Internal transport barrier discharges in JET and their sensitivity to edge conditions

Experiments in JET have concentrated oil steady state discharges with internal transport barriers (ITBs). The ITBs are formed during the current rise phase of the discharge with low magnetic shear (= r/q(dq/dr)) in the centre and with high additional heating power. In order to achieve stability against disruptions at high pressure peaking. which is typical for ITB discharges, the pressure profile call be broadened with all H mode transport barrier at the edge of the plasma. However, the strong increase in edge pressure during all ELM free H mode weakens the ITB owing to a reduction of the rotational shear and pressure gradient at the ITB location. In addition, type I ELM activity during the H mode phase leads to a collapse of the ITB with the input powers available in JET (up to 28 MW). The best ITB discharges are obtained with input power control to reduce the core pressure, and with the edge pressure of the plasma controlled by argon gas dosing. These discharges achieve steady conditions for several energy confinement times (tau (E)) with H97 confinement enhancement factors (tau (E)/tau (E).(ITER97scaling)) of 1.2-1.6 at line averaged densities of around 30 40% of the Greenwald density. Increasing the density by using additional deuterium gas dosing or shallow, pellet fuelling leads to a weakening of the ITB. In order to sustain ITBs at higher densities, type III ELMs should be maintained at the plasma edge. giving scope for future experiments in JET.