# Conceptual Design of a Compact Divertor Heat Load Simulation Device: HIT-PSI
## 2 Designs of the Device and Its Sub-Systems
### Plasma Source  
Both RF and cascaded arc sources have been experimentally demonstrated to satisfy the demand for the plasma source of a linear plasma device with ITER-like high heat flux by MPEX and Magnum-PSI [20,21]. However, the RF sources are typically of a lower density, and thus other assisting heating means are necessary to achieve a high plasma temperature. It is then a great technical challenge with notable cost. By contrast, cascaded arc sources are more advantageous since they can generate higher-density plasmas with a temperature of up to several eVs without extra heating. Consequently, we applied cascaded arc sources in HIT-PSI.  
The cascaded arc plasma source was invented in the 1960s by [PERSON] [25] and first constructed and optimized with three channels in 2009 [26] at Eindhoven University of Technology. It has also been experimentally explored at Harbin Institute of Technology for years [27]. The three-channel cascaded arc plasma source and its anode flange are shown in Figure 2. Three to five cascade plates made of pure copper are sandwiched between the anode flange and separated by Boron nitride gaskets, with water cooling. Molybdenum rings are inserted into the cascade plates to face the plasma. Three cathode pins made of tungsten are put on top. For argon gas discharge with no magnetic field, each current channel can reach 160 A, and the plasma density may exceed \(10^{19}\) m\({}^{-3}\).  
Multiple channels of the cascaded arc plasma cannot increase the plasma beam density, especially when a strong magnetic field exists to limit the mix of separated channel beams, resulting in an uneven plasma density. Therefore, either a single-channel or a three-channel cascaded arc plasma source has been applied to meet the requirement of various features of the plasma beam in different operation scenarios.  
The water-cooling architecture of cascade plates is a key factor for the long-term steady-state operation of the arc source. An optimized waterway designed by COMSOL Multiphysics is shown in Figure 3, which is efficient in terms of heat dissipation and water  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Facility** & **Source** & **Te** & **Ne** & **Particle Flux** & **B (T)** & **Heat Flux (MW/m\({}^{2}\))** & **Ref.** \\ \hline PISCES-B & RF & -20 & \(10^{19}\) & \(10^{22}\)-\(10^{23}\) & 0.2 & - & [7,8] \\ NACDIS-II & RF & -20 & \(10^{19}\) & \(10^{22}\)-\(10^{23}\) & 0.4 & - & [9] \\ STEP & LaB6 & -20 & \(10^{19}\) & \(10^{22}\)–\(10^{23}\) & 0.4 & - & [23] \\ Magnum-PSI & Cascaded arc & \textless{}10 & \(\geq\)\(10^{21}\) & \(\gtrsim\)\(10^{24}\) & 2.5 & \textgreater{}10 & [20] \\ MPEX & RF & \textless{}40 & \(10^{19}\) & \(10^{22}\)–\(10^{23}\) & 5.0 & \textgreater{}10 & [21] \\
**HIT-PSI** & **Cascaded arc** & **\textless{}5** & \(\geq\)\(10^{21}\)** & **\textgreater{}10\({}^{24}\)** & **2.0** & **\textgreater{}10** & \\ ITER Divertor & - & 1–10 & \(10^{21}\) & \(\gtrsim\)\(10^{24}\) & 5.0 & \textgreater{}10 & [24] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters of ITER required and existing/designed devices.  
Figure 2: Schematic drawing of a three-channel cascaded arc plasma source and its anode flange for HIT-PSI.  
pressure reduction. Complex waterway designs can be achieved to maximize water cooling efficiency by diffusion welding.