Heat load distribution on castellated W/Cu plasma facing components for divertor target in EAST*

In magnetic confinement fusion devices, the high heat flux coming from core plasma and depositing on the surface of plasma facing components (PFCs) is a concerned issue, in view of both engineering and physical perspectives. The EAST, as a superconducting tokamak similar to the future ITER, is installed the ITER-like castellated PFCs for both the upper and lower divertor in EAST. However, the previous results show the regular damage, i.e. melting, distribution along both toroidal and poloidal directions on these castellated PFCs, which may be attributed to the heat load distribution. Consequently, it is imperative to comprehend the characteristics of heat load distribution on castellated PFCs in the EAST.
Firstly, the PFC-flux code was employed to investigate the toroidal distribution of heat load on the PFC surface. By establishing an accurate model and setting reasonable boundary conditions, the heat load deposition pattern along the toroidal direction on cassette PFCs for the divertor was successfully simulated and analyzed. It was found that the heat load deposition pattern on a single PFC exhibits a significantly non-uniform distribution in the toroidal direction, and changes abruptly at chamfer positions in the gaps. The heat load deposition pattern for each PFC is generally consistent and shows periodic change. One of two edge parts of each PFC often be shadowed by a neighboring PFC. The heat flux on the surface of inner target and outer vertical target for both upper and lower divertor monotonically increases or decreases along the toroidal direction, whereas the heat flux of the outer horizontal target for the lower divertor is uniformly distributed, except for at chamfer positions. The change in the inclination angle of magnetic field lines is a primary reason for the non-uniform distribution of heat flux in the toroidal direction. Additionally, the chamfer sizes also affect the toroidal distribution of heat load. The toroidal distribution of heat flux obtained from PFC-flux simulation aligns with the toroidal distribution of temperature measured by IR cameras.
Secondly, to obtain the heat flux and its distribution on the lower divertor target, a 3D Finite Element Method model of the W/Cu flat-type component is constructed, incorporating actual thermal conditions. By utilizing the actual toroidal distribution profile (PFC-flux calculation) and assuming a varying peak heat flux (q∥) along with its poloidal decay parameters (decay length λq in the poloidal direction and the Gaussian spreading width S), a simulation temperature distribution can be successfully obtained using actual cooling conditions. And then, the simulation temperature distribution was compared with the temperature measured by a high-resolution IR camera (~1.1 mm/pixel). By adjusting the q, λq, and S, a very close temperature distribution to the temperature measured by IR diagnostic was obtained, and subsequently, the heat flux parameters were finally determined. Taking a discharge shot (#123059 with ~10MW heating power) as an example, the heat flux parameters were obtained as follows: q∥~216 MW/m², with λq~6.2 mm and S~1.2 mm, which are well in agreement with those form langmuir probes.
Such study of the distribution feature of heat load on castellated W/Cu PFCs for divertor targets can provide important data for engineering optimization and plasma operation in EAST, while also accumulate crucial experiences for other tokamaks.