Tearing mode stabilization by electron cyclotron resonance heating demonstrated in the TEXTOR tokamak and the implication for ITER

Abstract

Controlled experiments on the suppression of the m/n = 2/1 tearing mode with electron cyclotron heating and current drive in TEXTOR are reported. The mode was produced reproducibly by an externally applied rotating perturbation field, allowing a systematic study of its suppression. Heating inside the island of the mode is shown to be the dominant suppression mechanism in these experiments. An extrapolation of these findings to ITER indicates that the projected system for suppression of the tearing mode could be significantly more effective than present estimates indicate, which only consider the effect of the current drive but not of the heating inside the island.

Avoiding Tokamak Disruptions by Applying Static Magnetic Fields That Align Locked Modes with Stabilizing Wave-Driven Currents

“…By contrast, continuous ECCD on rotating islands is always on, but only stabilizing half of the time [6,13], and modulated ECH/ECCD on rotating islands is on and stabilizing half of the time, if properly phased [7,11].…”

“…Electron Cyclotron Heating (ECH) was also used for stabilization [8], but is predicted to scale unfavorably to large hot plasmas [9]. Two experiments combined magnetic perturbations -to produce the island-with ECH that stabilized it: in the first one the mode was born locked to a given phase and was stabilized by continuous ECH [10]; the second one controlled island rotation and stabilized the mode by modulated ECH [11].However, if the rotating island (typically a spontaneous, pressure-driven 'Neoclassical Tearing Mode') is not preempted or stabilized (due for example to late intervention, misalignment, or insufficient power being used for this purpose), or if the island does not ever rotate at all, it becomes necessary to suppress the locked mode. This capability was numerically modeled [12], experimentally tested [13], and is fully demonstrated here for the first time.…”

“…Without this capability, the locked mode would grow and promptly lead to a disruption. After that, only one last line of defense would remain, namely to mitigate the disruption, for instance by massive gas injection [1,14].Static, rather than rotating fields [11,13], are used here, permitting to align the plasma such that ECCD can be continuously deposited into the location of the locked mode where it has a stabilizing effect on it. By contrast, continuous ECCD on rotating islands is always on, but only stabilizing half of the time [6,13], and modulated ECH/ECCD on rotating islands is on and stabilizing half of the time, if properly phased [7,11].…”

Effect of Heating on the Suppression of Tearing Modes in Tokamaks

“…In TEXTOR, suppression by ECRH dominates over ECCD [5] due to the low current drive efficiency (low T e ).…”

“…In this Letter, only data are presented for which the ECRH power was deposited directly on the resonant q 2 surface. The appropriate launcher position is determined from a scan of the poloidal launcher angle, searching for optimum island suppression [5].…”