The challenge of climate change and the promise of CO2-free energy production are driving the development of new nuclear fusion reactor concepts that differ significantly from systems such as ITER or JT60-SA [R1]. These new fusion reactors push the technological boundaries by reducing investment and operating costs through the use of high-temperature magnets (HTS) to confine the plasma [R4]. These HTS promise to achieve high-intensity magnetic fields while operating at higher cooling temperatures, thereby reducing the complexity of cryogenic cooling, which is normally achieved by forced circulation of supercritical helium at approximately 4.5 K (see 1.8 K for WEST/Tore Supra) delivered by a dedicated cryogenic plant.

The pulsed operation of tokamaks induces a temporal variation in the thermal load absorbed by the cooling system. This operating scenario has led to the development of several load smoothing techniques to reduce the amplitude of these thermal load variations, thereby reducing the size and power of the cooling system, with beneficial effects on cost and environmental impact. These techniques use liquid helium baths (at approximately 4 K) to absorb and temporarily store some of the thermal energy released by the plasma pulse before transferring it to the cryogenic installation [R5].

The objective of this thesis is to contribute to the development of innovative concepts for the refrigeration of large HTS systems at temperatures between 5 and 20 K. It will include (1) the modeling of cryogenic system and cryodistribution architectures as a function of the heat transfer fluid temperature, and (2) the exploration of innovative load smoothing techniques in collaboration with the multidisciplinary 'Fusion Plant' team of the PEPR SUPRAFUSION project. The first part will involve the development and improvement of 0D/1D numerical tools called Simcryogenics, based on Matlab/Simscape [R6], through the implementation of physical models (closure laws) and the selection of appropriate modeling techniques to analyze and compare suitable architectural solutions. The second part will be experimental and will involve conducting load smoothing experiments using an existing cryogenic loop operating between 8 and 15 K.

This activity will be at the forefront of the nuclear fusion revolution currently underway in Europe [R3, R7] and the United States [R4], addressing a wide range of cryogenic engineering fields such as refrigeration technologies, superfluid helium, thermo-hydraulics, materials properties, system and subsystem design, and the design and execution of cryogenic tests. It will thus be useful for the development of new generations of particle accelerators using HTS magnets.

[R1] Cryogenic requirements for the JT-60SA Tokamak https://doi.org/10.1063/1.4706907]
[R2] Analysis of Cryogenic Cooling of Toroidal Field Magnets for Nuclear Fusion Reactorshttps://hdl.handle.net/1721.1/144277
[R3] https://tokamakenergy.com/our-fusion-energy-and-hts-technology/fusion-energy-technology/
[R4] https://tokamakenergy.com/our-fusion-energy-and-hts-technology/hts-business/
[R5] “Forced flow cryogenic cooling in fusion devices: A review” https://doi.org/10.1016/j.heliyon.2021.e06053
[R6] “Simcryogenics: a Library to Simulate and Optimize Cryoplant and Cryodistribution Dynamics”, 10.1088/1757-899X/755/1/012076
[R7] https://renfusion.eu/
[R8] PEPR Suprafusion https://suprafusion.fr/