSCs by creating structural defects in them using different techniques. These structural defects can be in the form of periodic pinning arrays or random pinning distributions (i.e., with different ordering states of a vortex lattice) to improve the maglev properties of the HTSCs.

In our experiments, the HTSCs are superconducting ceramics based on YBa2Cu3O7<sup>x</sup> (or Y123; production of LPI) and superconducting tapes of second generation (2G HTSC) based on GdBa2Cu3O7<sup>x</sup> (or Gd123; production of SuperOx, Ltd.). The obtained results have shown that these HTSCs can be successfully used to maintain a friction-free motion of the HTSC-sabots, and also to provide a required stability of the levitation height over the whole acceleration length due to the pinning effect. This becomes more viable because the critical temperature of Gd123 and Y123 is Tc 90 K, which is nearing the boiling point of nitrogen (Table 2).

The HTSC-sabot designs were in the form of an "open parallelepiped" (its cross section forms a trough) or in the form of a "hollow parallelepiped" (its cross section forms a square).


Table 2.

Parameters of the HTSC materials used in the experiments.

allows the acceleration experiments to be feasible at T80 K taking into account that such experiments at T18 K are unpractical inside a small test

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

Figure 8 shows the results of the demonstration tests: a set of freeze-frame shots of the acceleration process of the levitating HTSC-sabot at T = 80 K using the linear

The HTSC-sabot is trapped (Figure 8a) and accelerated in front of a magnetic traveling wave (Figure 8b–d). It reaches a velocity of 1 m/s and keeps this velocity on all sabot-track length of 22.5 cm (motion time is t = 0.22 s). The levitating drift is not observed. Technologically, this allows a convenient spacing of the multiple coils (also called a multiple-stage accelerator, Figure 7a), and leads to realizing very high velocities of the HTSC-sabot. It is necessary to highlight that in the experiments, we did not use a special driving body, and the obtained result of 1 m/s is due to the surface currents in the HTSC material itself arising from the magnetic field B1

Thus, a friction-free HTSC-sabot transport can be realized with the levitation devices using superconductors and permanent magnets. The continuous space range of the stable position of the levitated HTSC-sabot has been demonstrated by the experimental results. Features of the device concepts and their future applica-

Generally, superconducting material selection for sabot manufacturing is defined,

T = 18.3 K [1–3]. Superconductors are classified into two types [25], called type-I and type-II, based on their diamagnetic properties (magnetic susceptibility χ < 0). Type-I superconductors (low-temperature superconductors) are in the state which is called "perfect diamagnetism" (the Meissner effect at which the magnetic lines bend around the superconductors). As the applied magnetic field increases, so does the opposing magnetization until the field reaches the critical field BC, whereupon the superconductivity disappears. Since the type-I superconductors have the critical temperature TC < 10 K (i.e., their heating above 10 K destroys their superconductivity), they cannot be considered as candidates when developing a maglev transport system for application to IFE. Besides, type-I superconductors typically have the critical field

In the study, we use the samples from HTSCs, which are known to be type-II superconductors. They have two values of the critical magnetic field BC (called first BC1 and second BC2). Below BC1, type-II behaves similar to the type-I. When the applied magnetic field is between BC1 and BC2, the magnetic field penetrates the type-II superconductors in the form of quantized magnetic flux lines (either tube or vortices), and they become a mixture of the normal and superconducting states. Emphasize that inside each magnetic flux tube, superconductivity is locally

destroyed. Such materials can be subjected to much higher external magnetic fields and remain superconducting. This property is used for obtaining strong magnetic fields under the conditions of no thermal losses when the high currents are passing through HTSCs. In addition, the HTSC materials with critical temperatures in the range of 90–120 K have received a great deal of attention because they can be

The second issue under superconducting material selection is structural characteristics of HTSCs, which influence on the potentialities of their levitation properties. The authors of [13] revealed many interesting features related to this problem.

maintained in the superconducting state with liquid nitrogen (77 K).

first of all, by the temperature requirement for IFE targets which must be at

tions in the noncontact delivery system are discussed below.

chamber of the cryostat.

generated by the field coil.

