Overview of disruptions with JET-ILW

Overview of disruptions with JET-ILW

Overview of disruptions with JET-ILW 150 150 UKAEA Opendata

The paper presents an analysis of disruptions occurring during JET-ILW plasma operations covering the period from the start of ILW (ITER-like wall) operation up to completion of JET operation in 2016. The total number of disruptions was 1951 including 466 with deliberately induced disruptions. The average disruption rate of unintended disruptions is 16.1 %, which is significantly above the ITER target at 15 MA. The pre-disruptive plasma parameters are plasma current Ip = (0.82 – 3.38) MA, toroidal field BT = (0.98 – 3.4) T, safety factor q95 = (1.52 – 9.05), plasma internal inductance li = (0.58 – 1.86), Greenwald density limit fraction FGWL = (0.04 – 1.61), with 720 X-point plasma pulses from a subset of 1420 unintended disruption shots. Massive gas injection (MGI) has been routinely used in protection mode both to terminate pulses when the plasma is at risk of disruption and to mitigate against disruption effects. The MGI was mainly triggered by the n = 1 locked mode amplitude exceeding a threshold or by the disruption itself; either dIp/dt or the toroidal loop voltage exceeding threshold values. For mitigation purposes, only the locked mode was used as a physics precursor to trigger the MGI prior to disruption. Long lasting locked modes (≥ 100 ms) do exist prior to disruption in 75% of cases. Though, 10% of non-disruptive pulses have a locked mode which eventually vanished without disruption. The plasma current quench (CQ) may result in 3D equilibria, termed as asymmetrical disruptions, which are accompanied by sideways forces. Unmitigated VDEs generally have significant plasma current toroidal asymmetries. Unmitigated disruptions also have large plasma current asymmetries presumably because there is no plasma vertical position control during the CQ and so they too are subject to large vertical displacements. MGI is a reliable tool to mitigate 3D effects and correspondingly sideways forces during the CQ. The vessel structure loads depend on the force impulse and force time behaviour inclding rotation. The toroidal rotation of 3D equilibria is of particular concern because of potential resonance with the natural frequencies of the vessel components in large tokamaks such as ITER. The amplitude-frequency interdependence is presented.

Collection:
Journals
Journal:
Nuclear Fusion
Publisher:
IOP (Institute of Physics)
Published date:
20/05/2020