The role of presheaths in establishing anisotropy at the sheath edge

Presheaths provide ion acceleration to the Bohm velocity. We consider modifications to the parallel and perpendicular IVDFs associated with the presheath. Laser-induced fluorescence (LIF) data obtained with a diode laser near a negatively-biased plate give the transverse and parallel metastable ion velocity distribution function profiles (measured with respect to the normal to the plate) in a low temperature, low pressure, DC multidipole argon discharge plasma. For a neutral pressure of 0.3~mTorr, the transverse temperature increases along the presheath from 0.026~eV in the bulk plasma to 0.058~eV at the presheath sheath boundary. This result is compared with PIC code simulations\footnote{A. Meige et al., {\em Phys. Plasmas}, 14, 032104 (2007)} and experimental results\footnote{N. Claire et al., {\em Phys. Plasmas}, 13, 062103 (2006)} found in the literature. The general dependence on presheath characteristics on neutral pressure is discussed. Apparently contradictory experiments and PIC code results are resolved.