3.2 HTSC materials

64

values too low for practical applications.

PMG-system.

positioning accuracy of a sensitive element is 0.1 mm, the measuring range is +1 T, and the absolute error is +0.005 T. The maximum magnetic induction (0.33 T right at the track's surface) was for the linear PMG (Figure 11) along the acceleration

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

Note that the PMG optimization is the most critical problem of practical interest since it serves as a continuous magnetic track to generate the required magnetic field by rare-earth permanent magnets (made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B) and inserts of soft magnetic (ferromagnetic). Therefore, a feasibility study of the key technical issues such as influence of the PMG-fields of different configurations on the mechanical and timing performances of the HTSC transport process and active guidance due to different driving pulses

In this chapter, we present an analysis of dynamic behavior for two proposed PMGs consisting of different arrangements of the permanent magnets (different shape and size). The levitation experiments in specifically designed configurations (Figure 10) with strongly pinned superconductors (Gd123 and Y123) display a repeatable PMG operation, allowing a simultaneous demonstration of linear and lateral stability. We have also studied the issues of how the geometrical and loading characteristics of the HTSC-sabots can affect their levitation capability at different

The main idea in the PMG construction is that the magnetic track must allow the HTSC-sabots to move freely only in one direction in order to avoid any contact with a stronger magnetic field, which pushes out them and return them to their initial trajectory (according to the Eq. (3)). This is due to the fact that any spatial motion of the HTSCs will cause the magnetic flux tubes to move. A reasonable plan to prevent this effect in the lateral direction is to made the linear PMG with the magnet poles aligned antiparallel to each other (N-S-N) for producing a considerably strong gradient for a side-to-side motion (Figures 7, 8, 10a). The circular PMG consists of a disk of NdFeB permanent magnet (OD = 100 mm, ID = 50 mm, 5 mm thick) embedded in the soft ferromagnetic holder to realize a required distribution of the magnetic field along the width of the magnetic track (Figure 10b). Since the flux tubes are magnetic fields frozen in the superconducting material, the very superconducting material itself creates a force to inhibit any motion in relation to the magnetic field, and the HTSC-sabot remains "trapped" in its trajectory. This is an efficient scheme for optimizing the levitation and guidance forces which is

We have found that not only the linear PMG-systems (Figures 8 and 10a) but also the circular ones are promising candidates aimed at development of HTSCmaglev transport system for high-velocity target applications, target trajectory correction, and creation of a precise injector. The circular PMG testing under a typical optimization of the levitation stability has proven its robustness and efficiency. Figure 11 demonstrates the HTSC-sabot motion with different velocities over the circular PMG shown in Figure 10b. In the experiments, we use a mechanical

Acceleration of the HTSC-sabot (model #3, Figure 9c) over the linear (a) and circular (b) PMGs at T = 80 K.

direction OX for a magnetic track of 24-cm long.

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

(mechanical and electromagnetic) are under way.

considered as a base to perform the search of an optimal PMG.

constraints of the PMG cross section.

Figure 10.

67

Figure 9.

Usual HTSC-sabot designs used in the experiments. In (a): "open parallelepiped" (model#1); in (b): "hollow parallelepiped" (model#2 + polymer foam (1)); in (c): model#3—"hollow parallelepiped" (empty).

Here, we set the task of accelerating different HTSC-sabots (Figure 9) over different PMG-systems to study the stability of the main levitation parameters: the load capacity (mass of an object which HTSC-sabot can maintain), the space locking (three-dimensional stability of HTSC-sabot), and the gap between the HTSC-sabot and the magnetic track (levitating drift). The HTSC-sabot parameters are as follows:


In the experiments, the force F driving the diamagnetic Gd123-tapes is given by [27]:

$$F = \frac{\chi}{2\mu\_0} V\_S \frac{d B\_x^2}{d x},\tag{3}$$

where μ<sup>0</sup> is the permeability of vacuum, VS is the superconductor volume, x is the acceleration axis, B is the magnetic induction produced by the field coil (in our case B1). Since for diamagnetic χ < 0, then this force is contrariwise to the gradient of the magnetic induction in the x-direction. Therefore, the HTSC-sabot is pushed out from the area of a stronger magnetic field that defines its behavior in the PMGsystems.

