JET Scientists Announce Important Advance

April 2, 2020

On March 24 scientists working on the Joint European Torus (JET) at the Culham Center for Fusion Energy (CCFE) announced an important advance. it reads in part:

A vital discovery on how to force the extreme heatload away from a fusion reactor's exhaust system while maintaining high performance has been discovered in recent tests at the Joint European Torus (JET) device at Culham.

In machines like JET, which use the tokamak reactor concept, an exhaust system known as the 'divertor' removes both the extreme heat and impurity particles from the hot plasma fuel. One of the problems with scaling up JET (the EUROfusion flagship research device hosted at CCFE) to a larger machine like ITER - the international reactor-scale experiment being built in France - is that the divertor components in the latter device will not be able to take extreme levels of exhausted heat as they will damage the divertor. This is why the importance of the experiments carried out on JET at the start of the latest run of tests cannot be underestimated.

Scientists working on JET can spread the heat load across different tiles by moving the 'strike point' (specific divertor tiles which spread the heat), but in ITER the heat load will be much higher than in JET and the strike point cannot be moved. Therefore, in order not to melt the divertor, the power has to be radiated before reaching divertor tiles.

One solution to this problem is to use a gas as an impurity (often nitrogen) to cool down the plasma by radiating the heat over a wider surface area within the divertor region. But nitrogen can break down into additional compounds - leading to tritiated ammonia - and this is not compatible with JET's processing systems. The alternative is to use neon, but previously the level of heating power available on JET has meant this hasn't been able to achieve the same effects as nitrogen without affecting how well the plasma is confined in the tokamak; a key factor in fusion reactor performance.

However, taking advantage of increased heating power and additional neon being injected into the plasma, scientists have found the effects are just as good - something hugely positive for ITER.

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