# Conceptual Design of a Compact Divertor Heat Load Simulation Device: HIT-PSI
## 2 Designs of the Device and Its Sub-Systems
### Superconducting Magnets  
In a linear plasma device for PSI research, the role of the magnetic field is to confine the radial diffusion of the plasma beam and to limit the radius of the sputtered particle rotation to smaller than the re-deposited material sample. Meanwhile, previous experimental results of Magnum-PSI showed that the application of a strong magnetic field could improve the performance of the arc plasma source, where a higher ionization rate and a more effective rise of the plasma density and temperature approaching the target plate can be achieved [23; 28]. When the confinement magnetic field of the source reaches a \(\sim\)Tesla level, the heat flux can reach \(\sim\)10 MW/m\({}^{2}\). Thus, a superconducting magnet subsystem to generate a steady-state strong magnetic field is essential. The construction of superconducting magnets may take a major share of the budget. Therefore, the design should be well balanced among physical needs, engineering challenges, operating costs, and budget allowed. The design parameters then have to mostly meet the following needs:  
* Safety, which always comes first, including personnel and operational safety needs;
* An axial magnetic field of >2.0 T with great homogeneity;
* More than 1000 mm axial distance of beam transmission, for future experimental research with a radial size as large as possible, enough for the vacuum chamber to accommodate the beam;
* Radial windows left for pumping and diagnostic needs;
* A fast excitation for experimental flexibility;
* Also, a low cost for operation, maintenance, and construction. Taking the above factors into consideration, the superconducting magnet design, shown in Figure 4, adopts an integrated system immersed in a closed cryostat containing liquid helium and directly cooled by a cryocooler to form a zero-boil-off system. Only a tiny amount of liquid helium is needed to keep the steady-state operation of the magnets. The main body of a superconducting magnet is shown in Figure 5, with an inner diameter of 450 mm, a total length of 1804.4 mm, and a total weight of 348.7 kg. In addition, eight radially distributed room temperature ports with a diameter of 167.6 mm for each are reserved for diagnostics and vacuum pump connection. Figure 3: Temperature simulation result of an optimal design for the cooling waterway of cascade plates. The major parameters of the superconducting magnet subsystem are listed in Table 2. A total of five solenoid coils are utilized in the superconducting magnet subsystem to ensure the preferred axial homogeneity of the magnetic field with a possibility of flexible modification. Table 3 is for the coil parameters. The magnetic field distribution generated by the maximum operating current of 275 A is shown in Figure 6 by Comsol 2D axisymmetric modeling. Coils 1 and 5, symmetrically distributed on both sides of Coil 3, have the same parameters. So do Coils 2 and 4. Copper stabilized (Cu/Sc:4) NbTi conductor lines of totally 23.77 km, with an insulated rectangular cross-section of 1.65 \(\times\) 1.0 mm\({}^{2}\) and an insulating layer of 0.04 mm, are adopted. Due to the space left for room temperature bores between \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Number of coils & 5 \\ Outer diameter & 956 mm \\ Inner diameter & 450 mm \\ Axial length & 1804.4 mm \\ Maximum operating current & 275 A \\ Mean strength of the axial magnetic field & \textgreater{}2.0 T \\ Magnetic inductance & 35.66 H \\ Total stored energy & 1.348 MJ \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of superconducting magnet subsystem. Figure 4: The design plan of the superconducting magnet subsystem, with superconducting coils (**a**) wrapped with two layers of Dewar (**b**,**c**) and a layer of thermal insulation cold shield (**e**), and the magnets wrapped with magnetic shielding iron shield (**f**) with eight radially distributed room temperature ports (**d**), as well as a service tower (**g**) on the top.