Recycling and neutral transport in the DIII-D tokamak

Abstract

Several diagnostics have been used to characterize the edge plasma in the DIII-D divertor region and thereby to understand the particle recycling and neutral particle transport in an open divertor. An array of Langmuir probes mounted on the carbon divertor plates determines that generally the electron temperature is low ( T e ⋍ 10–20 eV ), and the electron density is high ( n e ⋍ 5 × 10 19 m −3 ). The edge plasma temperature and density are also measured by a moveable Langmuir probe mounted at the midplane of the plasma; the data from these shot-by-shot edge radial scans are then connected with the data for the core plasma obtained by Thomson scattering. The heat flux to the divertor plates is measured by an absolutely calibrated infrared camera; a plasma model is used to compare the heat flux with the measured T e and n e . The molecular neutral pressure at the edge of the plasma at several poloidal and toroidal locations is obtained from absolutely-calibrated pressure gauges; a gas puff enables in-situ gauge calibrations before each plasma shot. Typically, the divertor pressure is 10–50 times larger than that at the midplane. Time-resolved H α brightness measurements are obtained from an absolutely calibrated television camera viewing the divertor region from above; both strike points are viewed simultaneously. The emission from the inner and outer strike points are usually equal after the H-mode transition, but are often unequal after the first ELM in H-mode and during some phases of L-mode discharges. A tangential camera measures the falloff in emission from the X-point to the plasma midplane. These measurements have been modeled with the DEGAS neutral transport code specifically modified for DIII-D. The wall geometry, wall composition, measured magnetic geometry, and measured plasma parameters are inputs to the model. The code calculates the gauge pressure as a function of position and the H α television picture directly. During H-mode, we find a factor of two agreement in both the measured pressures and the absolute H α brightnesses with one exception: the measured divertor pressure is higher than the code prediction. However, we find that a local gas source located below the X-point equal to only 10% of the divertor recycling source will bring the divertor pressure into agreement with the data. A difference in the electron temperature and density at the two strike points has been used to model the asymmetric H α emissions. Comparisons have been made between the model and the data during H-mode periods of the discharge. The model has also been used to predict the influence of baffles to form a more closed divertor configuration.