Nonlinear Interaction of Drift and Kelvin Helmholtz (KH) mode in a linear plasma device

Experiments are carried out in a linear magnetized plasma device, IMPED (Inverse Mirror Plasma Experimental Device) at different ratios (R_m) of magnetic fields for source and main chamber. Various configurations of electric probes are used during experiment. Probes are moved radially across the plasma column to measure density, temperature and plasma potential and their fluctuations. From the mean profiles it is found that plasma has density as well as potential gradient. Density gradient is responsible for driving Drift Waves (DW). Intrinsic potential gradient in presence of magnetic field generates poloidal plasma flows. Shear in poloidal flow triggers KH instability. Mean plasma profiles indicate the driving source of the instability. The spectral analysis of the potential and density fluctuation as a function of radius is used to characterize the instabilities. It is observed that, at higher R_m only DW in the range of 1-2 kHz and at lower R_m only KH in the range of 4-5 kHz exists. However, both DW and KH coexist in the intermediate range of R_m. At higher R_m, density gradient is stronger and drives the DW. As R_m is lowered, the gradient of E-field increases which drives the KH instability. In the intermediate range of R_m, not only both coexist but also they start to interact. This interaction is found to be nonlinear in wavenumber and frequency selection rules, f₊= f_KH + f_DW, f_- = f_KH – f_DW, and k₊ = k_KH + k_DW, k_- = k_KH - k_DW. Further, higher order spectral analysis of fluctuations, i.e. auto-bicoherence confirms the nonlinear interaction. The detailed experimental results of excitation and nonlinear interaction of DW and KH modes will be presented.