A simple model of current ramp down in the ITER tokamak

TL;DR

This paper models the current ramp down in the ITER tokamak using a simple cylindrical geometry approach, assessing MHD stability and feasibility of a 60-second ramp.

Contribution

It introduces a straightforward model for ITER's current ramp down and analyzes MHD stability, especially the tearing mode risks during the process.

Findings

60-second ramp down is feasible if plasma is hot enough initially

Faster ramp downs risk exciting tearing modes and causing disruptions

The m=2/n=1 tearing mode is the main stability concern

Abstract

The controlled ramp down of the toroidal plasma current in the ITER tokamak is simulated using a simple model that employs cylindrical geometry. The magnetohydrodynamical (MHD) stability of the plasma throughout the whole current ramp is also calculated. The only potentially unstable MHD mode is the m=2/n=1 classical tearing mode. The envisioned 60 second ramp down of the plasma current in ITER is found to be perfectly feasible, provided that the plasma is sufficiently hot at the start of the ramp. However, attempts to ramp down the current on a significantly faster time scale are predicted to excite 2/1 tearing modes that are likely to lock to the vacuum vessel, and trigger a disruption.