Study of the spectrum effect on the threshold of resonant magnetic perturbation penetration on J-TEXT

The spectrum effect on the penetration of resonant magnetic perturbation (RMP) is studied with upgraded in-vessel RMP coils on J-TEXT. The poloidal spectrum of the RMP field, especially the amplitudes of 2/1 and 3/1 components, can be varied by the phase difference between the upper and lower coil rows, ∆ϕ = ϕ_top-ϕ_bottom, where ϕ_top and ϕ_bottom are the toroidal phases of the n = 1 field of each coil row. The type of RMP penetration is found to be related to ∆ϕ, including the RMP penetration of either 2/1 or 3/1 RMP and the successive penetrations of 3/1 RMP followed by the 2/1 RMP. For cases with penetration of only one RMP component, the penetration thresholds measured by the corresponding resonant component are close for various ∆ϕ. However, the 2/1 RMP penetration threshold is significantly reduced if the 3/1 locked island is formed in advance. The changes in the rotation profile due to 3/1 locked island formation could partially contribute to the reduction of the 2/1 thresholds.