### 3.3 PMG-systems

The characteristics of the permanent magnets composing the PMGs are very important for their performance in terms of levitation force and stability. The PMGsystems of different configuration were used in the experiments (Figures 8, 11–14). Our goal was to demonstrate not only the levitation stability of the HTSC-sabots, but also their transport over the PMGs with a guidance force resulting in either linear or circular motion in a "tight space."

The PMGs [4, 5] were constructed on the basis of neodymium permanent magnets with an axial magnetization (MIDORA, Ltd.). A magnetometer with a sensitivity of 280 mV/T was applied with the following performance data: the

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

positioning accuracy of a sensitive element is 0.1 mm, the measuring range is +1 T, and the absolute error is +0.005 T. The maximum magnetic induction (0.33 T right at the track's surface) was for the linear PMG (Figure 11) along the acceleration direction OX for a magnetic track of 24-cm long.

Note that the PMG optimization is the most critical problem of practical interest since it serves as a continuous magnetic track to generate the required magnetic field by rare-earth permanent magnets (made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B) and inserts of soft magnetic (ferromagnetic). Therefore, a feasibility study of the key technical issues such as influence of the PMG-fields of different configurations on the mechanical and timing performances of the HTSC transport process and active guidance due to different driving pulses (mechanical and electromagnetic) are under way.

In this chapter, we present an analysis of dynamic behavior for two proposed PMGs consisting of different arrangements of the permanent magnets (different shape and size). The levitation experiments in specifically designed configurations (Figure 10) with strongly pinned superconductors (Gd123 and Y123) display a repeatable PMG operation, allowing a simultaneous demonstration of linear and lateral stability. We have also studied the issues of how the geometrical and loading characteristics of the HTSC-sabots can affect their levitation capability at different constraints of the PMG cross section.

The main idea in the PMG construction is that the magnetic track must allow the HTSC-sabots to move freely only in one direction in order to avoid any contact with a stronger magnetic field, which pushes out them and return them to their initial trajectory (according to the Eq. (3)). This is due to the fact that any spatial motion of the HTSCs will cause the magnetic flux tubes to move. A reasonable plan to prevent this effect in the lateral direction is to made the linear PMG with the magnet poles aligned antiparallel to each other (N-S-N) for producing a considerably strong gradient for a side-to-side motion (Figures 7, 8, 10a). The circular PMG consists of a disk of NdFeB permanent magnet (OD = 100 mm, ID = 50 mm, 5 mm thick) embedded in the soft ferromagnetic holder to realize a required distribution of the magnetic field along the width of the magnetic track (Figure 10b). Since the flux tubes are magnetic fields frozen in the superconducting material, the very superconducting material itself creates a force to inhibit any motion in relation to the magnetic field, and the HTSC-sabot remains "trapped" in its trajectory. This is an efficient scheme for optimizing the levitation and guidance forces which is considered as a base to perform the search of an optimal PMG.

We have found that not only the linear PMG-systems (Figures 8 and 10a) but also the circular ones are promising candidates aimed at development of HTSCmaglev transport system for high-velocity target applications, target trajectory correction, and creation of a precise injector. The circular PMG testing under a typical optimization of the levitation stability has proven its robustness and efficiency. Figure 11 demonstrates the HTSC-sabot motion with different velocities over the circular PMG shown in Figure 10b. In the experiments, we use a mechanical

Here, we set the task of accelerating different HTSC-sabots (Figure 9) over different PMG-systems to study the stability of the main levitation parameters: the load capacity (mass of an object which HTSC-sabot can maintain), the space locking (three-dimensional stability of HTSC-sabot), and the gap between the HTSC-sabot and the magnetic track (levitating drift). The HTSC-sabot parameters

Usual HTSC-sabot designs used in the experiments. In (a): "open parallelepiped" (model#1); in (b): "hollow parallelepiped" (model#2 + polymer foam (1)); in (c): model#3—"hollow parallelepiped" (empty).

