Non-resonant Particle Heating due to Collisional Separatrix Crossings

We observe plasma heating when a pure ion column is ``sloshed'' back and forth across a trapping separatrix, with heating rate larger than expected from simple collisional viscosity. Here, an externally applied theta-symmetric ``squeeze'' potential creates a velocity separatrix between trapped and passing particles, and weak collisions at rate $\nu_{c}$ cause separatrix crossings. The trapped particles are repeatedly compressed and expanded (by $\delta L$ at rate $f_{sl}$) whereas the passing particles counter-stream and Debye shield the resultant potential variations. LIF diagnostics clearly show the separatrix energy $E_{sep} (r)$, in close agreement with $(r,z)$ Boltmann-Poisson equilibrium calculations. With $\nu_{c} \ll 2\pi f_{sl} \ll 2\pi f_{plas}$, simple bounce-averaged transport theory of the separatrix boundaries layer predicts heating scaling as $\dot{{T}}/T\propto (\delta L/L)^{2} f_{sl} \sqrt {\nu_{c} /f_{sl} } \quad V_{sq}^{2} /T^{2}$, distinct from bulk-viscosity heating scaling as $\nu_{c}^{1} $. Experiments corroborate the scalings with $f_{sl} $ (and hence $\nu_{c}$), with $\delta L$, and with $V_{sq} $, and give overall quantitative agreement with theory within a factor-of-two.