Sawtooth control using electron cyclotron current drive in the presence of energetic particles in high performance ASDEX Upgrade plasmas

摘要

Sawtooth control using steerable electron cyclotron current drive (ECCD) hasbeen demonstrated in ASDEX Upgrade plasmas with a significant population ofenergetic ions in the plasma core and long uncontrolled sawtooth periods. Thesawtooth period is found to be minimised when the ECCD resonance is swept tojust inside the q = 1 surface. By utilising ECCD inside q = 1 for sawtoothcontrol, it is possible to avoid the triggering of neoclassical tearing modes,even at significnatly higher pressure than anticipated in the ITER baselinescenario. Operation at 25% higher normalised pressure has been achieved whenonly modest ECCD power is used for sawtooth control compared to identicaldischarges without sawtooth control when neo-classical tearing modes aretriggered by the sawteeth. Modelling suggests that the destabilisation arisingfrom the change in the local magnetic shear caused by the ECCD is able tocompete with the stabilising influence of the energetic particles inside the q= 1 surface.