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We report on studies of detached plasma operation in the UMIST linear system (ULS). The ULS, designed to study a range of edge physics issues relevant to tokamak divertors, is capable of producing plasmas with electron densities and temperatures in the range 10 17 –10 19 m −3 and 2–15 eV, respectively. Previous studies of the interaction between the hydrogen plasma and low-pressure hydrogen gas have identified a regime where molecular activated recombination (MAR) processes dominate plasma losses. Here we report on studies in which the upstream plasma parameters are varied such that three-body and radiative electron–ion recombination (EIR) dominates. Initial modelling of the recombination region is undertaken using a simplified version of the one-dimensional electron energy and continuity equations. We determine the factors that govern the threshold between MAR and EIR dominated detached regimes in terms of upstream plasma parameters and compare this with predictions based on the competing effects of electron cooling and recombination.

We describe an experiment, the UMIST Linear System (ULS), in which a hydrogen plasma stream, guided by a longitudinal magnetic field, is injected through a diaphragm containing an orifice into a separately-pumped target chamber in which the neutral hydrogen pressure can be raised to a maximum of 8 mTorr. The stream is about 6 mm in diameter, has an electron temperature of up to 15 eV and an ion flux of 3×1018 s-1; it is supersonic with Mach number up to M≈3. We have studied both the passage of the stream through the orifice and the interaction of the supersonic plasma with neutral hydrogen in the target chamber. We find that transmission is incomplete even when the orifice diameter is five times that of the plasma; we attribute this to the presence of ion trajectories which extend well outside the visible plasma and are intercepted by the diaphragm. In the target chamber, the stream does not broaden, but the ion flux decreases approximately exponentially with distance, with a scale length of the order of the mean free path for momentum transfer in ion-neutral collisions, and much less than that expected for other processes, such as charge exchange or electron-ion recombination. Elastic collisions alone cannot decrease the flux, but would lead to a large accumulation of slow ions in thermal equilibrium with the neutral gas, which must be limited by some other loss process: collisional diffusion and electron-ion recombination are too slow, leading to a density approaching 1020 m-3. The observed density is of the order of 1018 m-3, requiring a process with a rate of 10-100 times faster. Calculated rates for molecular-activated recombination (MAR) of the slow ions are of this order, and the predicted density agrees with our observations to order of magnitude.

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.