Edge Pedestal Characterisation in High Triangularity Advanced Tokamak Scenarios with Impurity Seeding at JET
The optimisation of performance in scenarios characterised by Internal Transport Barriers (ITB) leads, naturally, in the direction of combining the enhanced core confinement with a reduced edge transport, i.e. with an H-mode edge barrier. The principle advantage is improved bootstrap current generation due to the edge pressure gradient: one of the best targets for JET steady-state operation is considered to be a plasma with an ITB, whose foot is located at a relatively large radius, superimposed on a strong edge pedestal. Unfortunately, this has so far proven to be an elusive goal. The cold pulse due to large periodic edge pedestal relaxations, type I ELMs, often tends to penetrate deep into the plasma and reach the foot of the ITB, causing a loss of the improved confinement inside the ITB. In addition, plasmas with a strong edge density pedestal have reduced toroidal rotational shear, leading to a substantial decrease of the E¥B shearing rate and increased difficulty in triggering and sustaining the ITB. It is, therefore, important and advantageous to develop an operational regime featuring smaller ELMs, to minimise their effects on the ITB dynamics whilst maintaining as strong an edge pedestal as is compatible with the requirement of smaller ELMs.