Plasma current asymmetries during disruptions in JET

Abstract

A key feature of disruptions during vertical displacement events, discovered in JET in 1996, is the toroidal variation in the measured plasma current Ip, i.e. the plasma current asymmetries, lasting for almost the entire current quench. The unique magnetic diagnostics at JET (full set of poloidal coils and saddle loops recorded either from two toroidally opposite or from four toroidally orthogonal locations) allow for a comprehensive analysis of asymmetrical disruptions with a large scale database. This paper presents an analysis of 4854 disruptions over an 18 year period that includes both the JET carbon (C) wall and the ITER-like (IL) wall (a mixed beryllium/tungsten first wall). In spite of the Ip quench time significantly increasing for the IL-wall compared to C-wall disruptions, the observed toroidal asymmetry time integral (∼ sideways force impulse), did not increase for IL-wall disruptions. The Ip asymmetry has a dominantly n = 1 structure. Its motion in the toroidal direction has a sporadic behaviour, in general. The distributions of the number of rotation periods are found to be very similar for both C- and IL-wall disruptions, and multi-turn rotation was sometimes observed. The Ip asymmetry amplitude has no degradation with rotation frequency for either the C- or IL-wall disruption. Therefore dynamic amplification remains a potentially serious issue for ITER due to possible mechanical resonance of the machine components with the rotating asymmetry.

Disruptions in ITER and strategies for their control and mitigation

“…These asymmetries give for example rise to sideways and/or tilting forces on the VV. Of special concern is rotation during asymmetric events as observed at JET [26] or NSTX [27] as this could potentially cause dynamic amplification of loads. This issue is currently addressed by 3D MHD modelling [28] and by a dedicated effort within ITPA.…”

Effects of asymmetric vertical disruptions on ITER components

“…The perturbation is modeled with an n = 1, m = 1 kink mode, obtained by a time varying horizontal displacement dx(t) and a time varying tilt around the x axis d x (t) [13]; the time evolution of the two quantities is assumed to be pulsed, starting at t = 690 ms and whose duration is 338 ms.…”

Analytical model of wall forces produced by kink perturbations in tokamaks

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Evaluations for ITER give worrisome numbers at the level of hundred MN. [12][13][14][15][16][17][18][19][20][21] There are several approaches [2][3][4][5]7,9,17,19,[21][22][23][24] to calculating the instability-induced electromagnetic forces on the vessel components in tokamaks, but some issues remain unresolved. 13,20 One of them was recently raised in Refs.…”

“…[12][13][14][15][16][17][18][19][20][21] There are several approaches [2][3][4][5]7,9,17,19,[21][22][23][24] to calculating the instability-induced electromagnetic forces on the vessel components in tokamaks, but some issues remain unresolved. 13,20 One of them was recently raised in Refs. 17 and 21 where an analytical model of the wall force produced by kink modes was developed that predicted a strong dependence of such force on the growth rate c of the mode and the resistive wall penetration time s w .…”