Exploring the impact of ion velocity distribution functions on plasma-sheath formation

Measuring the change in the sheath structure under the influence of background ion flows will facilitate a better understanding of the kinetic form of the generalized Bohm criterion. A dedicated device, the Sheath Experiment & Exploration Device (SEED), was constructed at the University of Wisconsin – Madison to study the impact of ion velocity distribution functions on plasma-sheath formation. SEED consists of two chambers (source and target), where separate plasmas can be produced within their respective liners by thermionic electrons emitted from the heated filaments. Ion flows are generated by applying a higher potential bias on the anode of the source chamber than the target chamber. A 15-cm-diameter, biased stainless steel plate is placed on the target chamber axis to create various boundary conditions. Diagnostic tools such as Langmuir probes (LP), emissive probes (EP), and laser-induced fluorescence (LIF) are implemented to measure the spatial profiles in front of the plate. Preliminary measurements show a plasma potential of 2-3 V, plasma parameters of T_e ~ 2-3 eV and n_e ~ 10⁸~10⁹ cm^-3.