Identifying the location of the OMP separatrix in DIII-D using power accounting

In order to identify reliable scalings for the scrape-off layer (SOL) power width it is necessary to know the location of the separatrix in divertor tokamaks as accurately as possible, specifically its location at the outside midplane (OMP) the standard reference location. Two methods are described which use power accounting to improve the accuracy of identifying the location of the OMP separatrix. The first uses the infrared-measured deposited power profile at the outer target as the primary input, the ' method'. The second uses the measured power input to the SOL, obtained by subtracting the power radiated from inside the separatrix from the total heating power, the ' method'. These two power accounting methods are illustrated with the examples of 21 H-mode DIII-D discharges. High spatial resolution Thomson scattering measured profiles of ne and Te for the main SOL near the OMP are also used as primary input to the analysis; only between-edge localized mode data are used here. The Thomson profiles are used to calculate the electron parallel conducted heat flux profiles which are then matched to the measured and by adjusting the location of the OMP separatrix relative to that of the Thomson data. For these attached discharges, it is found that the values of given by the two power accounting methods agree to within ∼1 mm of each other and also to within ∼1 mm of the values given by the 'standard DIII-D method' described by Porter et al (1998 Phys. Plasmas5 1410). The shifted results in only modest changes to the values of ne and Te at the OMP separatrix relative to the 'standard' values, increasing by 8% and by 20%.