# Conceptual Design of a Compact Divertor Heat Load Simulation Device: HIT-PSI
## 2 Designs of the Device and Its Sub-Systems
### Superconducting Magnets
Figure 5: Sketch of superconducting magnet dimensions without external magnetic shielding. Coil Pairs 2/4 and 1/5, Coils 1 and 5 have more layers to improve homogeneity. Both the source and the target are placed near room temperature bores for easy observation and diagnosis. The plasma density of the beam generated by the cascaded arc plasma source exhibits a Gaussian distribution, and the radial full width at the half maximum (FWHM) of the plasma density is less than 50 mm due to the confinement of the strong magnetic field [19]. Moreover, the magnetic field distribution in the main plasma transmission region (enclosed in Figure 6 by red dots), with a length of 1300 mm along the axis and a radius of 50 mm, is illustrated in Figure 7. The average magnetic intensity in this  
Furthermore, Table 4 lists the inductance matrix of five coils. To obtain better experimental adaptability, the ramp time of the coil current needs to be minimized. For the excitation voltage distribution of 30.4%/8.1%/20.0%/8.1%/30.4%, the ramp-up time of 32.7 min at 5 volts and 54.5 min at 3 volts is aimed to reduce impacts while still meeting adequate technical standards. The cryogenic cooling arrangements, including current leads, cryocooler, gas supplement valves, liquid helium injection ports, auxiliary helium interfaces, pressure measurement interfaces, bursting membranes, exhaust valves, and coil protection circuit accessories, are all integrated into the service tower (Figure 4g) located in the upper middle part of the magnet. Current leads are made of REBCO materials. The cryocooler uses an SRDK-415D Sumitomo Gifford-McMahon one with a first-stage cooling capacity of 42 W at 50 K and a second-stage capacity of 1.8 W at 4.2 K, while the estimated heat loss is 1.5 W at 4.2 K.  
This superconducting magnet subsystem with a lot of radial windows is a new challenge for design and construction. Extra attention to its stability and safety is particularly necessary. The first and most needed task is to control the occurrence of quench. There are many possible causes for quench, and for this integrated design of a multi-coil superconducting magnet, an enormous electromagnetic force exists between the coils. The force may cause coil movements, resulting in asymmetrical twists to symmetrically designed coils, and thus result in catastrophic repercussions. The axial and radial Lorentz force distributions between different coils are shown in Figure 8. The resultant force of Coils 2/4, Coils 1/5, and Coil 3 are 247.4 kN, 128.8 kN, and 16.4 kN, respectively. To increase the rigidity of the magnet, coils are all wound on the same specifically designed reinforced cylindrical coil carrier with a wall thickness of 8 mm. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Coil 1** & **Coil 2** & **Coil 3** & **Coil 4** & **Coil 5** \\ \hline
**Coil 1** & 9.56 & & & & \\
**Coil 2** & 0.44 & 1.47 & & & \\
**Coil 3** & 0.37 & 0.76 & 4.61 & & \\
**Coil 4** & 0.05 & 0.06 & 0.76 & 1.47 & \\
**Coil 5** & 0.06 & 0.05 & 0.37 & 0.44 & 9.56 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Inductance matrix of the five coils. Figure 8: Axial (**left**) and radial (**right**) Lorentz force distributions of the coils. There are 15 resistors placed outside the heat-insulating theater in three groups to provide quench protection. The quench protection circuit diagram is depicted in Figure 9. As quench occurs, the energy is lost to resistance, converting to heat and releasing immediately. As seen in Figure 10, the current drops rapidly within 6 s after the quench occurs. More detailed information on quench protection simulation and experimental test results will be presented in future works. The magnetic stray field is shielded by external iron walls. The 5.5 cm-thick iron walls are constructed outside the superconducting magnet.