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

TL;DR

This study demonstrates that steerable electron cyclotron current drive (ECCD) can effectively control sawtooth oscillations in high-performance ASDEX Upgrade plasmas with energetic ions, improving stability and pressure limits.

Contribution

It introduces a method of using ECCD just inside the q=1 surface to suppress sawteeth and avoid neoclassical tearing modes in plasmas with energetic particles.

Findings

ECCD inside q=1 reduces sawtooth period effectively.

Operation at 25% higher normalized pressure achieved with ECCD control.

Modeling shows local magnetic shear changes destabilize sawteeth despite energetic particles.

Abstract

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