Analysis of the Influence of Different ICRF Heating Scenarios on the Performance of Optimised Shear Discharges in JET

Optimised shear discharges, i.e. discharges with an Internal Transport Barrier (ITB), are normally heated by a combination of Neutral Beam Injection (NBI) and Ion Cyclotron Range of Frequency (ICRF) heating in JET. Although the NBI power in general dominates, ICRF power has been found to be important for obtaining good performance. Several ICRF heating scenarios have been tested, including the use of hydrogen and He as a minority species, on and off-axis heating and different phasings of the ICRF antennas. Here, we concentrate on analysing the influence of the different phasings of the ICRF antennas on the performance. In JET the ICRF antennas can be phased to launch toroidally directed waves. In general, the performance of optimised shear discharges has been found to improve when the phase difference between the currents in the JET four strap antennas has been set to +90, which corresponds to waves propagating along the current, as compared to -90 corresponding to propagation antiparallel to the current. Smaller differences have been observed between +90 and symmetric phasings. A possible theoretical explanation for these experimental results could be the presence of an ICRF induced spatial pinch of the resonating ions. Theory predicts that the turning points of trapped resonating ions are driven inward/outward when the waves propagate parallel/antiparallel to the plasma current [1,2], and experimental evidence for this has been observed [2]. An important consequence of the ICRF induced pinch is a peaking of the fast ion pressure profile and a concomitant peaking of the collisional heating of the background plasma when the waves propagate along the current and a broadening for antiparallel propagation. The current driven by the fast ions is also modified. In optimised shear discharges one would intuitively expect the fast ion pressure, and thus the heating of the background plasma, within the ITB to play a significant role. It is therefore important to establish whether the ICRF induced spatial pinch is strong enough to explain the experimental results.