This work discusses the possible mechanisms that have led to the degradation of the pedestal pressure height and pedestal stability of low triangularity peeling-ballooning (PB) limited pla-smas from JET with the carbon wall (JET-C) to JET with the ITER-like wall (JET-ILW). The work shows that the lower pedestal stability of JET-ILW is not directly caused by the presence of a metal wall, but rather due to the operational constraints that require operation with high gas fuelling. Recent results have suggested the possible role of the distance between the density and temperature pedestal positions (nepos − Tepos, also called the 'relative shift') with increasing gas fuelling rate on the PB stability of JET-ILW baseline plasmas (Stefanikova et al 2018 Nucl. Fusion58 056010). The work further extends the analysis of the role of the relative shift, showing that it plays an important role in the difference between the pedestal performance of JET-C and JET-ILW. Moreover, the work also shows that the pedestal density neped, Zeff, and pedestal pressure width wpe play an important role in this difference. The pedestal structure and stability have been studied both experimentally and by modelling. The modelling shows that the changes in nepos − Tepos, neped, Zeff, and wpe are sufficient to explain the differences in the pedestal performance between JET-C and JET-ILW PB limited discharges. A hypothesis describing the possible mechanisms leading to the degradation of the pedestal pressure and stability from JET-C to JET-ILW in PB limited plasmas is put forward.
Dimensionless scalings of confinement, heat transport and pedestal stability in JET-ILW and comparison with JET-C