Three-dimensional distortions of the tokamak plasma boundary: boundary displacements in the presence of saturated MHD instabilities

Abstract

The three-dimensional plasma boundary displacement induced by long-lasting core magnetohydrodynamic (MHD) instabilities has been measured in JET, MAST and NSTX. Only saturated instabilities are considered here since transient rapidly growing modes which degrade confinement and act as potential triggers for disruptions bring more fundamental concerns than boundary displacements. The measured displacements are usually small, although in extreme cases in MAST when the rotation braking is strong, a significant global displacement can be observed. The instability most likely to saturate and exist for many energy confinement times whilst distorting the boundary of ITER is the saturated internal kink, or helical core, which can be found in plasmas with a wide region of low magnetic shear such as the hybrid scenario. This mode can lead to non-negligible boundary displacements. Nonetheless, the boundary displacement resultant from core MHD instabilities in ITER is predicted to be less than ±1.5% of the minor radius, well within tolerable limits for heat loads to plasma-facing components.

Influence of plasma boundary shape on helical core/long-lived mode in tokamak plasmas

“…The magnitude of the distortion strongly depends on the minimum value of the safety factor q min . 3,9,10,13 In evaluating the dependence, here, q min is varied by changing a in q ¼ ð0:7 þ ð0:7 þ aÞs Àð1 þ aÞs 4 Þ À1 with keeping qðq ¼ 0Þ and qðq ¼ 1Þ as is done in Ref. 9.…”

“…For instance, the ITER hybrid scenario has a q-profile that is nearly flat and close to unity in the core region, 1 leading to, for example, a better dependence of turbulent transport on b because of weak magnetic shear in the core. 2 A helical distortion of the magnetic axis is observed for such a q-profile in some tokamaks, JET, MAST, and NCSX testing the hybrid scenario, 3 that is called as the snake, 4 the helical core, and the longlived mode. Analysis on helical cores was extended to DIII-D, Alcator C-Mod, and JT-60U.…”

“…[5][6][7][8] In the presence of a helical core, although the amplitude of helical displacement of plasma is largest at the magnetic axis, the displacement is finite even at the edge region, resulting in the fast-ion loss incorporated with toroidal ripple effects. 3 The appearance of a helical core is understood as a bifurcation of a magnetohydrodynamic (MHD) equilibrium from an axisymmetric one to a non-axisymmetric one. [9][10][11][12][13] The bifurcated equilibrium is regarded as a non-axisymmetric solution to the MHD equilibrium equation, which has an ðm; nÞ ¼ ð1; 1Þ helical structure at the core, where m and n are the poloidal and toroidal mode numbers, respectively.…”

The impact of the fast ion fluxes and thermal plasma loads on the design of the ITER fast ion loss detector

“…The distortion of the OMP is expected to be less than ± 1.75% of the minor radius for 15 MA H-mode plasmas, i.e. about ± 3.5 cm around the nominal OMP position [6,7]. This perturbation (n = 3 toroidal mode in ITER) will not be static and it will rotate toroidally with the frequency of a few Hz.…”

Helically localized ballooning instabilities in three-dimensional tokamak pedestals

“…These 3D boundary displacements have been studied in multiple machines and reviewed in Ref. [15], which concludes that the magnitude of the edge displacement scales linearly with the applied MPs calculated by vacuum field modeling. However, in some cases, it was observed that vacuum modeling drastically underestimated the 3D displacement due to the stable kink response [16] [17].…”