A comparison of the influence of plasmoid-drift mechanisms on plasma fuelling by cryogenic pellets in ITER and Wendelstein 7-X
Efficient plasma core fuelling is a key issue for achieving steady-state scenarios in large magnetic confinement fusion devices. At present, the most promising technique to achieve this goal is the injection of cryogenic pellets. In this paper, the injection of nominal pellets into standard plasma
scenarios in the tokamak ITER and the stellarator Wendelstein 7-X is studied by modelling analysis of the pellet ablation and particle deposition characteristics, focussing on the evaluation of the expected differences in pellet plasmoid drifts in tokamaks and stellarators. Since the efficiency of the damping-drift mechanisms is predicted to depend on the magnetic configuration, device-specific characteristics are expected for the temporal evolution of the plasmoid drift acceleration. For instance, plasmoid-internal Pfirsch-Schlü ter currents dominate the drift damping process for stellarators, while
plasmoid-external currents are more relevant for tokamaks. Also, relatively larger drifts are in principle expected for W7-X due to higher field gradients in relation to machine dimensions. However, shorter plasmoid-internal charge reconnection lengths result in the plasmoid drift damping due to internal Pfirsch-Schlüter currents being more effective than in a tokamak. Therefore, the average relative drift displacement during the whole plasmoid homogenization phase may a priori be comparable in both magnetic configurations.