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

• Model #1, "open parallelepiped": Gd123 tape thickness is 0.5 mm, length inside

• Model #2, "hollow parallelepiped": Gd123 tape thickness is 0.3 mm, internal

• Model #3, "hollow parallelepiped": Gd123 tape thickness is 0.3 mm, internal

In the experiments, the force F driving the diamagnetic Gd123-tapes is given

where μ<sup>0</sup> is the permeability of vacuum, VS is the superconductor volume, x is the acceleration axis, B is the magnetic induction produced by the field coil (in our case B1). Since for diamagnetic χ < 0, then this force is contrariwise to the gradient of the magnetic induction in the x-direction. Therefore, the HTSC-sabot is pushed out from the area of a stronger magnetic field that defines its behavior in the PMG-

The characteristics of the permanent magnets composing the PMGs are very important for their performance in terms of levitation force and stability. The PMGsystems of different configuration were used in the experiments (Figures 8, 11–14). Our goal was to demonstrate not only the levitation stability of the HTSC-sabots, but also their transport over the PMGs with a guidance force resulting in either

The PMGs [4, 5] were constructed on the basis of neodymium permanent magnets with an axial magnetization (MIDORA, Ltd.). A magnetometer with a sensitivity of 280 mV/T was applied with the following performance data: the

<sup>F</sup> <sup>¼</sup> <sup>χ</sup> 2μ<sup>0</sup> VS dB<sup>2</sup> x

, total mass is 0.97 g (together with filling of polymer

dx , (3)

, total mass is 0.59 g (no polymer filling).

is 25 mm, width inside is 8 mm, height is 4 mm, total mass is 1.26 g;

are as follows:

Figure 9.

by [27]:

systems.

66

3.3 PMG-systems

linear or circular motion in a "tight space."

sizes are 4 � <sup>4</sup> � 24 mm<sup>3</sup>

sizes are 4 � <sup>4</sup> � 30 mm<sup>3</sup>

foam which mass is 0.38 g);

external disturbances are considerable, vertical and lateral displacements of the levitating body may occur simultaneously. In this connection, it was very important

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

Our findings (Figures 8, 10–13) have shown that a basic phenomenon responsible for the levitation stability is the flux pinning effect inherent in the interaction between a type-II superconductor and a permanent magnet. For specifically designed configurations of PMGs (peculiar distribution of the magnetic field), the flux-pinning is tending to enhance the stability of HTSC-sabot levitation, and strongly pinned superconductors (Gd123) display high stability, allowing the demonstration of striking effects, such as vertical, lateral, or inverted levitation. They look like they are pinned to a magnetic track so they can stably levitate over permanent magnets without any active control. Thus, the HTSCs can be designed to enhance the effect called "enhanced flux pinning." It is of a great importance for target trajectory correction during its delivery inside the assembly of "HTSC-sabot +

An idea of using the magnetic field to control the "HTSC-sabot + target" trans-

Along the magnetic track (or acceleration length, both linear and circular), there are no magnetic field changes, which allows the HTSC-sabots to move forth and back with no energy loss. Normal to the acceleration length, the magnet poles are aligned antiparallel to each other (N-S-N, see, for example, Figure 7b) that produces a considerably strong gradient along the width of the magnetic track. This gradient prevents the motion of HTSC-sabots, and they remain located in the transverse direction. In other words: (1) flux-pinning makes the HTSC-sabot motion trapped in the space within a PMG field; (2) flux-pinning makes the HTSCsabot orientation fixed in the space so that they will not reorient themselves without any external action (so-called a three-dimensional locking of type-II superconductors). This process of locking by height and orientation reduces any undesirable wobble during HTSC-sabot movement. Thus, the obtained results indicate that we have an effective set of tools (quantum levitation and quantum locking) for a noncontact acceleration of the HTSC-sabots in the mutually normal magnetic fields

