Non-resonant Particle Heating due to Collsional Separatrix Crossings

We observe weak plasma heating when a pure ion column is ``sloshed'' back and forth across a partial trapping barrier, and coherent laser diagnostics characterize the resulting particle distributions. 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 $\delta L$) whereas the passing particles counter-stream and Debye shield the resultant potential variations. The LIF diagnostics then clearly determine the separatrix energy $E_{sep} (r)$, since the trapped and passing particle distributions are in-phase and out-of-phase with the plasma motion. The measured $E_{sep} (r)$ is in agreement with that calculated from a $(r,z)$ Boltzmann-Poisson equilibrium solution. Theory predicts heating from separatrix crossings scaling as $\nu_c^{1/2} E_{sep}^2 ( \delta L / L )^2 $, distinct from bulk viscosity heating scaling as $\nu_c^1 $. Experimental scalings with density and temperature will allow direct comparison to theory.