Micro-structuring of tungsten surface for mitigation of ELM induced thermal fatigue

Micro-structuring of tungsten surface formitigation of ELM induced thermal fatigueA. Terra *, G. Sergienko, A. Kreter, M. Rasiński, M. Wirtz, Th. Loewenhoff, G. Pintsuk, D. Dorow-Gerspach, Y. Mao, D. Schwalenberg, L. Raumann, J. W. Coenen,S. Brezinsek, B. Unterberg, and Ch. LinsmeierForschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung, 52425 Jülich, [email protected] micro-structured tungsten is a new concept trying to address one of the main issues of tungsten as high heat flux (HHF) plasma facing material (PFM), which is its brittleness and its propensity to crack formation under pulsed, ELM like, heat loads [1, 2]. With power densities between 100 MW/m² and 1 GW/m², cumulative thermal fatigue adds up for each pulse and show damage like roughening, subsequent cracking and even melting may occur, all influenced by the base temperature. This represents a serious issue for the use of tungsten as HHF-PFM, as in future tokamaks, such as ITER, about 108 ELMs are expected to occur during the operational lifetime. This is a clear limitation to exploitation and brings concerns in term of maintenance and costs.In the past, several approaches have tried to overcome this brittleness issue, e.g. alloying tungsten with others elements [3] or introducing pseudo-ductility by the additions of fibers thus creating composites [4], or even by specific grain size and shape. However all these approaches show concerns in term of durability because they may loose their effectiveness with time, neutron irradiation or high temperature (recrystallization), a concern that does not applies to micro-structure tungsten. The current investigation of both, reference and micro-structured tungsten, was performed in the PSI-2 facility [5] with a combined steady state deuterium plasma (5.1×1025 D+ m-2, 51 eV, 180°C, 150 min) loading with sequential laser pulses (up to 7.104 pulses of 0.68 GW/m2, 3 mm spot diameter, 23 J, 1 ms pulse duration, up to 25 Hz pulse frequency). Recrystallized fibers were used for micro-structured tungsten sample, in order to isolate the intrinsic material properties contribution to the performance already demonstrated. On contrary to reference tungsten sample, the micro-structured sample, despite recrystallized material, did not revealed any damage. To confirm these results, micro-structured and reference tungsten samples have been loaded with up to 106 pulses of 0.55 GW/m2 and 0.5 ms pulse length in the high power electron beam facility JUDITH 2 and characterized by metallographic means. The results of both experiments will be discussed and compared

Micro-structured tungsten transient thermal fatigue resilience orders of magnitude improvement

31st Symposium on Fusion TechnologyMicro-structuredtungsten transient thermal fatigue resilienceorders of magnitudeimprovementA.Terra *;G.Sergienko;A.Kreter;M.Rasiński;M.Wirtz;Th.Loewenhoff;G.Pintsuk;D.Dorow-Gerspach;Y.Mao;D. Schwalenberg;L. Raumann;J.W.Coenen;S.Brezinsek;B.Unterberg;and Ch.LinsmeierForschungszentrum Jülich GmbH, Institut für Energie-und Klimaforschung,52425 Jülich, [email protected] micro-structuredtungsten is a new concept trying to addressone of the main issuesof tungsten as high heat flux (HHF)plasma facing material(PFM), which is itsbrittleness and its propensity to crackformationunder pulsed, ELM like,heat loads [1, 2].With power densities between 100 MW/m²and 1GW/m², cumulativethermal fatigueadds up for each pulse and showsdamage like roughening. Subsequent cracking and even meltingmayoccur,all influenced by thebase temperature. This representsa serious issue for the use of tungsten as HHF-PFM, as in future tokamaks,such as ITER, about 108ELMs are expected to occur during the operationallifetime. This is a clear limitation to exploitation and brings concerns in term of maintenance and costs.In the past, severalapproacheshave tried to overcomethis brittleness issue, e.g.alloying tungsten with others elements[3]or introducingpseudo-ductility bythe additions of fibersthus creatingcomposites[4], or even by specific grain size and shape. However all these approaches show concerns in termofdurability because they may losetheir effectiveness with time, neutron irradiation or high temperature (recrystallization),a concern that does not applyto micro-structure tungsten[5]. The microstructure tungsten concept already showed and confirmedanimprovement by a factor 105in its ability to handle ELM like events [5, 6]The currentinvestigationofboth, reference and micro-structured tungsten, was performedin the PSI-2 facility[7]with a combined steady state deuterium plasma (5.1×1025D+m-2, 51 eV, 180°C, 150 min)loading with sequential laser pulses (up to 1.105pulses of 0.68 GW/m2, 3mm spot diameter, 23 J,1 ms pulse duration,up to 25 Hzpulse frequency).Recrystallized fibers were usedfor micro-structured tungsten sample, in order to isolate the intrinsic material properties contribution to the performance already demonstrated. Contrary to the reference tungsten sample, the micro-structured sample, despite recrystallized material, did not revealany damage. To confirm these results, micro-structured and reference tungstensampleshave been loadedwith up to 106pulses of 0.55GW/m2and 0.5ms pulse length in the high powerelectron beam facility JUDITH2and characterized by metallographic means.The results of both experiments will be discussed and compared.[1] Th. Loewenhoff, et al., Nuclear Fusion 55 (2015)123004, doi.org/10.1088/0029-5515/55/12/123004[2] M Wirtz, et al., Physica Scripta T170(2017), 014066, doi.org/10.1088/1402-4896/aa909e[3] Ch. Linsmeier, et al., Nucl. Fusion 57 (2017) 092007, doi.org/10.1088/1741-4326/aa6f71[4] J. W. Coenen, et al., Nuclear Materials and Energy 15 (2018) 214-219, doi.org/10.1016/j.nme.2018.05.001[5] A. Terra, et al., Nuclear Materials and Energy 19(2019) 7, https://doi.org/10.1016/j.nme.2019.02.007[6] A. Terra, et al., A Terra et al 2020 Phys. Scr. 2020 014045, https://doi.org/10.1088/1402-4896/ab4e33[7] A. Kreter,et al., Fusion Science and Technology 68(1)(2015) 8-14, doi.org/10.13182/FST14-90