MHD and Gyro-kinetic Stability of JET Pedestals

TL;DR

This study investigates the stability of JET plasma pedestals, revealing how fueling levels affect pedestal behavior and identifying the dominant MHD and micro-instability modes during the ELM cycle.

Contribution

It provides a comprehensive analysis of MHD and gyro-kinetic stability in JET pedestals, highlighting the effects of fueling on pedestal width, pressure gradient, and mode stability.

Findings

Pedestal width decreases with low fueling during ELM cycle.

Both plasmas become unstable to finite-n ideal MHD peeling-ballooning modes at ELM end.

Micro-tearing modes are present but sub-dominant to ion temperature gradient modes.

Abstract

The pedestal profile measurements in high triangularity JET plasmas show that with low fuelling the pedestal width decreases during the ELM cycle and with high fuelling it stays constant. In the low fuelling case the pedestal pressure gradient keeps increasing until the ELM crash and in the high fuelling case it initially increases then saturates during the ELM cycle. Stability analysis reveals that both JET plasmas become unstable to finite-n ideal MHD peeling-ballooning modes at the end of the ELM cycle. During the ELM cycle, infinite-n ideal MHD ballooning modes and kinetic ballooning modes are found to be locally stable in most of the steep pressure gradient region of the pedestal owing to the large bootstrap current, but to be locally unstable in a narrow region of plasma at the extreme edge. Unstable micro-tearing modes are found at the JET pedestal top, but they are sub-dominant to ion temperature gradient modes. They are insensitive to collisionality and stabilised by increasing density gradient.