The production of cryogenic solids by desublimation is involved in numerous scientific and technological applications, including tokamak fusion technologies. In magnetic confinement fusion devices such as ITER, large solid hydrogen pellets are produced and accelerated as part of the Shattered Pellet Injection (SPI) disruption mitigation strategy. Recent studies have demonstrated the feasibility of producing ITER-scale pellets by cryogenic desublimation, while also highlighting the significant impact of fabrication conditions on pellet quality and launchability [3, 4]. Nevertheless, the relationship between cryogenic process parameters and the resulting material properties is not fully understood.
 
This work presents an experimental investigation into how cryogenic fabrication parameters influence the formation of solid hydrogen produced by in-situ desublimation inside a cryostat developed at CEA-IRIG/DSBT for ITER Disruption Mitigation System studies [4, 5]. The study will focus on three key process variables that govern cryogenic growth conditions: temperature, pressure, and gas flow rate.
 
A dedicated experimental facility allows cylindrical solid hydrogen samples to be produced under controlled conditions that are representative of ITER pellet manufacturing (solid hydrogen of Ø = 28.5 mm and L = 57 mm produced at 5 Kelvin). Parametric campaigns are conducted by independently varying temperature (5 K, 8 K and 10 K), pressure (between 12 mbar and 72 mbar) and gas flow rate in order to quantify their impact on desublimation kinetics and cryogenic solid growth. Particular attention is given to the formation of structural porosity, density gradients and cryogenic snow structures, all of which have recently been identified as key features affecting pellet integrity and launch behavior [3].
 
Preliminary results reveal a strong dependency of the final characteristics of solid hydrogen on fabrication parameters. Variations in deposition temperature, pressure and gas flow rate have a significant impact on pellet morphology, structural homogeneity, detachment pressure and the population of in-flight debris. These findings support the hypothesis that cryogenic growth conditions play a critical role in determining the performance of desublimated hydrogen solids.
 
[1] Lehnen M. et al., R&D for Reliable Disruption Mitigation in ITER, Proc. IAEA FEC, 2018, https://inis.iaea.org/records/178bs-7kz12
[2] Baylor L.R. et al., Shattered pellet injection technology design and characterization for disruption mitigation experiments, Nucl. Fusion 59 (2019) 066008, https://doi.org/10.1088/1741-4326/ab136c
[3] Zoletnik S. et al., Production and launch studies of cryogenic pellets for the ITER disruption mitigation system, Nucl. Fusion 64 (2024) 096033, https://iopscience.iop.org/article/10.1088/1741-4326/ad6676
[4] Manzagol J. et al., Simulation and developments for large pellet formation and acceleration for shattered pellet injection of the ITER DMS, Fusion Engineering and Design 191 (2023) 113665, https://doi.org/10.1016/j.fusengdes.2023.113665
[5] Berton J. et al, Impact force measurements of solid cryogenic pellets for Disruption Mitigation Systems. Available at SSRN: http://dx.doi.org/10.2139/ssrn.6792893
 

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