Direct preemptive stabilization of neoclassical tearing modes by electron cyclotron current drive in the DIII-D low-torque ITER baseline scenario

Abstract

We report the first preemptive direct stabilization of $m,n = 2,1$ neoclassical tearing modes (NTMs) in DIII-D low-torque ITER baseline scenario plasmas by electron cyclotron waves. These experiments strongly support that the observed stabilization is achieved by replacing the missing bootstrap current in the island by electron cyclotron current drive (ECCD). These plasmas (with and without ECCD) are stable to 2,1 NTMs when the differential rotation between the q = 1 and q = 2 surfaces ( $\Delta f_{1,2}$ ) is sustained above a critical value ( $\Delta f_\textrm{CRIT}\approx1$ kHz), but those evolving to low differential rotation ( $\Delta f_{1,2}\lt \Delta f_\textrm{CRIT}$ ) routinely develop a disruptive 2,1 island at the time and frequency of a sawtooth precursor. Preemptive, local and direct stabilization by ECCD is tested by scanning the current drive amplitude and location inside and outside of the q = 2 rational surface shot by shot. Analysis of only low differential rotation time windows, i.e. when stabilization from $\Delta f_{1,2}$ is absent, shows focused ECCD at q = 2 prevents the onset of 2,1 NTMs. These results are important, as they give the first demonstration of disruptive NTM control by ECCD in the plasma scenario planned for ITER.

Keywords

• tearing
• tokamak
• stability