Hybrid Compared to Baseline ELMy H-mode Confinement in JET
This paper compares the confinement in JET low and high triangularity hybrid and baseline ELMy H-mode regimes in a large database of 112 plasmas and identifies the location and source for the enhanced confinement factors in the hybrid plasmas. JET hybrid plasmas feature enhanced performance, and typically have bN ~ 2.5-3, H98 < 1.5 compared to baseline H-mode plasmas, which typically have bN ~ 1.5-2, H98 ~ 1. The higher global pressure achieved in hybrid plasmas is obtained through a tight coupling of the core and pedestal pressures. The confinement in hybrid plasmas is therefore strongly linked to the obtainable pedestal confinement. A comparison of the EPED1 predictive pedestal model with the experimental pedestal pressures shows good agreement of the model predictions with the measured data, offering a possible explanation for the tight link between global and pedestal pressure through the stabilising effect of high b on the pedestal MHD stability. The electron kinetic profiles do not show a net profile peaking of the electron pressure, although the electron density and temperature show respectively decreasing and increasing trends with collisionality. Hence the confinement benefit from the electron profile peaking as reported in previous studies is not confirmed by the current study. Both low and high triangularity hybrid plasmas feature Ti/Te > 1 across the entire profile leading to a confinement benefit through the ions. Additional performance is obtained in the low triangularity hybrid plasmas through increased ion temperature profile peaking at low collisionality. Possible candidate parameters responsible for this enhanced ion temperature peaking have been identified as Ti/Te, s/q and rotational shear.