Comparison of hybrid and baseline ELMy H-mode confinement in JET with the carbon wall

Abstract

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 b N~2 .5-3, H 98 < 1.5 compared to baseline H-mode plasmas, which typically have b N~1 .5-2, H 98 ~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 [most recently in M. Maslov et al., Nucl. Fusion 49] is not confirmed by the current study. Both low and high triangularity hybrid plasmas feature T i /T e > 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.

Pedestal study across a deuterium fuelling scan for highδELMy H-mode plasmas on JET with the carbon wall

“…On average there is good agreement between the model results and the experimental pre-ELM measurements where the average ratio of predicted pedestal height to observed pedestal height is 1.04±0.22 and the average ratio of predicted pedestal width to observed pedestal width is 1.01±0.23. This spread is within the range of EPED predictive accuracy as observed in a wider multi machine comparison [20,52,53,60]. However the JET results do show a number of differences in comparison to studies on other machines.…”

First operation with the JET International Thermonuclear Experimental Reactor-like wall

“…This database has been compared with a similar one which was establish in JET-C [65]. In JET-C, baseline ELMy Hmode plasmas achieved good normalised confinement with H98 ≈ 1 in un-fuelled plasmas.…”

The impact of poloidal asymmetries on tungsten transport in the core of JET H-mode plasmas

“…In addition, hybrid scenarios have regularly lower densities and higher values of normalized plasma pressure b N ¼ hbi=ðI p =aBÞ (where hbi is the volume average of 2l 0 p/B 2 measured in %, a is the minor radius in m, B is the magnetic field in T and I p is the plasma current in MA). 24 The interest of the experimental conditions regularly achieved in baseline scenarios is that the W accumulation is usually determined in a radial window which is not as centrally localized as observed in hybrid scenarios, where the behaviour of the W transport is tracked by the behaviour of the background density and temperature profiles very close to the magnetic axis (r/a 0.3). This is shown in Fig.…”