Zonal Flows 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. 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 transport². Low-frequency zonal flows (LFZF) and high-frequency geodesic acoustic mode (GAM)^2,4 have been observed and studied in several toroidal devices, However, several features related to the nature of these modes are still not fully understood. LFZF and GAM have been studied in ADITYA-U tokamak using poloidaly and toroidally distributed Langmuir probe arrays. Detailed analysis of density and floating potential fluctuations revealed the presence of LFZFs and GAM-like modes in typical ADITYA-U discharges. It has been observed that although the LFZFs are electrostatic, the GAM-like modes are triggered in high MHD activity plasma discharges of ADITYA-U tokamak. Magnetic oscillations (m/n=2/1) above a threshold of 3 G drive a similar frequency oscillation in the edge floating potential and ion saturation current fluctuations exciting a GAM-like mode with m/n=2/1 for floating potential and m/n=1/1 for ion saturation current fluctuation. A radial electric field in order of 1-3 kV/m up to a length scale in order of 1 cm, can be observed outside the magnetic island. When the magnetic island grows in size up to 5-6 cm, this radial electric field modifies the edge dynamics. This explains the existence of a threshold in magnetic fluctuation’s amplitude for this coupling to trigger. The radial electric field fluctuation induces a similar mode structure in floating potential and poloidal velocity fluctuation causes a GAM-like structure in density fluctuation. The excitation of this GAM-like mode coincides with a decrease in broadband turbulence, associated transport, and an increase in confinement. The complete analysis of LFZFs and GAM-like modes in ADITYA-U plasma will be presented in this paper.