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A deuterium “airbag” that protects against the potentially harmful effects of plasma instabilities

​A collaboration involving the CEA-IRFM, the European consortium EUROfusion, Iter and American partners has shown that it is possible to protect successfully the internal structures of a fusion reactor against damage caused by very energetic electrons that escape the fusion plasma following a major instability.

Published on 30 April 2021

To produce nuclear fusion reactions, it is not only necessary to heat hydrogen isotopes (deuterium and tritium) to some 150 million degrees, but also to keep the resulting plasma away from the walls of the reactor and therefore "confine" it by means of magnetic fields.

However, instabilities which can lead to energy deposition on the walls may occur in the plasma composed of ions and electrons at very high temperatures. The plasma then disappears almost instantaneously in a phenomenon called disruption. In some cases, electrons escaping from the plasma (referred to as "runaway electrons") are accelerated to relativistic speeds in the moments following the disruption and then accelerate  other electrons in their wake, in a sort of "avalanche" process.

The speed at which this avalanche of runaway electrons develops increases exponentially with the size of the tokamak. In the upcoming international fusion experiment Iter, these electrons could deposit almost as much energy on an extremely small surface as is contained in the fusion plasma itself (on the order of a hundred megajoules)!

The preferred method for protecting against potential damage caused by disruptions is to inject heavy atoms (argon or neon), but this promotes the appearance of energetic electrons. Consequently, the solutions applied so far could be counterproductive for larger tokamaks such as Iter.

During a one-time experiment conducted at the DIII-D tokamak operated by General Atomics in the United States, physicists observed that a substantial injection of deuterium could dissipate the energy of decoupled electrons very quickly. This idea published in 2018 was taken up, expanded upon and confirmed in several experiments at the JET in the UK in 2019 and 2020. The scientists demonstrated that the "clean" dissipation of energy from decoupled electrons was possible, without any measurable heat load on the reactor's internal components, provided that deuterium is massively injected in the form of ice shards immediately after the disruption.

Using computer simulations, the researchers detailed how two physical processes contribute to this protective effect. The deuterium atoms increase the instability of the decoupled electron beam and promote the spreading of the energy deposit, while also driving out of the plasma the impurities that participate in the reacceleration of the electrons.

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