2 research outputs found
Pedestal evolution physics in low triangularity JET tokamak discharges with ITER-like wall
The pressure gradient of the high confinement pedestal region at the edge of tokamak plasmas
rapidly collapses during plasma eruptions called edge localised modes (ELMs), and then
re-builds over a longer time scale before the next ELM. The physics that controls the evolution
of the JET pedestal between ELMs is analysed for 1.4 MA, 1.7 T, low triangularity, δ = 0.2,
discharges with the ITER-like wall, finding that the pressure gradient typically tracks the ideal
magneto-hydrodynamic ballooning limit, consistent with a role for the kinetic ballooning
mode. Furthermore, the pedestal width is often influenced by the region of plasma that has
second stability access to the ballooning mode, which can explain its sometimes complex
evolution between ELMs. A local gyrokinetic analysis of a second stable flux surface reveals
stability to kinetic ballooning modes; global effects are expected to provide a destabilising
mechanism and need to be retained in such second stable situations. As well as an electronscale electron temperature gradient mode, ion scale instabilities associated with this flux
surface include an electro-magnetic trapped electron branch and two electrostatic branches
propagating in the ion direction, one with high radial wavenumber. In these second stability
situations, the ELM is triggered by a peeling-ballooning mode; otherwise the pedestal is
somewhat below the peeling-ballooning mode marginal stability boundary at ELM onset. In
this latter situation, there is evidence that higher frequency ELMs are paced by an oscillation
in the plasma, causing a crash in the pedestal before the peeling-ballooning boundary is
reached. A model is proposed in which the oscillation is associated with hot plasma filaments
that are pushed out towards the plasma edge by a ballooning mode, draining their free energy
into the cooler plasma there, and then relaxing back to repeat the process. The results suggest that avoiding the oscillation and maximising the region of plasma that has second stability
access will lead to the highest pedestal heights and, therefore, best confinement—a key result
for optimising the fusion performance of JET and future tokamaks, such as ITER.EURATOM 633053EPSRC EP/K504178/1EPSRC EP/L01663X/1Plasma HEC Consortium EPSRCV EP/L000237/