Direct observation of tokamak plasma responses to the externally applied rotating helical magnetic field in the small tokamak HYBTOK-II

Abstract

Direct observations of tokamak plasma responses to an externally applied rotating helical magnetic perturbation (RHMP) have been performed on the small tokamak device HYBTOK-II in order to clarify the penetration process of the RHMP into tokamak plasmas. Not only electromagnetic but also electrostatic fluctuations have been measured with small magnetic probes and a four-pin Langmuir probe, which were inserted inside the plasma. As regards the observed radial profiles of the RHMP in the plasma, the low Doppler-shifted frequency of RHMP produces the amplification of RHMP in the plasma due to magnetic island formation, while the expected attenuation of RHMP in the plasma has been found in the case of the high Doppler-shifted frequency. It is also found that the observed radial electric fields with the four-pin Langmuir probe have strong dependences on the driving frequency of the RHMP and the toroidal magnetic field. A theoretical calculation of the Alfvén wave propagation driven by local helical coils in HYBTOK-II has also been carried out. The observed electric fields are discussed in terms of the Alfvén resonance and the growth of magnetic islands.