Tearing modes are the resistive magnetohydrodynamic (MHD) modes, which occur in all conventional tokamaks. They are known to degrade plasma confinement by creating magnetic islands which often lead to plasma disruption if their growth is not controlled. They have been extensively studied both theoretically and experimentally, as controlling them is foremost priority for every tokamak, including ITER and future large size tokamaks. In the high diamagnetic-drift frequency regime, the drift mode couple to tearing modes resulting in drift-tearing modes. The growth rate of Drift-Tearing modes decreases as its real frequency increases. In ADITYA as well as in ADITYA Upgrade tokamak, the frequency spectra of Mirnov signal show multiple frequency bands identified as drift tearing modes. Interestingly, the higher frequencies have precisely been found to be the integral multiples of the fundamental frequency. Further analysis reveals that these frequencies do not belong to different modes but harmonics of a single mode. These harmonics are also observed in the density as well as impurity emission. It has been observed that the harmonics generation is strongly correlated with the presence of high energetic electrons, known as runaway electrons, in the plasma. When the magnetic island grows in size, the runaway electrons get trap in these islands. And as the linear growth rate of tearing modes increases in presence of REs, the island size increases further due to the presence of runaway electrons, leading to harmonics generation.