The HTSC-sabot design is a vital point in the process of its transport with levitation. The most striking examples are the HTSC-sabots, the shape of which

• Model #4, "hollow parallelepiped + 5 wings": Gd123 tape thickness is 0.3 mm,

) + 5 wings (12 12 mm<sup>2</sup>

), total mass is 1.46 g.

port is very attractive for the following reasons. In HTSC-maglev, the stable transport with levitation is caused by a combination of the Meissner effect (quan-

As it is mentioned above, in HTSCs the magnetic field is not excluded completely, and the superconductor tries to keep the magnetic flux or vortexes pinned in weak areas (e.g., grain boundaries or other defects). Energetically, this means that the vortexes favor to be located in the bulk of HTSCs where defects exist. Any spatial displacement of the HTSCs causes the magnetic flux motion. Using the vortex physics [26] under the PMG construction, we have succeeded in controlling the magnetic field so that there are some directions where the HTSCsabots can move, and there are some directions where the HTSC-sabots remain

tum levitation) and the flux pinning in HTSCs (quantum locking).

to study the levitation stability of the HTSC-sabots.

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

target." Several remarks should be made here.

"trapped" (located in a "tight space").

generated by the field coil and the PMG-system.

corresponds to Model #4 and Model #5 (Figure 14):

Model #2 (4 <sup>4</sup> 30 mm<sup>3</sup>

3.4 HTSC-sabot design

69

Figure 11.

Different velocities of the HTSC-sabot (model #3, load capacity: a piece of wood) over the circular PMG at T = 80 K. In (a): internal orbit; in (b): external orbit.

driving pulse to simply change the initial conditions related to placing the HTSCsabot on different trajectories. The findings accessible from these experiments are the bases to provide the conditions when the HTSC-sabot can be accelerated in a stable orbit under different load capacities, which is directly related to the safe operation and design of the whole system. Below, we present the experimental results obtained in this area. Figures 12 and 13 show a stable acceleration of HTSCsabot (Model #1) over the linear and circular PMGs with several samples surrogating both spherical and cylindrical targets.

In both cases, after being disturbed during motion, the HTSC-sabot has a disturbance-induced velocity that, in general, can result in changes in the levitation and guidance forces and can cause some serious malfunctions. If these induced

Figure 12.

Acceleration of the HTSC-sabot over a linear PMG at T = 80 K. In (a): HTSC-sabot moves along the magnetic track; in (b): 1—HTSC-sabot is on the middle of the track, 2—HTSC-sabot exit out of the track (load capacity: 5 spherical targets of 0.6 mg each).

#### Figure 13.

Acceleration of the HTSC-sabot over the circular PMG at T = 80 K. In (a): before acceleration; in (b) and (c): during acceleration (load capacity: three cylindrical targets of 1.1 g each); in (d): before acceleration; in (e) and (f): during acceleration (load capacity: two cylindrical targets of 1.1 g each are placed in the same HTSCsabot in such a way that one of them is vertical, and the other is horizontal).

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

external disturbances are considerable, vertical and lateral displacements of the levitating body may occur simultaneously. In this connection, it was very important to study the levitation stability of the HTSC-sabots.

Our findings (Figures 8, 10–13) have shown that a basic phenomenon responsible for the levitation stability is the flux pinning effect inherent in the interaction between a type-II superconductor and a permanent magnet. For specifically designed configurations of PMGs (peculiar distribution of the magnetic field), the flux-pinning is tending to enhance the stability of HTSC-sabot levitation, and strongly pinned superconductors (Gd123) display high stability, allowing the demonstration of striking effects, such as vertical, lateral, or inverted levitation. They look like they are pinned to a magnetic track so they can stably levitate over permanent magnets without any active control. Thus, the HTSCs can be designed to enhance the effect called "enhanced flux pinning." It is of a great importance for target trajectory correction during its delivery inside the assembly of "HTSC-sabot + target." Several remarks should be made here.

