Interaction of MHD Activity with Edge Plasma Turbulence and its Consequences on Plasma Confinement in ADITYA-U Tokamak

Plasma turbulence is known to be one of the strong driving mechanisms of energy and particle transport, degrading the plasma confinement in tokamaks. In addition to the free energy associated with the gradients in temperature, density, and potential, magnetic fluctuations also play a key role in modifying the turbulence. In ADITYA-U tokamak, a coupling between magnetic fluctuations and edge plasma turbulence is observed in high MHD plasma discharges. It is seen that the MHD modes, mainly the m/n = 2/1, beyond an amplitude threshold value of 0.3- 0.4%, excite coherent oscillations in the density and floating potential having the same frequency as the MHD mode. Interestingly, the mode investigation of the excited mode reveals that the coherent mode in edge potential fluctuation has a mode number of m / n = 2 / 1 whereas the edge density fluctuation has a mode number of m / n = 1 / 1. Beyond the threshold, the coupled power fraction between the MHD modes and density and potential fluctuations scales linearly with the magnitude of fluctuations.

Furthermore, the growth rates of coherent oscillations in density and potential are also found to be dependent on the growth rate of MHD modes, however, the mode numbers of the excited mode are found to be independent on the MHD mode frequency. The MHD-induced coherent modes result in a reduction in edge turbulence and associated transport. This is contrary to the expectation of a detrimental effect on confinement due to increased amplitudes of the MHD modes. When the MHD activity amplitude starts to decrease, the total power in edge plasma fluctuations increases substantially indicating enhanced turbulence due to decrement in magnetic island width. This observation of energy transfer between the magnetic and edge plasma fluctuations suggests the existence of a channel for energy transfer between the two. In this paper, we report the detailed experimental investigation of this phenomenon in ADITYA-U tokamak's high MHD activity regime. The experimental findings unfold potential areas for further research to better understand nonlinear dynamics involved in interactions between edge plasma turbulence and MHD activity.