A Fast Double Null Control Concept Enabling Compact Fusion Reactors

The STEP prototype power plant (SPP) will utilize a double null diverted (DND) plasma topology to manage very high divertor heat fluxes within a compact design. The DND configuration provides advantages for divertor power balancing and H-mode access while it is connected, i.e., $\delta R_{\text {sep}}$ is small. Due to vertical instabilities and limitations of active vertical control, the plasma may deviate from a connected DND resulting in imbalanced divertor heat loads and potentially loss of H-mode, both compromising the viability of the plasma scenario. To mitigate this, we propose a novel double null balance (DNB) control scheme to control $\delta R_{\text {sep}}$ in conjunction with plasma vertical position control, preventing disconnection. In this study, we test an implementation of DNB control on TCV (tokamak à configuration variable). This controller demonstrates greatly improved control of $\delta R_{\text {sep}}$ , maintaining connection during vertical steps of up to +/−3 cm. The controller also demonstrates effective balancing of radiated power during these excursions, preventing radiated power spikes to the inner divertor regions. These positive results are a very encouraging proof of concept for the DNB control, warranting further study and consideration for potential integration into the SPP and other future double-null power plant designs.