Spatial structure of detached plasmas in the ULS divertor simulator

The UMIST Linear system (ULS) is a linear plasma device designed to study plasma and atomic physics issues relevant to tokamak divertors. The ULS produces a steady state plasma beam which interacts with neutral gas in a target chamber. Dependent on the upstream conditions, either electron-ion recombination (EIR) or molecular activated recombination (MAR) may dominate. Here we report on detailed studies of the plasma spatial structure in both regimes. A specially designed optical spectroscopy probe is used to measure the visible emission, with spatial resolution less than 5 mm. Atomic and molecular lines are identified, and the results are interpreted using a collisional-radiative model, enabling the MAR rate to be calculated. One dimensional plasma modelling also demonstrates a transition between EIR and MAR dominated regimes, as the upstream conditions are varied.