Effect of symmetry-breaking on the MHD edge stability limit of tokamak plasmas

Abstract

Abstract We demonstrate for the first time that symmetry-breaking in tokamak plasmas reduces the window of stable edge operational space, leading to a lower achievable edge pressure gradient. Predictive simulations with the linear magnetohydrodynamic stability code CASTOR3D show a reduction of the critical pressure gradient by up to 30%, in agreement with experimental observations. The analysis has been extended to experimental plasmas considering, for the first time, ion diamagnetic drift effects in realistic non-axisymmetric (3D) tokamak geometry. The 3D geometry in plasmas with edge localised modes (ELMs) increases the growth rate of edge instabilities compared to the axisymmetric case. Further reducing the edge pressure gradient eliminates the instabilities corresponding to an experiment with suppressed ELMs, reproducing the empirically observed threshold for ELM suppression. Our findings highlight the importance of predictions of the ELM occurrence in full 3D geometry for future tokamak devices with intentionally and unintentionally broken axisymmetry.