The first IAEA Technical Meeting on Plasma Disruptions and their Mitigation, jointly organized with the ITER Organization and held virtually last week, made substantial progress towards reliable and successful operation of ITER: the international tokamak reactor scale experiment being assembled in France to demonstrate the scientific and technological feasibility of fusion power.
Nuclear fusion bears the promise of an almost inexhaustible source of carbon emission-free energy through a process akin to that occurring in the sun. Fusion reactions taking place in plasmas – the hot and charged state of matter made of free electrons and ions – generate massive amounts of energy. Scientists and engineers worldwide are seeking to harness this virtually inexhaustible supply of energy to generate electricity.
At present, the most successful technology for confining hot, high-pressure plasmas is a donut-shaped arrangement of magnets called tokamak. However, in tokamaks, instabilities can develop under certain operational conditions or as a consequence of a loss of plasma control. These instabilities eventually lead to the rapid loss of thermal and magnetic energy, a phenomenon known as plasma disruption.
Plasma disruptions have two distinct phases which expose key tokamak components, such as first wall panels, divertor, vacuum vessel and magnetic coils, to thermal and mechanical loads. These phases are: thermal quench – heat flux causing erosion of the tokamak in-vessel components; and current quench – eddy and halo current, causing mechanical and thermal loads to the components. In the second phase of the disruption, the formation of high energy electrons, so called runaway electrons, is possible. These electrons can cause intense localized melting and material erosion and can negatively affect the integrity of cooling water channels.
In reactor scale tokamaks like ITER, the number of mitigated disruptions must be limited. This requires reliable plasma control and effective disruption avoidance schemes. Disruption mitigation, as a last resort, will be essential to reducing thermal and mechanical loading, in order to guarantee the lifetime of in-vessel components.
During the four-day IAEA event, over 120 participants discussed experimental, theoretical and modelling work in the field of plasma disruptions with special emphasis on developing a solid basis for avoidance and mitigation strategies in ITER and next generation fusion devices.
“The avoidance and mitigation of disruptions is one of the key challenges for ITER and future planned tokamak devices on the reactor scale,” said Michael Lehnen, Scientific Coordinator at ITER and Chair of the meeting. “It can only be addressed in a timely and effective way through internationally coordinated efforts, such as this meeting.”
ITER disruption mitigation system
An ongoing major international effort is the refinement of the design of the ITER disruption mitigation system. Recent experiments at DIII-D (USA), JET (EU), and KSTAR (Korea) tokamaks have demonstrated many of the requirements for effective disruption mitigation at ITER by the shattered pellet injection scheme – a method through which pellets made of frozen hydrogen and neon are injected into the plasma as small fragments after being shattered. The injection of the fragments dissipates plasma thermal and magnetic energy through photonic radiation and raises the density thus supressing runaway electron formation.
“The scale of the international disruption mitigation research program has increased dramatically in the past few years to meet the huge challenge of designing an effective disruption mitigation system for ITER. The shattered pellet injection research program has expanded from a single device for the past decade (DIII-D) to three new installations in 2019 (JET, J-TEXT, and KSTAR), with two more installations planned in 2020 and 2021 (HL-2A and ASDEX-U). In addition, a robust international program, both experimental and modelling, has grown to pursue a deep understanding of innovative methods for mitigating runaway electrons. This IAEA Technical Meeting provided a central forum to collate, compare, and derive plans from this rapidly expanding body of disruption mitigation research,” said Nick Eidietis, Scientist at General Atomics, USA.
