The efficiency of generating a helical current in magnetic islands for the purpose of suppression of neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) is studied experimentally in the ASDEX Upgrade tokamak. It is found that the efficiency of generating helical current by continuous current drive in a rotating island drops drastically as the width $2d$ of the co-ECCD driven current becomes larger than the island width $W$. However, by modulating the co-ECCD in phase with the rotating islands $O$ point, the efficiency can be recovered. The results are in good agreement with theoretical calculations taking into account the equilibration of the externally driven current on the island flux surfaces. The result is especially important for large next-step fusion devices, such as ITER, where $2d>W$ is expected to be unavoidable during NTM suppression, suggesting that modulation capability should be foreseen.

• Received 21 July 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.025005