Role of the separatrix density in the pedestal performance in deuterium low triangularity JET-ILW plasmas and comparison with JET-C

A reduction of the pedestal pressure with increasing separatrix density over pedestal density (n esep/n eped) has been observed in JET. The physics behind this correlation is investigated. The correlation is due to two distinct mechanisms. The increase of n esep/n eped till ≈0.4 shifts the pedestal pressure radially outwards, decreasing the peeling-balloning stability and reducing the pressure height. The effect of the position saturates above n esep/n eped ≈ 0.4. For higher values, the reduction of the pedestal pressure is ascribed to increased turbulent transport and, likely, to resistive MHD effects. The increase of n esep/n eped above ≈0.4 reduces ∇n e /n e, increasing η e and the pedestal turbulent transport. This reduces the pressure gradient and the pedestal temperature, producing an increase in the pedestal resistivity. The work suggests that the increase in resistivity might destabilize resistive balloning modes, further reducing the pedestal stability.