Matter Injection in EU-DEMO: The Preconceptual Design

EU-DEMO will be the next step in Europe after ITER on the path toward a fusion power plant. The matter injection systems have to provide the requested material in order to establish, maintain, and terminate the burning plasma. Their main function is to fuel the plasma, but other tasks are addressed as well like delivering matter for generating sufficient core radiation and divertor buffering. In the preconceptual design phase performed from 2014 to 2020, the matter injection systems, in particular pellet injection and gas injection, have been assessed. This work describes the main findings and state of the art of the matter injection systems at the transition from the preconceptual design phase to the conceptual design phase

Testbed for the Pellet Launching System for JT-60SA

As part of the European contribution to the large size superconducting tokamak project JT-60SA, a new Pellet Launching System (PLS) is designed and built. The aims of the PLS are to provide efficient fuelling to the plasma and to control and mitigate Edge Localised Modes (ELMs). Two pellet sources, one for fuelling pellets, one for pacing pellets, are delivering pellets to a centrifuge launcher. The centrifuge enables precise launch of pellets according to already proven control schemes. Furthermore, this system opens a way towards a test bed for the EU-DEMO fuelling system. The new PLS has to be completed and commissioned first at the IPP Garching pellet lab and then to be shipped to QST Naka site after having demonstrated its performance. This dedicated test bed has been set up, providing suitable vacuum conditions to operate the PLS in similar conditions (except magnetic field and radiation). Maximum hydrogen throughput is about 400 mbar·L/s per pellet source. Safety issues must be considered for hydrogen inventory of pellet sources (∼100 bar·L each). In a first step, the pellet sources will be put on a test vessel providing inherent safety by a huge volume (10 m³) which makes sure that the hydrogen concentration is below 1% under all circumstances. A hydrogen safety survey prior to assembly confirmed the concept to be followed by an assessment after the installation in order to get the required license for operation. The PLS as a whole, for the time being equipped with two pellet sources, is to be certified according to explosion prevention rules (ATEX) as a product to be shipped to Naka site. To obtain this, an appropriate declaration of explosion zones inside the vacuum system and the use of suitable and certified equipment is mandatory. Such, the integration of this system can be planned and assessed on a clear technical and regulatory basis

Admixed pellets for fast and efficient delivery of plasma enhancement gases: Investigations at AUG exploring the option for EU-DEMO

Gas and pellet injection are envisaged for particle fuelling in EU-DEMO. The gas system will provide edge and divertor fuelling and any further gas species required for operation. Pellets, mm-sized bodies formed from solid hydrogen fuel, are designed for efficient and fast core fuelling. However, they can also be employed for a more efficient delivery of plasma enhancement gases, by admixing them with the fuelling pellets. To check this option for EU-DEMO, explorative investigations have been performed at ASDEX Upgrade (AUG). The AUG system produces ice in a batch process sufficient for about 100 pellets, initially designed for operation with pure H$_2$ or D$_2$. On a trial basis, pellet formation was tested using an H$_2$/D$_2$ mixture and admixtures containing small amounts (up to 2 mol%) of N$_2$, Ar, Kr or Xe in the D$_2$ host. A homogeneous and reproducible ice composition was found for the H$_2$/D$_2$ = 1:1 case. For all the admixed gases, a depletion of the admixture in the ice with increasing atomic number is observed. Nevertheless, the fast and efficient delivery of admixed pellets was clearly demonstrated in dedicated plasma experiments at AUG. Detailed investigations showed that the Ar supplied via admixed pellets has a higher radiation efficiency and a faster radiation rise than an Ar/D$_2$ gas puff. Furthermore, Ar density measurements in a discharge with admixed pellet injection show reasonable agreement with findings of a fading admixed species’ concentration along the ice rod and assumptions on the pellet ablation location in the plasma. Investigations performed at the Oak Ridge National Laboratory with a large batch extruder using up to 2 mol% Ne in D$_2$ confirmed that production of much larger ice quantities can be achieved. These initial explorative investigations clearly reveal the great potential of admixed pellets, although they also demonstrate that further technology efforts are required before their benefits can be utilized