Understanding the dynamics of the inductive plasma formation and its application to create doublet shaped plasma in the TCV tokamak

The dynamics of the plasma formation in TCV are revisited with the goal of improving reliability and developing new scenarios such as the creation of doublet configurations. A database for the plasma formation scenarios in TCV reveals that 15$\%$ of the attempts to form a plasma fail during the burn-through phase. Plasma formation dynamics are greatly affected by the difference between programmed and obtained plasma current ramp rates that can lead to oscillations in $I_P$ when the $I_{P}$ feedback control is activated. This mismatch in $I_P$ also propagates into the radial position control. Failed burn-throughs occur when the Ohmic heating power is insufficient either since $I_P$ rises too slow or due to a combined effect of the $I_P$ feedback oscillations and a regularly occurring MHD instability. Several strategies to improve the present plasma formation scenario have been implemented. Based on the improved understanding of the plasma formation dynamics, a strategy has been developed to create and control a doublet configuration by merging of two droplet-shaped plasma requiring simultaneous breakdown at two locations.