The extensive studies of disruptions at the COMPASS tokamak will help to improve the models for ITER
This joint effort has provided the first experimental evidence of a physical limit to the flow
of electric currents between the plasma and tokamak components during these events. This finding
will help scientists to refine models and provide improved predictions of plasma dynamics and the
associated forces on in-vessel components during ITER disruptions.
In tokamaks, disruptions are sudden losses of the thermal and magnetic energy stored within
the plasma, which occur when operating near plasma stability limits or when systems malfunction and
plasma control is lost. Disruptions, particularly on large devices like ITER, can lead to high
power fluxes and mechanical forces on in-vessel components, which can impact their lifetime. A
major contributor to these loads are the electric currents that circulate between the plasma and
the components during the "current quench" phase of the disruption when the plasma current
collapses. The amplitude and distribution of these electric currents (the so-called halo currents),
together with the strong magnetic fields always present in tokamaks, determine the local mechanical
stresses on the in-vessel components and thus need to be understood in detail to refine
expectations for ITER. Physicists from IPP in Prague working on the COMPASS tokamak and from the
ITER Organization have collaborated to perform a series of careful experiments designed to measure
the halo currents circulating between the plasma and surrounding components during disruption
current quenches. The result is a unique database of high spatial resolution measurements of these
currents.
Figure 1. Locally measured halo current during disruptions in
COMPASS versus local plasma flux (both measured in Mega-Amperes per square m) over a range of
plasma currents
The experiments, reported in this recent
publication, have demonstrated (Figure 1)
that the local value of the halo current cannot exceed the local value of the plasma particle flux
to the in-vessel components. This physics limit, already known for decades to apply under standard
plasma conditions in the cool plasma edge where magnetic field lines intersect solid surfaces, has
been found on COMPASS during these extremely transient disruptive phases using arrays of dozens of
tiny, finely spaced electric sensors known as Langmuir probes embedded in the divertor target
surfaces (see LPA and LPB in Figure 2). This probe system captured the halo and the plasma flux
simultaneously for the first time during purposely triggered disruptions. The experiments also
confirmed previous findings elsewhere that the global value of the halo current is proportional to
that of the current flowing in the plasma before the disruption. This, together with the new limit
identified in these experiments, means that the area of in-vessel components over which the halo
current flows grows with increasing current when the limit is at work. This spreads the halo
current across the in-vessel components and lowers the local stresses compared to what would be
predicted if such a limit were not applied.
Figure 2. A disruption seen by a visible camera (blue colours on
the left) overlaid with the electric current measuring probes (LPA and LPB) located in the COMPASS
divertor targets