Studies of detached plasma in the ULS divertor simulator

Summary form only given. We report on experimental and modelling studies of "detached " plasma operation in the UMIST Linear System (ULS) divertor simulator. The ULS is a device designed to study a range of edge plasma physics issues relevant to tokamak gas target divertors and is capable of producing steady-state plasmas with electron densities and temperatures in the range 10/sup 17/-10/sup 19/ m/sup -3/ and 2-15 eV respectively; this plasma is made to flow into a separate gas target chamber into which a variety of gases can be introduced. Previous studies of detached plasmas in the ULS have centred on the interaction between hydrogen plasma and low pressure (< 10 mTorr) neutral hydrogen gas and have identified a regime in which molecular activated recombination processes appear to be the dominant plasma loss mechanism (MG Rusbridge et al, Plas. Phys. Cont. Fus. 42, 588 (2000)). Here we report on further studies in which the upstream plasma parameters are varied such that three-body and radiative electron-ion recombination (EIR) of hydrogen plasmas can be dominant. Spectroscopic and Langmuir probes data have demonstrated the resulting highly non-equilibrium distribution of excited neutral states resulting from these recombining plasmas. Evidence of hysteresis is found in the transition between the two modes (EIR and MAR) of recombination. Initial modelling of the recombination region in the target chamber is being undertaken using simplified one-dimensional electron energy balance and continuity equations (Krasheninnikov et al, Phys. Plas. 4, 1644 (1997)). We determine the factors that govern the threshold between MAR and EIR dominant detached regimes in terms of the upstream plasma parameters. We discuss the significance of these results for future divertor simulator research.