# Conceptual Design of a Compact Divertor Heat Load Simulation Device: HIT-PSI
## 3 Numerical Simulation Device Capability with SOLPS  
The capacity of the device is simulated by SOLPS-ITER [30], which has been widely used in linear plasma device simulation [31; 32; 33; 34]. SOLPS-ITER is a code package for tokamak edge plasmas simulations with two main parts, B2.5 for solving multi-fluid simulation with Braginskii equations and EIRENE for neutral particles and molecular ions transport simulation by Monte Carlo methods. In our simulation, we applied a 2D cylindrical coordinate system (R, Z) with axial symmetry, and the corresponding mesh grid is shown in Figure 14. The lower line is the symmetry axis in a symmetrical boundary condition with zero particle density flux. The quasi-orthogonal mesh region with a high resolution denotes plasma, with grids determined by magnetic field lines axially (in blue). The boundary condition on the  
Figure 13: Schematic drawing of the rotatable angle target system and the facing plasma part. outermost line has a decay range of 0.5 cm. The front and rear end in the axial direction are the Bohm boundaries, and energy transmission factors for electrons and ions are 1.0 and 1.5, respectively. The green triangular mesh of a lower resolution is for the EIRENE, where the recycling rate at the two pumping surfaces (bold red lines) is 0.95, and the rest are wall boundaries. There is an open boundary at the target end (assuming Z = 0 at the source, here Z = 1.33 m). The drift effect is ignored, and Braginskii equations are solved only along the direction of the magnetic field. Two sets of anomalous transport coefficients: \(D_{\perp}=0.3\) m\({}^{2}\)/s, \(\chi_{e,i}=0.9\) m\({}^{2}\)/s (Case 1) and \(D_{\perp}=1.0\) m\({}^{2}\)/s, \(\chi_{e,i}=2.0\) m\({}^{2}\)/s (Case 2) are adopted, corresponding to strong and weak magnetic fields, respectively. Plasma parameters at the source location are shown in Figure 15a, which are obtained by adjusting the external particle source applied heating power to the source surface (Z = 0). Slight differences in the two cases are due to various grids corresponding to the magnetic fields generated by 275 A and 75 A coil currents. Figure 14: The simulation mesh grid distribution. The source and target positions are selected with a distance of 1.33 m.  
Figure 15: The electron density/temperature radial distributions at the source (**a**) and target (**b**) locations in the two cases. Figures 15b, 16 and 17 show the simulation results of the two cases. It can be seen that the general features of the two cases are basically the same, with very similar radial distributions of the electron temperature at the target end. The plasma density drops faster after leaving the source surface in Case 2 than that in Case 1, due to the more significant radial diffusion coefficient in a weaker field. In addition, one can find that when \(Z<1.1\) m, the electron temperature drops from 4 eV to 1 eV, accompanied by a gentle electron density bump. This region is called the'recombination front [29]', where the temperature is between 1 eV to 5 eV, causing ionization to dominate and leading to an electron density increase, but then electron density starts to decrease due to energy relaxation between ions and electrons. When \(Z>1.1\) m, the electron temperature drops to \(\sim\)1 eV, and thus recombination is dominant, resulting in a significant electron density drop. In addition, due to the existence of the pumping surface, the neutral particle pressure does not increase obviously along the axial direction. Figure 16: Simulation results of 2D distributions of electron density of Case 1 (**a**) and Case 2 (**b**), and electron temperature of Case 1 (**c**) and Case 2 (**d**). The source position in Figure 14 corresponds to \(Z=0\).