Weak external helical fields can act as local resonant disturbances in a tokamak plasma. Such fields can therefore be used as 'active magnetic probes' in the study of MHD instabilities in tokamak plasmas. For this purpose, ℓ = 2/n = 1 and ℓ = 3/n = 1 helical windings have been installed on the HT-6B and HT-6M tokamaks. Diagnostic signals (using magnetic probes, soft X-ray detectors, spectroscopy, electron cyclotron emission, etc.) from MHD instabilities have been obtained and studied during discharges with programmed resonant helical fields (RHFs). It was found that an optimized value of the ℓ = 2 RHF strongly suppressed not only the m = 2/n = 1 magnetic perturbation but also the 3/1, 4/1 and 1/1 modes. It was unexpected that the ℓ = 3 RHF could suppress all m = 2,3,4 modes, as well as (and even more strongly) the m = 1 mode. The ℓ = 2 and ℓ = 3 RHFs could also substantially modify the sawtooth oscillation. During these processes, all MHD modes with different m numbers were tightly coupled, which indicates a global coherent mode structure over the whole plasma rather than individual mode behaviour.
Direct observation of tokamak plasma responses to the externally applied rotating helical magnetic field in the small tokamak HYBTOK-II