One focus of the last two experimental campaigns at JET was to investigate methods that can be used to mitigate disruptions. Disruptions are events during which the plasma loses all its energy within a very short period of time. Their effects are more severe for fusion reactors with a metal wall than they are for the traditionally used carbon walls. Metal may melt when impacted by extreme heat or by very intense particle beams. Furthermore, fusion plasmas in metal vessels contain fewer impurity ions and therefore radiate less energy. Consequently, the plasma must lose its energy via alternative routes – often resulting in strong heat loads and forces on the wall.
The plasma-facing components (PFCs) of ITER are made of the metals beryllium and tungsten and therefore ITER must take great care to avoid or at least mitigate disruptions in order to minimise repair down times. It falls on JET with its ITER-Like Wall to investigate methods to demonstrate their ability to offer effective disruption mitigation in ITER.

Massive gas injection

One issue with disruptions in metal wall vessels is the purity of the plasma and its resulting inability to lose energy via radiation. Erosion, due to both physical and chemical processes, causes atoms from carbon PFCs to enter the plasma. These impurity ions absorb plasma energy and radiate it. Metal walls erode less due to an almost complete absence of chemical processes and thus the plasma lacks radiating impurities. A suitable method of mitigating disruptions in metal wall vessels is therefore the injection of large amounts of gas such as neon or argon. Once inside the plasma, the gas ions enable energy radiation. If the gas is injected just at the onset of a disruption, it helps to trigger a more controlled, less powerful disruption. There are currently two methods of massive material injection used in tokamaks: Disruption Mitigation Valves which incorporate large amounts of gas and the injection of large pellets which are shattered against a target plate. JET employs the first method, using a fast valve developed at FZ Jülich, and currently features two such valves.

Mitigating heat loads and forces

Using one or two of these valves, JET carries out experiments designed to characterise the disruption mitigation by massive gas injection. It has been found that the effectiveness of this method decreases in plasmas with high thermal energy content. Further experiments will be needed to fully understand the reasons behind this. The radiation hitting the first wall is toroidally and poloidally asymmetric due to the localised gas injection. This asymmetry can be minimised by the simultaneous use of multiple valves. JET will add a third valve next year which will thus allow further investigation of this aspect. The forces resulting from so-called vertical displacement events, during which the plasma shifts vertically, and halo currents, which develop when the plasma touches the wall and which partly flow in the metal wall, have been reduced by massive gas injection. However, the resulting fast plasma current decay leads to larger eddy currents. These must be controlled in ITER as they can lead to unacceptably high loads on in-vessel blanket modules. Experiments at JET will continue to further optimise massive gas injection for mitigating heat loads and forces.

Suppressing runaway-electrons

JET experiments have also investigated the effects of a massive gas injection on the suppression of so-called runaway electrons. These very intense and highly energetic electron beams develop during disruptions and can cause local melting of metal walls. The experiments have shown that a gas injection just at the onset of a disruption can prevent these runaways. They have also shown that timing is critical: if the gas injection is delayed, runaways occur. On behalf of ITER, JET took these experiments one step further and deliberately produced runaway electrons in order to find ways to eliminate them. However, a massive gas injection did not have a significant effect on an already developed beam of runaway electrons. Similar observations have been made in earlier experiments at smaller tokamaks. Now the reasons for this effect need to be investigated further.

Meanwhile the third mitigation valve that has been installed and JET offers even more ITER-like conditions for testing the application of massive gas injections to mitigate disruptions and to suppress runaway electrons.