MHD triggered GAM in ADITYA-U tokamak

Plasma turbulence is known to be one of the strong driving mechanisms of energy and particle transport which can degrade the plasma confinement in tokamaks. Hence, it is important to understand different physical processes which drive and control turbulence. It has been observed that the self-generated axis-symmetric structures such as zonal flows play a key role in controlling the turbulence and associated transport [1, 2]. Low-frequency zonal flows (LFZF) and high-frequency geodesic acoustic mode (GAM) have been observed and studied in several toroidal devices [2]. The effect of MHD modes on GAMs is also an important aspect that is being explored in several experiments in devices like TCABR and STOR-M tokamak [3,4].



Signatures of coupling between the MHD oscillations and fluctuations in edge density (n_e) and floating potential (V_f) have been observed in ADITYA-U tokamak. Systematic analysis of a large discharge database (~100) indicates that these coherent edge fluctuations are triggered only when the MHD amplitude exceeds a threshold value otherwise the fluctuation frequencies are in a broadband range. Detailed analysis of these MHD triggered fluctuations using toroidally and poloidaly distributed Langmuir probe arrays revealed that the poloidal mode number (m) for n_e fluctuations is ≈ 1.156 and for V_f it is ≈ 0.03 which is a characteristic of GAMs_. These results indicate that these MHD oscillations are the origin of GAMs in ADITYA-U tokamak. In this paper, we will present a detailed analysis of this coupling and the resulting GAMs and their characteristics.