Gas puff induced drift waves in ADITYA and ADITYA-U Tokamak

The gradients in the radial profiles of density and temperature gives rise to various instabilities which aid in the transport of the particles across the magnetic field lines in tokamaks. These instabilities, signature of whose are embedded in fluctuations observed in various physical quantities like plasma potential, density, etc. are required to be understood properly for mitigating them. Here, we report the excitation of a low frequency mode in the density and floating potential fluctuations in presence of gas puffing in typical discharges of ADITYA-U tokamak, a medium sized air core limiter tokamak. The fluctuations in electron density are measured with microwave interferometry and Langmuir probes. To characterize the observed fluctuations, the Fourier spectrum analysis has been performed on the time-scale data of these fluctuations. The low frequency modes can be characterized as drift waves excited by an external power drive, the ohmic transformer power supply in our case. However, the growth rates calculated from analytical expressions are too less to be observed, as seen in the analysis of density fluctuations also. Incidentally, the presence of gas puff seems to modify the growth rate of the mode. The introduction of neutral particles by the gas puff seems to play a dominant role in the excitation of these modes