Mechanical and Materials Development of Radio-Frequency Contact for the ITER Ion Cyclotron Resonance Heating Antenna

Abstract : Ion Cyclotron Resonance Heating (ICRH) is one of the most important plasma heating methods in magnetically confined fusion experiments. In ITER, two ICRH antennas are designed to supply 20 MW of Radio-Frequency (RF) power at 40-55 MHz to heat the plasma. RF sliding contacts are used in the antennas to allow their remote handling assembly and to improve their maintainability, as well as to absorb the thermal expansion of the RF conductors during operations. One of the RF contacts is designed to be operated at 2.25 kA in steady-state (1200 s), with a current density of 4.8 kA/m. With such current levels, high heating occurs at the contact area which threatens the structural and material safety of the RF contacts and constrain their life time. In addition, before operation of the ITER ICRH antennas, all the in-vessel structures will be baked at 250°C during thousands of cumulated hours for outgassing. In CEA, R&D work on RF contact development has been carried out for 10 years. Recently, Ag-coated CuCrZr louvers RF contact prototype based on Multi-Contact LA-CUT commercial contact configuration was tested on TITAN test-bed. 1500 A, 1200 s steady-state operation was achieved. However, due to burn failure, the RF contact prototype couldn’t reach 1200 s steady-state under 2 kA as expected. In order to improve the performances of the RF sliding contacts to match ITER requirements, failure mechanisms of RF contacts during RF operations were analyzed and possible materials or coated systems that can be used for RF sliding contacts compatible with the ITER environment have been studied in detail within this thesis work. The effects of material selection, cooling parameter and contact resistance on louvers temperature have been modelled and simulated through finite element methods. Moreover, functional coatings like Ag, Au, Rh and their alloys were manufactured by electroplating on 316L and CuCrZr, which are commonly used as base materials on tokamak. By mimicking the ITER baking conditions, the coated samples were thermal aged under vacuum at 250ºC for 500 h, after which the materials properties evolution such as hardness, grain size and adherence was characterized. In addition, the coating life time has been evaluated through cross-sectional diffusion characterizations. In order to evaluate the electrical and tribological behavior of the coated material pairs, a dedicated and innovative test bed was designed during this thesis and used successfully. On this test bed the samples can be heated up to 350ºC and the vacuum can reach 10-5 Pa. Static contact resistance as well as transient contact resistance/friction coefficient of sliding pairs can be measured. Sliding and electrical tests of uncoated 316L/CuCrZr pair and coated pairs Ag/Rh and Au-Ni/Rh were carried out. The relationships between vacuum condition, temperature, contact force and the static contact resistance have been studied and an optimal contact force was selected. Under this optimal contact force, the sliding tests were launched and the evolutions of friction coefficient as well as contact resistance were analyzed. Through wear characterizations, the life times of the coatings were evaluated. Finally, based on the first tribological results obtained on commercial Au-Ni and Au-Co alloy coatings, the feasibility of depositing solid lubricant containing Au-Ni and Au-Co composite coatings to minimizing wear and friction coefficient was evaluated. Au-Ni/a-C and Au-Co/WS2 composite coatings were successfully developed by using magnetron sputtering and electroplating methods respectively. Their tribological performance under ITER relevant vacuum condition had been studied.
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Zhaoxi Chen. Mechanical and Materials Development of Radio-Frequency Contact for the ITER Ion Cyclotron Resonance Heating Antenna. Electromagnetism. l'Université Toulouse 3 - Paul Sabatier, 2018. English. ⟨tel-02052920⟩

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