The application of RMP is a feasible method to control MHD instabilities such as ELMs and TMs. The research of plasma response to RMP is important to understand the underlying physics. The differential phase Δφ which is defined by the phase difference between the current of the top and bottom RMP coils plays an important role in the plasma response [1]. The magnetic island will form in plasma when the amplitude of RMP exceeds a critical threshold. Recently, the experiments of the Δφ scan were carried out on J-TEXT to investigate the plasma response to RMP. The signal from the High Field-Side (HFS) locked mode detector shows that there is a near-linear plasma response before RMP field penetration. The dependence of response amplitude on Δφ is very close to that of the vacuum 2/1 RMP component on Δφ. It implies that the 2/1 screening response dominates in these lower RMP amplitude cases.

The density scaling of 3/1 RMP field penetration threshold was carried out in different Δφ (180°&270°). Successfully 3/1 RMP penetration is observed in a much wider electron density range (0.5~4×10¹⁹ m^-3) compared to the previous J-TEXT experiment [2]. The 3/1 RMP penetration threshold has been observed to vary non-monotonically with the increase of density on J-TEXT, in a similar manner as the density scaling of the 2/1 RMP penetration threshold [3]. Although the RMP current is different, the density scaling with penetration thresholds represented by the vacuum 3/1 RMP component are very close for these two Δφ. It suggests that the 3/1 RMP component dominates the 3/1 RMP penetration process while other components have few effects.

RMP;field penetration;plasma response