Pedestal Study in Dense High δ ELMy H-mode Plasmas on JET with the Carbon Wall

We present the results from a new fuelling scan database consisting of 15 high triangularity (δ ~ 0.41), Type I ELMy H-mode JET plasmas. As the fuelling level is increased from low, (ГD ~ 0.2 x 1022el/s, ne,ped/ngw = 0.7), to high dosing (ГD ~ 2.6 x 1022el/s, ne,ped/ngw = 1.0) the stored thermal energy shows no degradation. The ELM frequency decreases as density increases due to increased inter-ELM losses as shown by a slower build-up of stored energy for a high fuelling pulse in comparison to a low fuelling pulse. Consequently the ELMs at higher fuelling are referred to as 'mixed Type I/II' as opposed to 'pure Type I'. These findings are consistent with previous studies, however the pulses in the new database are better diagnosed and most notable have pedestal measurements provided by the JET High Resolution Thomson Scattering (HRTS) system. We continue by presenting, for the first time, the role of pedestal structure, as quantified by a least squares mtanh fit to the HRTS profiles, on the performance across the fuelling scan. The pedestal pressure increases throughout the ELM cycle for low and high fuelling pulses. The pedestal width narrows and peak pressure gradient increases during the ELM cycle for a low fuelling pulse, whereas at high fuelling the pedestal width and peak pressure gradient saturates towards the latter half of the ELM cycle. An ideal MHD stability analysis shows that both low and high fuelling plasmas move from stable to unstable approaching the ideal ballooning limit of the finite peeling ballooning stability boundary. On DIII-D and MAST the pedestal width increases whilst the peak pressure gradient remains constant during an ELM cycle, as expected from the leading pedestal model, EPED. Therefore the new JET fuelling scan database is used for a comparison with EPED predictions. EPED self-consistently predicts the pre-ELM width and height with an accuracy of 20%. On average EPED agrees well with experimental measurements however when presented as a function of pedestal density experiment and model show opposing trends. The experimental pre-ELM pressure pedestal height increases by ~20% and the width widens by ~55%, in poloidal flux space, for a high fuelling pulse in comparison to low fuelling pulse. The increase in pedestal pressure at high fuelling is attributed to the wider pedestal as the steep pressure gradient can be sustained over a large region at the plasma edge. In contrast EPED predicts a decrease of 25% in pedestal pressure and a decrease in pedestal width of 20% in flux space. We give two possible explanations for the disagreement. First, EPED under predicts the critical density, which marks the transition from kink-peeling to ballooning limited plasmas. Second, the stronger broadening of the experimental pedestal width than predicted by EPED is an indication that other transport related processes contribute to defining the pedestal width such as enhanced inter-ELM transport as observed at high fuelling, for mixed Type I/II ELMy pulses.
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