Impact of the new TCV baffled divertor upgrade on pedestal structure and performance

A new set of carbon tiles, neutral beam heating optics and gas baffles were installed on TCV during thebaffled divertor upgrade in early 2019. The installation of the baffles allows a deconvolution of the roles ofmain chamber and divertor neutral pressure on the H-mode pedestal structure. This physical barrier allowsrelatively high neutral pressures to be constrained to the divertor, thus preventing neutrals from entering themain chamber and potentially degrading core confinement. This study presents the experimentally measuredand modelled pedestal heights and structure for a series of H-mode discharges prior to and after this upgrade.Increased pedestal performance at high divertor neutral pressure was observed after the baffled divertorupgrade. This was consistent across all triangularities and outer target locations investigated and is attributedto higher pedestal top temperatures being maintained at high gas injection rates. ASTRA simulations indicatedbeam heating power coupled to the plasma did not significantly vary after the baffled divertor upgrade oras a function of divertor neutral gas pressure. Analysis of the pedestal structure exposed a strong correlationbetween pedestal performance and the density pedestal position prior to and after the baffled divertor upgrade.The baffled divertor upgrade limited the outward shift of the density pedestal, thus maintaining higher pedestalperformance at high divertor neutral pressures. Stability analysis indicated the majority of discharges studiedwere within 25% of the stability boundary. No correlation was found between the distance from the stabilityboundary and pedestal performance or structure. Comparison with the EPED1 model indicated that TCVdischarges do not have a fixed dependence between pedestal𝛽𝜃and pedestal width. A large variation inthe EPED1 relating parameter was observed and found to vary with the density pedestal position.