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New experiments at JET with the ITER-like wall show for the first time that ITER-relevant low field side resonance first harmonic ion cyclotron resonance heating (ICRH) can be used to control sawteeth that have been initially lengthened by fast particles. In contrast to previous (Graves et al 2012 Nat. Commun. 3 624) high field side resonance sawtooth control experiments undertaken at JET, it is found that the sawteeth of L-mode plasmas can be controlled with less accurate alignment between the resonance layer and the sawtooth inversion radius. This advantage, as well as the discovery that sawteeth can be shortened with various antenna phasings, including dipole, indicates that ICRH is a particularly effective and versatile tool that can be used in future fusion machines for controlling sawteeth. Without sawtooth control, neoclassical tearing modes (NTMs) and locked modes were triggered at very low normalised beta. High power H-mode experiments show the extent to which ICRH can be tuned to control sawteeth and NTMs while simultaneously providing effective electron heating with improved flushing of high Z core impurities. Dedicated ICRH simulations using SELFO, SCENIC and EVE, including wide drift orbit effects, explain why sawtooth control is effective with various antenna phasings and show that the sawtooth control mechanism cannot be explained by enhancement of the magnetic shear. Hybrid kinetic-magnetohydrodynamic stability calculations using MISHKA and HAGIS unravel the optimal sawtooth control regimes in these ITER relevant plasma conditions.

Extensive linear and non-linear gyrokinetic simulations, including kinetic electrons, collisions, flow shear, realistic geometry, electromagnetic effects, impurities, as well as perpendicular and parallel magnetic fluctuations, and linear MHD analyses performed respectively with the GENE and MISHKA codes, have shown for the first time that the large population of fast ions found in the plasma core under particular heating conditions has a strong impact on core microturbulence and edge MHD. In particular, nonlinear electromagnetic stabilization of Ion Temperature Gradient (ITG) turbulence can be very much enhanced by fast ion pressure gradients. These results can explain the improved ion energy confinement regime observed in L-mode ion heat transport studies at the JET tokamak which manifests itself as a reduction of the ion temperature stiffness and which, until now had not been reproduced by nonlinear gyrokinetic simulations. The same effect has been shown to be important in high- hybrid scenarios from JET and JT-60U with a large population of fast ions. Up to 4 times of lower ion heat flux for the same R/LTi is obtained in these plasmas when the fast ions contribution is taken into account. This can explain the higher core ion temperature gradients obtained in these regimes as the flow shear is found to play a much weaker role. In addition to the core transport stabilization, the fast ions have been seen to favorably impact edge transport. The high core total pressure achieved due to fast ions modifies the Shafranov-shift leading to a pedestal pressure which can increase by 10% when the extra core pressure is taken into account. Therefore, a virtuous circle starts in these plasmas when the fast ions increase the total core pressure without increasing the turbulence drive, even reducing the ITG microturbulence. This leads to an improved edge pedestal pressure by means of the increased Shafranov-shift in a manner unachievable by simple thermal pressure, which is strongly limited by microturbulence. This work was supported by EURATOM and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission.