## 2 High-Field-Gradient Magnet Structures  
### Electromagnetic Parameters of the Gradient Magnet  
Table 3 lists the basic design prerequisites for gradient magnets. The original standard required that the central magnetic field strength should exceed 10 T to effectively control crystal growth behavior in high magnetic fields. Secondly, in order to control the simulated gravity, it is necessary to calculate the distribution and extreme point of the product of the magnetic field and the field gradient on the z-axis, which is expressed in its functional form as B (dB/dz).  
In order to form a 0-2 g gravity simulation field, the second standard stipulates that the maximum gradient absolute value along the central axis should reach 400 T\({}^{2}\)/m.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \begin{tabular}{c} **Central** \\ **Magnetic Field** \\ **(**Abolute Value**)** \\ \end{tabular} & \begin{tabular}{c} **Magnetic Field Gradient** \\ **Extreme Value** \\ **(Absolute Value)** \\ \end{tabular} & \begin{tabular}{c} **Room-Temperature** \\ **Diameter** \\ \end{tabular} & \
\begin{tabular}{c} **System Outer** \\ **Diameter** \\ \end{tabular} &
\begin{tabular}{c} **Coeling Method** \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 3: Design prerequisites for the gradient magnet.  
The remaining indicators are determined by experimental parameters. As new crystal growth research continues to unfold, new requirements are put forward for the size of the operating space.  
As shown in Figure 1, the magnet coil is intended to be wound using an odd-even layer winding technique. To enhance the magnetic field gradient and reduce the external diameter of the magnet, an additional coil with reverse current is added in the longitudinal direction.  
The magnet utilizes a zero-liquid helium evaporation system for cooling. Given the necessity for a room-temperature aperture of \(\geq\)200 mm, the installation of a cold screen and vacuum chamber is imperative between the room-temperature aperture and the inner diameter of the magnet. Drawing from practical experience, a minimum space of at least 20 mm needs to be allocated on one side. Therefore, the predetermined diameter for the magnet's cooling aperture is set at 246 mm.  
While satisfying the specified magnet parameters, the design places emphasis on the combined structure of the magnet coils, involving several variables: the axial distance (d\({}_{1-4}\)) between coils 1 and 4; the deviation distance (d\({}_{1-23}\)) between the centers of coil 1 and coils 2 and 3, between the single-layer turns, and between the layers of coil 4, as well as between the single-layer turns and the layers of coils 1, 2, and 3. According to engineering practice, one must note the distance between coils 1 and 2 and the gaps between coil 3, which consist of the thickness of the insulation layer, the thickness of the preload layer, and that of the coil gap. Under the premise of leaving a certain margin, these three types of thickness are all fixed values, and the space for optimization is small. Therefore, in this design, the gaps between coils 1, 2, and 3 do not need to be optimized. The ultimate goal of optimization is to minimize the cost of wire usage for the coils. Table 4 presents the crucial electromagnetic parameters obtained after optimizing the magnet. The optimization iteration of electromagnetic field design is carried out step by step according to the following rules (Figure 2).  
Figure 1: Electromagnetic scheme of gradient magnet.