Preliminary assessment of vertical instability with blanket in CFETR

This paper summarizes the preliminary assessment results of plasma vertical instability, based on six highly elongated plasma equilibria in the Chinese Fusion Engineering Test Reactor (CFETR) presently under design in China, by considering the stabilization effect of vessel and blanket modules. The double-layer vacuum vessel is found to be too far away from the plasma to provide effective passive stabilization, in terms of reducing the vertical instability growth rate. The blanket modules (BMs) are found to provide good stabilization, thanks to the associated conducting structures, first wall, manifold and back-plates being much closer to the plasma. The CFETR BMs, with reference material conductivity, have a good capability of slowing down growth rate to a level below 12.6 s−1. On the other hand, the vessel + BMs penetration time of τw = 1081.2 ms and the blanket-alone penetration time of τb = 879 ms are both much longer than the most unstable growth time of τg = 79.4 ms, resulting in a strong shielding of external magnetic fields. The effect of BMs conductivity on the mode growth rate and estimate of the penetration time of conducting structures are performed to allow better prediction of the vertical instability in CFETR. The BMs mid-plane stabilizing structures are found to have very weak impact on the mode growth rate and the field penetration, and thus can be removed from consideration. The substantial difference between τw and τg indicates that vertical control will be lost before external radial field penetrates into the plasma region and interacts with plasma. As an important conclusion, this study recommends a design of in-vessel coils, powered by fast power supplies for controlling the vertical instability in CFETR.