Experimental observation of Coexistence and Nonlinear Interactions of Low-Frequency Instabilities in a Magnetized Linear Plasma Device.

The Inverse Mirror Plasma Experimental Device (IMPED) is a magnetized, linear plasma machine designed to study low-frequency (ω< ω_ci ~50 kHz) waves and instabilities. By altering plasma control parameters such as pressure and magnetic field, IMPED allows for explicit variation of radial mean plasma profiles for density n(r), temperature T_e(r), and plasma potential V(r). A unique feature of IMPED is the ability to control these profiles through the change in ratio of the main chamber (B_m) to the source chamber (B_s) magnetic field, termed as R­_m (=B_m/B_s). The background and fluctuating plasma parameters are measured using various configurations of multiple in-situ electric probes at different spatial locations (r, θ, z). These measurements reveal local gradients in profiles of n(r), Te(r), V(r), which can inherently excite low-frequency primary instabilities. Intrinsic excitation and coexistence of Drift Waves (DW), Rayleigh-Taylor (RT), and Kelvin-Helmholtz (KH) instabilities and nonlinear interaction for RT-DW is observed. These instabilities can exist independently or coexist without significant interaction at lower pressures (5e-5 to 8e-5 mbar). However, as pressure increases (>2.5e-5 mbar), the nonlinear interactions become more pronounced. An increase in B_m (>500 G) results in a higher radial electric field (E_r ~130V/m), inducing a sheared poloidal flow that enhances the dominance of the KH mode. Also, Steeper density gradients develop with B_m, influencing the dynamics of RT (7-9 kHz) and DW (1-2 kHz) modes. Further, we report nonlinear mode interactions categorized by frequency (f₁= f₂ + f₃) and wavenumber (k₁= k₂ + k₃) matching conditions. Critical to these findings is the role of R_m, which governs the location of maxima in profile gradients, influencing the dynamics of RT-DW non-linear modes. Detailed experimental analysis and results providing insights into underlying instability dynamics will be presented.