The role of lid layers on the performance of von Karman type dynamos

The von Karman sodium (VKS) experiment is intended to investigate the hydromagnetic dynamo effect in a highly turbulent flow. Although much effort has been spent in the optimization of the flow structure and the lateral boundary conditions in order to minimize the critical magnetic Reynolds number, no dynamo effect has been seen in the recent experiments. The measured induced magnetic fields deviate significantly from the computed ones. In order to understand these experimental result, we focus on the role of lid layers, i.e. layers of liquid sodium between the impellers and the end walls of the cylinder. It is shown that static lid layers yield a moderate increase of the criticial Rm by approximately 12 per cent. A more dramatic increase by 100 until 200 per cent can occur when some rotational flow is taken into account in those layers. Possible solutions of this grave problem are discussed for the real dynamo facility.