An idea of using the magnetic field to control the "HTSC-sabot + target" transport is very attractive for the following reasons. In HTSC-maglev, the stable transport with levitation is caused by a combination of the Meissner effect (quantum levitation) and the flux pinning in HTSCs (quantum locking).

As it is mentioned above, in HTSCs the magnetic field is not excluded completely, and the superconductor tries to keep the magnetic flux or vortexes pinned in weak areas (e.g., grain boundaries or other defects). Energetically, this means that the vortexes favor to be located in the bulk of HTSCs where defects exist. Any spatial displacement of the HTSCs causes the magnetic flux motion. Using the vortex physics [26] under the PMG construction, we have succeeded in controlling the magnetic field so that there are some directions where the HTSCsabots can move, and there are some directions where the HTSC-sabots remain "trapped" (located in a "tight space").

Along the magnetic track (or acceleration length, both linear and circular), there are no magnetic field changes, which allows the HTSC-sabots to move forth and back with no energy loss. Normal to the acceleration length, the magnet poles are aligned antiparallel to each other (N-S-N, see, for example, Figure 7b) that produces a considerably strong gradient along the width of the magnetic track. This gradient prevents the motion of HTSC-sabots, and they remain located in the transverse direction. In other words: (1) flux-pinning makes the HTSC-sabot motion trapped in the space within a PMG field; (2) flux-pinning makes the HTSCsabot orientation fixed in the space so that they will not reorient themselves without any external action (so-called a three-dimensional locking of type-II superconductors). This process of locking by height and orientation reduces any undesirable wobble during HTSC-sabot movement. Thus, the obtained results indicate that we have an effective set of tools (quantum levitation and quantum locking) for a noncontact acceleration of the HTSC-sabots in the mutually normal magnetic fields generated by the field coil and the PMG-system.

#### 3.4 HTSC-sabot design

The HTSC-sabot design is a vital point in the process of its transport with levitation. The most striking examples are the HTSC-sabots, the shape of which corresponds to Model #4 and Model #5 (Figure 14):

• Model #4, "hollow parallelepiped + 5 wings": Gd123 tape thickness is 0.3 mm, Model #2 (4 <sup>4</sup> 30 mm<sup>3</sup> ) + 5 wings (12 12 mm<sup>2</sup> ), total mass is 1.46 g.

driving pulse to simply change the initial conditions related to placing the HTSCsabot on different trajectories. The findings accessible from these experiments are the bases to provide the conditions when the HTSC-sabot can be accelerated in a stable orbit under different load capacities, which is directly related to the safe operation and design of the whole system. Below, we present the experimental results obtained in this area. Figures 12 and 13 show a stable acceleration of HTSCsabot (Model #1) over the linear and circular PMGs with several samples surrogat-

Different velocities of the HTSC-sabot (model #3, load capacity: a piece of wood) over the circular PMG at

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

In both cases, after being disturbed during motion, the HTSC-sabot has a disturbance-induced velocity that, in general, can result in changes in the levitation and guidance forces and can cause some serious malfunctions. If these induced

Acceleration of the HTSC-sabot over a linear PMG at T = 80 K. In (a): HTSC-sabot moves along the magnetic track; in (b): 1—HTSC-sabot is on the middle of the track, 2—HTSC-sabot exit out of the track (load

Acceleration of the HTSC-sabot over the circular PMG at T = 80 K. In (a): before acceleration; in (b) and (c): during acceleration (load capacity: three cylindrical targets of 1.1 g each); in (d): before acceleration; in (e) and (f): during acceleration (load capacity: two cylindrical targets of 1.1 g each are placed in the same HTSC-

sabot in such a way that one of them is vertical, and the other is horizontal).

ing both spherical and cylindrical targets.

T = 80 K. In (a): internal orbit; in (b): external orbit.

capacity: 5 spherical targets of 0.6 mg each).

Figure 13.

68

Figure 12.

Figure 11.

