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Cryogenic pellets to mitigate ITER plasma disruptions

ITER plasma can be subject to very strong instabilities, called disruptions, which could damage the tokamak. Researchers at IRIG have entirely designed and built TYFANIES, a full-scale test bench able to produce and accelerate cryogenic pellets at 1 km/s to fragment them in the plasma and mitigate the effects of disruptions. These centimeter-sized cryogenic pellets are produced in an "in situ condensation" cell cooled by liquid helium, a technological "innovation" of these researchers.

Published on 7 June 2022
Cryogenic pellets injectors are a key element of tokamaks for plasma control either for fueling during thermonuclear fusion reactions by magnetic confinement, or for reactor protection in case of high-energy plasmas disruptions. For this last application, the plasma, subjected to very strong instabilities, like solar eruptions, called disruptions, risks to damage the first walls of the plasma chamber and the mechanical structure of the tokamak. Thus, disruptions must be detected and instantly mitigated. It is therefore crucial to develop an efficient system to mitigate the effects of disruptions in high-energy plasmas.

This is the objective of the DMS (Disruption Mitigation System) project for ITER, which proposes the massive injection of centimeter-sized cryogenic pellets (Hydrogen, Deuterium and Neon) fragmented at the entrance of the plasma chamber. To this end, 27 pellet injectors will eventually be installed on ITER to allow the safe controlled operation of high-energy plasmas.
A scale 1 test bench (Photograph 1) has been entirely designed and built at IRIG by the Low Temperature Systems Department in the framework of the "Technology Fundamental Studies" contract in response to a need expressed by ITER. This test bench, named TYFANIES, is now operational. For this purpose, cryogenic pellets are produced in an " in situ condensation" cell cooled by liquid helium, a past technological "innovation" of the DSBT adapted for the larger size pellets for DMS needs (diameters from 10 to 30 mm and lengths from 20 to 60 mm).

Photograph 1: Overview of the TYFANIES pellets injection bench for ITER-DMS.

They are then accelerated up to 1 km/s with a single stage gas gun. The characterization of the observed phenomena (dimensions, velocity and trajectory of the pellets) is performed in a vacuum optical diagnostic line until the experimental chamber, simulating the ITER plasma chamber.
With the acceleration of the first centimetric hydrogen pellets to 1 km/s (Photograph 2) in the test bench, a major milestone in the project has been reached in the first quarter of 2022. Experimental campaigns are now underway to study the formation and propulsion conditions of the pellets and to maximize their reproducibility, and thus ultimately ensure the reliability of the ITER protection system. 

Photograph 2: Centimetric hydrogen pellet measured at 920 m/s.

Once these parameters of formation and acceleration of the pellets are mastered, additional studies will be carried out on the bench in order to study their fragmentation at the entrance of the plasma in order to go towards a maximum efficiency of the DMS system. In parallel, technological developments (extrusion source, propulsion system) are also foreseen to answer the next calls of project in the field of cryogenic pellets injectors for magnetic confinement fusion reactors.

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