Symmetry Breaking and the Inverse Energy Cascade in a Plasma

The application of electrostatic bias to both low density plasma with coherent fluctuations and high density plasma with turbulent fluctuations confined by a magnetic dipole are investigated. Previously, electrostatic biasing of low density plasma was symmetric, drove rapid plasma rotation, and excited the centrifugal interchange instability. This research investigates the application of non-symmetric bias and the influence of broken symmetry on strongly turbulent plasmas. Non-symmetric bias is applied through either point biasing or an equatorial array spanning the device. In both cases, the spatial symmetry of applied bias dramatically effects the plasma fluctuations. With bias applied, the plasma achieves a new equilibrium characterized by amplified low order modes and diminished amplitude of higher order modes. Although the turbulent spectrum changes, the RMS fluctuation level is unchanged by the bias. Bias also causes the turbulent electrostatic fluctuations to coalesce into a quasi-coherent mode and the appearance of increased coherence. The effect of bias configuration is also seen to change the measured levels of nonlinear coupling. Non-symmetric biasing increases nonlinear coupling while symmetric biasing leaves the coupling unchanged. These results represent the first experimental demonstration of symmetry breaking driving the inverse energy cascade in a quasi-two dimensional plasma system. The application of dynamic and rotating electrostatic bias as well as plans for applying turbulent feedback are discussed.