
Introduction : We investigate the possibility to carry out exploratory work on cryogenic systems capable to deliver the cooling power to high temperature superconducting-based fusion devices (tokamaks, stellerators) operating at cooling temperatures higher than the classical applications (typically 4.5 K with supercritical helium – i.e. for ITER, JT60-SA [R1], DTT) and spanning from ~5 K to 20 K (i.e. in the SPARC tokamak [R2]). Additionally, the higher temperatures of the coolant require reconsidering the thermal load smoothing techniques and architectures typical of the pulsed plasma operation in tokamak devices for optimal operation and design-to cost assessments. The main question is whether a cryogenic plant and its main sub-systems (i.e. the circulators, the thermal buffer) can be designed to perform these functions with a good thermodynamic efficiency under these new operating temperatures [R3].
Technical context : The challenge of the global warming and the production of CO2-free energy spurred the development of new and bold concepts of nuclear fusion reactors which differ substantially from systems such as ITER (in development) or JT60-SA (recently commissioned [R1]). These new fusion reactors push the technological boundaries for lower capital and operational cost by using (among others) HTS-based magnets to confine the plasma [R4]. These high temperature superconducting materials promise to achieve high strength magnetic fields while being operated at higher cooling temperatures to reduce the cumbersome complexity of the cryogenic cooling which is normally carried out by means of a forced circulation of supercritical helium at ~4.5 K (or lower) delivered by a dedicated cryo-plant.
The tokamaks, in particular, work by a pulsating plasma operation thus leading to the temporal variation of the thermal load absorbed by the refrigerating system. This operating scenario lead to the development of several load smoothing techniques to reduce the amplitude of the thermal load variations thus reducing the size and power of the refrigeration system with beneficial effects on the cost and the environmental impact. Such techniques use liquid helium bathes (at ~4 K) to absorb and temporarily store some of the thermal energy released by the plasma pulse before delivering it to the cryo-plant [R5].
The implementation of HTS-based magnets operating at high temperature can alter substantially the architecture of the refrigeration system in several multi-disciplinary areas such as:
- High speed turbomachinery for the refrigeration process as long as the circulation of the coolant
- Transient thermal-hydraulics for the thermal buffering method
- Command and control for optimal operations
Objective of the activity : The goal of the activity is to contribute to the development of innovative concepts for the refrigeration of large HTS systems at temperatures between 5 K and 20 K. It will involve the conceptual design of the cryo-plant and cryo-distribution architectures based on the temperature of the coolant, as long as of the buffering techniques with steady state balance-of plants Excel™ models. The results will also be used to assess the feasibility of the architectures and the availability of critical sub-systems (i.e. the preliminary sizing of high speed rotating machinery such as compressors and circulators by means of Balje diagrams may be foreseen). Sensitivity simulations on the boundary conditions and the main driving parameters of the cryogenic plant will also be performed. The promising concepts may be further analysed by means of a system level tool developed by the DSBT based on Matlab™/Simscape™ [R6] called Simcryo.
The activity will be divided in several phases:
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Phase 1: Bibliographic review. The phase will focus initially on the collection, analysis and synthesis of the existing literature on the topics by the Candidate as long as its training to use the DSBT in-house numerical tools. A magnetic fusion system reference case will be defined based on this analysis
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Phase 2: Innovation. During this phase the creativity of the candidate (supported by the Tutors) will let be free to propose innovative refrigeration system configurations, simulate them on simplified models and rank them based on objectives criteria and the performance results
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Phase 3: Consolidation. Based on the previous phase output and assessment the most promising configuration will be further investigated possibly by using the in-house simulation tool. The quantitative evaluation of their performance will be accompanied with the preliminary sizing of its inner components in order to identify future axis of technological development and maturation.
- Phase 4: Synthesis. The entire activity will be completed by the detailed synthesis of the work performed, the results and the outcomes as long as the perspectives.
The topic of the internship will be at the forefront of the nuclear fusion revolution currently taking place in Europe [R3][R7] and in the United States [R4] addressing a broad range of cryogenic engineering domains such as helium, hydrogen thermal-hydraulics, material properties, system and sub-system design, plant balance and operations with the possibility to explore creative and unconventional approaches. The learning will be directly applicable within the fusion domain and/or the sprawling activities related to the cryogenic hydrogen-based low carbon emissions mobility systems (aircrafts, heavy trucks, boats).
Candidate requirements : Candidate required competences: Curiosity and autonomy.
Fluids mechanics and thermodynamics. Knowledge of cryogenic processes, Numerical modelling capabilities in Excel™, Matlab™, Simulink/Simscape will be an added value.
References : [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/

Contact : Davide Duri
davide.duri@cea.fr