Chaotic Neoclassical Separatrix Dissipation in Parametric Drift-Wave Decay

Experiments and theory characterize a parametric decay instability between plasma drift waves when the standard nonlinear mode coupling is modified by chaotic dissipation on a separatrix. Experimentally, we utilize pure electron plasma columns with a central electrostatic ``squeeze'' barrier. We launch a large-amplitude $m_\theta=2$, $k_z=0$ dioctron mode, and observe it decay into an exponentially growing $m_\theta=1$, $z$-anti-symmetric ``Trapped Particle Diocotron Mode.'' Measurements of the growth rates $\Gamma_1$ and relative mode phases $\Delta \Theta_{12}$ during exponentiation accurately characterize both the standard nonlinear coupling term and the enhanced dissipation due to chaotic neoclassical transport. Here, the $m=2$ pump wave dynamically ``ruffles'' the separatrix, causing chaotic separatrix crossings. Similar enhancements are predicted and observed when the ruffle is static and the plasma drifts along the separatrix.\footnote{Dubin, Kabantsev, Driscoll, Phys Plas 19, 056102 (2012).} This novel chaotic dissipation is essentially {\it independent} of collisionality, and may dominate in the low-collisionality regimes of toroidal fusion plasmas where trapped particles and separatrices are endemic.