Interaction of externally applied helical field with tokamak plasma

Previous analyses for the interaction between a rotating helical field and the plasma column is mostly limited within vacuum magnetic field model. In the practical situation, the response of plasma to the externally applied h elical field, i.e. shielding current near the resonant surface and current profile modification due to island formation, often play important roles. R ecently, a strong deviation from a vacuum field model is experimentally obs erved in CSTN-IV device in Nagoya University. In this work, simulations usi ng a reduced set of resistive MHD equations in cylindrical geometry are car ried out and an attempt is made to find the plasma response effects on the externally applied helical field. We show that the deviation from a vacuum model is mainly comes from the perturbed current due to a helical deformati on for low S plasma (or for low frequency case). For high S, or high freque ncy case, the penetrating process is also affected by the shielding current near the resonant surface.