A task force to face the challenge
When operating tokamaks of the size of ITER, one of the key systems to ensuring reliable and successful operation from the very first campaign onward is the disruption mitigation system.
Preliminary layout of eight injectors in one-third of an equatorial port plug. The installation includes (from right to left) the propellant gas valve, the pellet cryostat, a pellet integrity monitor, the primary vacuum gate valve, and a bellows at the closure plate of the port plug for mechanical decoupling from the vacuum vessel. At the end of the flight tube inside the port plug, a bend will ensure breaking the pellet into fragments for better assimilation inside the plasma.
A disruption occurs when an instability grows in the tokamak plasma to the point where there is a rapid loss of the stored thermal and magnetic energy.
This rapid loss can also accelerate electrons to very high energy (so-called "runaways"). The disruption mitigation system has to protect the plasma-facing components against the heat and forces that arise during the disruption, and at the same time it must tame the runaway electrons that—if generated—could lead to melting of the first wall and leaks in the water cooling circuits.
The ITER Council Science and Technology Advisory Committee (STAC) characterized disruptions at its May 2018 meeting as "a serious threat to ITER's mission." Although long recognized as a key plant system for ITER, the disruption mitigation system has proven challenging to design because of the complex physics involved in stopping runaway electrons. Experts concluded at the Disruption Mitigation Workshop hosted by the ITER Organization in 2017
that "immediate decisive action must be taken to directly support research into solutions to outstanding critical issues relating to the specification and performance of the disruption mitigation system (see the ITER Technical Report here
To face the challenge and to ensure that the ITER system will fulfil its purpose the ITER Disruption Mitigation Task Force has been established. An extensive program has been defined that focuses on refining system design specifications and on performing engineering work for industrialization of the technology. Besides intensive work on improving the design of the selected technology for ITER, work that explores new techniques or new disruption mitigation strategies is highly encouraged. If better alternatives are available, later upgrades of the system will be possible should the present strategy fall short of ITER's needs.
The international task force team is led by Michael Lehnen from ITER, who has worked in the field of disruptions for more than 10 years and coordinated the first disruption mitigation experiments at JET. The task force work is distributed over three groups. Experiments will aim on validating the ITER shattered pellet injection scheme (see more below) on today's tokamaks. The group is led by Nick Eidietis (General Atomics, USA) who is also the co-chair of the task force. He is a well-known expert in the field, coordinating the disruption mitigation work at the DIII-D tokamak. Technology, led by Uron Kruezi (ITER) and Nick Balshaw (CCFE, United Kingdom), has the focus on improving the technique itself and making it fit for the demanding environment in ITER. Theory & Modelling development is essential for the interpretation of the experiments and the extrapolation to ITER; Eric Nardon (CEA, France) and Akinobu Matsuyama (QST, Japan) will coordinate several work packages with a special eye on the physics of runaway electrons.
The concept that has been chosen for the ITER disruption mitigation system is based on a technology called shattered pellet injection, a technique developed at the Oak Ridge National Laboratory and pioneered at the DIII-D tokamak. Pellets enable the injection of massive quantities of neon and deuterium into the plasma in the form of solid ice. To ensure that the plasma can assimilate these quantities, the pellets are shattered into small pieces just before they enter the vacuum vessel.
The largest pellets for ITER disruption mitigation are larger than a wine cork, with a diameter of 28 mm. Despite this "enormous" size for a cryogenic pellet, several of these have to be fired at the same time to reach the required quantities to stop the worst-case runaway electron beam in ITER. A new layout of the ITER disruption mitigation system has been proposed that allows the injection of up to 32 pellets from equatorial ports at three different toroidal locations and three pellets from different upper ports. Although the equatorial port plugs are of substantial size, the designers are challenged when it comes to integration.
At its most recent meeting, the ITER Council approved the ITER Organization's strategy to tackle disruption mitigation and now the Task Force can get off the ground. To gain more confidence in whether the shattered pellet injection technique and especially the ITER disruption mitigation system design fulfils its aim, several tokamak experiments are planned and in preparation.
Next year will be a productive time with more and more tokamaks joining the shattered pellet injection community. Amongst the new members are JET, where commissioning of the shattered pellet injection system has already begun, and KSTAR, where the effect of dual pellet injection in a similar configuration to that in ITER can be studied.
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