Effect of neon seeding on the pedestal structure and stability in the JET-ILW deuterium JET-ITER baseline scenario using both ideal and resistive MHD

Abstract In this work, the effect of neon seeding in the JET-ILW JET-ITER baseline scenario on pedestal structure, global stability and local stability is studied. Increased neon seeding leads to an increase in the pedestal electron and ion temperature while reducing the density. The combined effect leads to an increase in the pedestal top pressure. The dataset exhibits a mix of small edge localized mode (ELM)-like events and large ELMs. As the neon seeding is increased the frequency of the large ELMs goes down until the ELMs disappear completely. Infinite-n ballooning stability analysis shows that the seeded shots are all unstable to infinite-n ballooning modes at the bottom of the pedestal which could explain the small ELM-like events. Both ideal and resistive magnetohydrodynamic (MHD) stability analysis of the pedestal is performed and it is shown that resistive MHD can well describe the pre-ELM pedestal profiles for the large ELMs. The impact of the seeding on the resistive MHD stability is mainly tied to two competing effects. Firstly, increasing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>Z</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>eff</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> leads to an increased resistivity which destabilizes the resistive modes. Secondly, the diamagnetic frequency of the pedestal changes due to changes in the pedestal profiles that has a stronger impact on the resistive modes compared to the ideal ones.