Mechanism of vertical displacement events in JT-60U disruptive discharges

Abstract

Enhanced vertical displacement events (VDEs), which are frequently observed in JT-6OU disruptive discharges, are investigated using the Tokamak Simulation Code (TSC). The rapid plasma current quench can accelerate the vertical displacement, owing to both the up/down asymmetry of the eddy current distribution arising from the asymmetric geometry of the JT-6OU vacuum vessel and the degradation of magnetic field decay index n, leading to high growth rates of positional instability. For a slightly elongated configuration (n = -0.9), the asymmetry of attractive forces on the toroidal plasma plays a dominant role in the VDE mechanism. For a more elongated configuration (n = -1.7), the degradation of field decay index n plays an important role on VDEs, in addition to the effect of asymmetric attractive forces. It is shown that the VDE characteristics of a highly elongated configuration with a rapid plasma current quench can be dominated by the field decay index degradation. It is also pointed out that both the softening of current quenches as was experimentally developed in the JT-6OU tokamak, and the optimization of the allowable elongation of the plasma cross-section are critical issues in the development of a general control strategy of discharge termination.

Generation and termination of runaway electrons at major disruptions in JT-60U

“…For the controlled shutdown obtained by the plasma configuration control [14] placing the vertical position of the current centre around 'the neutral point' [24,25], q eff easily increases above 8 due to the faster ramp-down of the plasma current. When I γ is lower than 50 s −1 , runaway electron generation is not observed, as shown in Fig.…”

Configuration and perturbation dependence of the Neutral Point in JET

“…In non-symmetric configurations, the existence and location of a NP is not obvious. In the past [1][2][3], the existence of the NP, as defined above, has been [4] and Alcator C-Mod [5]. These works have shown that the direction of vertical plasma movement consequent to a radiative collapse depends on the initial vertical position of the plasma.…”

Comparison benchmark between tokamak simulation code and TokSys for Chinese Fusion Engineering Test Reactor vertical displacement control design

“…The TSC [1] and TokSys [2] are two widely used 2D simulation codes. They have been successfully used on several tokamaks in reproducing experimental VDE shots and/or designing vertical control systems, such as PBX, [1] DIII-D, [2,3] TFTR, [4] JT-60U, [5] EAST [6][7][8] and ITER. [9] The TSC is a full non-linear free boundary simulation code that accurately models the transport time-scale evolution of an axisymmetric plasma, including the plasma interactions with the passive and active feedback systems.…”