On the role of different phasings of the ICRF antennas in optimized shear discharges in JET

In discharges with internal transport barriers produced by combined NBI and ICRF heating using the hydrogen minority scheme in JET, confinement and fusion performance are strongly affected by the direction of propagation of the ICRF waves. When the waves propagate along the plasma current, the formation of an internal transport barrier is prompter and the neutron yield is up to a factor of two higher than that for propagation against the current. An ICRF induced pinch of resonating trapped ions is put forward as a candidate for explaining the observations. Simulation results are presented which show that this effect is strong enough to provide a credible explanation for the experimental results.