Disruptions in tokamaks

Abstract

Disruptions and related Vertical Displacement Events pose a major problem to the design and operation of future tokamak reactors. The cause and dynamics of disruptions will be described for the many different scenarios that these violent events can follow. Possibilities will be discussed to avoid or at least to ameliorate the damaging effects of disruptions. IntroductionDisruptions have been with us as long as tokamaks exist. In the early years they were mainly regarded as a nuisance and the l i i t e d time and spatial resolution of the diagnostics of those days prevented much progress in undentanding. In present-day machines of the size of ET, TFTR, and JT-6OU disruptions are more than a nuisance, the are able to cause considerable structures, melting or erosion of plasma facing components and short circuits in extemal supplies. The situation will be even worse in future burning fusion devices, like ITER, for which forces and heat loads are foreseen which are two orders of magnitude larger than in present-day machines. Therefore disruptions feature hkh on the physics R&D pmgramme damage. Forces up to 2 MN and heat loads up to 2 MJ/m T have caused deformation of of ITER [f]. The sophistication of present-day diagnostks enabl& us to make such-physics studies fruitfuL Classification of observed disruption scenariosThe chain of events altogether called 'disruption' is sketched in figure 1: initiating event, precursor, thermal quench and current quench. Many different initiating events, different precursor scenarios and current quench scenarios are possible. These could not always be distinguished in the past, which caused confusion. Until recently it was thought that there was only one physics mechanism underlying the thermal quench nonlinear interaction between low-mode number tearing modes, seeHowever, with the better diagnostics it has become clear that also for this very short duration event there are at least two different types of thermal quenches with different physics mechanisms: *) The aulhor acts mekly as rappimew on Le state-of-the-art in tokamak disruptions as established during the ITER Workshop on Disruptions and Vertical Displacement Events organized as a " i c a l Meeting of the ITER Disruptions, Plasma Conaol and MHD Expen Group m Garching, Germany, Febnrary 13-17.1995. Much of Le material presented is the unpublished work of participants which has been kindly made available to the author.

20 years of research on the Alcator C-Mod tokamak

“…The mechanical and thermal stresses along with the generation of large populations of relativistic electrons via avalanche amplification become unacceptable for large devices. 369 Performance trade-offs, which can place the design operating point near operational limits for current, pressure, or density, come with increased risk of disruption. Several of these issues can be non-trivial for C-Mod as well-its high field and very high plasma current density can lead to large forces, up to 600 kN (120 000 lbs) when the sudden loss of plasma current drives eddy and halo currents in its thick-walled, low resistance vacuum vessel.…”

Density limits in toroidal plasmas

“…This has the effect of shrinking the current channel and destroying the MHD stability of the discharge [57,108]. The growth of MHD fluctuations and the termination of the discharge have been studied numerically [109][110][111][112][113][114][115] and experimentally [116,55,56,[117][118][119][120].…”

The ITPA disruption database

“…Large instabilities can cause a tokamak discharge to rapidly terminate, releasing the stored thermal and magnetic energy in a sequence called a disruption [1]. The high heat flux and mechanical loads transmitted to the vessel during a disruption have the potential to erode the first wall and stress critical mechanical components [2,3].…”