Understanding the dynamics of the inductive plasma initiation in the TCV tokamak

The dynamics of the plasma initiation in TCV are revisited with the goal of improving the reliability and efficiency of conventional plasma initiation scenarios and develop new scenarios such as a simultaneous breakdown at two locations for the creation of a doublet shaped plasma. Plasma initiation is divided into three phases: (i) avalanche/breakdown, (ii) plasma burn-through and (iii) plasma current ramp-up. The breakdown is initiated by creating a favourable magnetic configuration at an optimal neutral gas pressure (~10 -4 mbar) at the time when the loop voltage is close to its maximum value (~10V). The particularly low vessel resistivity (~50μΩ) results in vessel eddy currents that significantly alter the magnetic configuration during the breakdown phase. A method to estimate the eddy currents and reconstruct the poloidal magnetic field distribution up to the time of breakdown with the help of magnetic measurements was developed [1]. Visible light emission measurements using a fast framing camera are consistent with breakdown being initiated in the vicinity of the largest poloidal magnetic field null in the vessel, confirming the accuracy of the magnetic reconstruction. By improving the compensation for the stray field generated by the ohmic coil and the vessel eddy currents, breakdown was observed to occur at earlier times. Surprisingly, early breakdown was found to increase the likelihood of failure (plasma current extinction) in the burn-through phase. Failure during burn-through is caused by a mismatch between experimental open-loop plasma current ramp rate and the current control reference leading to strong oscillations in the plasma current once plasma current control feedback is activated. Based on this improved understanding of the dynamics of plasma initiation in TCV, possible solutions are proposed to improve it and results for some of the preliminary tests are shown.