• Model #5, "hollow parallelepiped + 5 wings" on a Gd123 tape (35 <sup>12</sup> 0.3 mm<sup>3</sup> ): total mass is 1.97 g.

Nevertheless, Figure 15a shows that the Model #4 (the Model #2 in assembly with five wings) as an independent target carrier is inefficient. The wings keep the Model #4 "nonlevitated" so that it comes into contact with the magnetic track. However, if using the same Model #5 (the Model #4 placed on a superconducting Gd123 tape), the levitation effect occurs again (Figure 15b), because the Model #5 is a combination of the levitating Gd123 tape and the Model #4 as a load capacity.

body, the acceleration parameters which are of interest for IFE (injection velocity Vinj = 200 m/s) become unsuitable for the laboratory-scale tests. Therefore, the POP experiments (Figure 8) have been carried out with HTSC-sabots (Gd123) without any MgB2-coils in their design. The HTSC-sabot (Model #2) is accelerated using the magnetic field B1 generated by the field coil (ARP). The acceleration process maintenance is caused by the surface currents induced in the bulk Gd123 itself due to ARP, which results in arising the driving force along the acceleration length. The HTSC-sabot obtains a velocity of 1 m/s and keeps it over the whole magnetic track

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

Below, we discuss the issue related to a multiple-stage accelerator. The first problem is as follows: what characteristics of MgB2-cables are required to reach the required lower limit on the injection velocity Vinj = 200 m/s. We list below some

• MgB2 critical temperature is TC = 39 K, which is twice above than for Nb3Sn,

• MgB2 possesses high values of the critical current at a rather small sensitivity to

• simple chemical composition and low cost of the initial components for its

• achievement of typical magnetic fields more than 1.5–2.0 T at lower capital

• due to a weak anisotropy of the critical properties, the MgB2-cables can be well-shaped that is of a great importance for optimizing the current density in superconducting coils. Besides, the MgB2-cables can be of round or rectangular cross section and have a small weight. These are most important parameters

For estimations of the acceleration length, La, for a multiple-stage accelerator with a superconducting driving body (in our case MgB2-cables), we use the follow-

where Msab is the mass of the "HTSC-sabot + target" assembly, RFC is the field coil radius, RSC is the radius of the superconducting coils (RFC/RSC = 5), Fpin is the pinning force density, JC is the critical current density, which depends on the magnetic induction in the coil center B0 and superconductor temperature TS. The value of JC (defined as the current density where the pinning force and the Lorentz force become equal) determines the onset of resistivity [28–30]. In (4), a difficulty arises in calculation of La because only knowing the relationship between JC and B0,

, Fpin ¼ JCð Þ� B0; TS B0, (4)

for practical applications of MgB2-cables in the HTSC-sabot design.

� Msab FpinVS

• stability of MgB2 characteristics under the conditions of radiative environment;

investments and at lower cost of commercial operation;

• MgB2 is a promising superconductor for applications in the temperature range 15–20 K which meets the temperature tolerance for the IFE targets which must be at T = 18.3 K before the laser shot to obtain the maximum

distinguish features of MgB2, which are important for our study [28, 29]:

(22.5 cm). This is a demonstration of the one-stage accelerator.

and four times above than for Nb-Ti;

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

intergranular contacts;

synthesis;

ing ration [8]:

71

La <sup>¼</sup> <sup>π</sup>

<sup>2</sup> � <sup>V</sup><sup>2</sup> inj � RFC RSC

energy yield from the fusion reaction [1–3];

These results can be explained by a special mapping of the magnetic lines which are bending around the Model #4 creating the magnetic field close to the second critical value, BC2, or even more this value (T = 80 K is close to TC = 92 K). At a step-like surface relief (critical bending of the magnetic lines), the magnetic field is able to considerably slip through the HTSC material of the Model #4, and the normal cores of vortexes begin to adjoin and then the volume superconductivity disappears. In other words, under roughening of a surface, the number of vortices becomes so numerous that there is no space left for superconductivity, and the superconducting material becomes less and less diamagnetic.
