Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade
Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade
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ASDEX 升级中 ELM 瞬时熔化钨的实验
DOI：
10.1088/1741-4326/aa9a05
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发表时间：
2018
期刊：
影响因子：
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通讯作者：
P. Vondracek
中科院分区：
文献类型：
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作者：
K. Krieger;M. Balden;J. Coenen;F. Laggner;G. Matthews;D. Nille;V. Rohde;B. Sieglin;L. Giannone;B. Göths;A. Herrmann;P. de Marne;R. Pitts;S. Potzel;P. Vondracek
ASDEX Upgrade 研究了源自边缘局域模式 (ELM) 的功率瞬变导致钨的重复熔化。使用偏滤器操纵器 II (DIM-II) 系统将钨样品在外偏滤器靶板上暴露于 H 模式放电（Herrmann 等人 2015 Fusion Eng. Des. 98–9 1496–9）。设计为 JET 托卡马克单独实验中使用的几何形状的近似复制品（Coenen 等人 2015 J. Nucl. Mater. 463 78–84；Coenen 等人 2015 Nucl. Fusion 55 023010；Matthews 等人 2016 Phys. Scr. T167 7），这些样品具有未对准的前缘和倾斜的边缘分别为山脊。两种结构都突出到默认目标板表面上方，因此接收平行功率通量的比例增加。通过将外部撞击点移动到样品位置来引发 ELM 的瞬时熔化。测量到的电流从样品到容器电位的时间演变证实了瞬时熔化。电流大小和对表面温度的依赖性为热电子发射作为驱动熔体运动的替代电流的主要来源提供了强有力的证据。在两个样品几何形状暴露后观察到的不同熔化模式支持 MEMOS 熔化运动代码中使用的热电子发射模型，该模型假设热电子净电流在浅磁场到表面角度时强烈减少（Pitts 等人 2017 Nucl. Mater. Energy 12 60–74）。对回收样品的暴露后异位分析显示，暴露表面区域的钨再结晶深度可达数毫米。熔体层传输到较少暴露的表面区域导致重新凝固碎片的棘轮堆积，并从表面已经扩大的晶粒延伸出区域生长。
Repetitive melting of tungsten by power transients originating from edge localized modes (ELMs) has been studied in ASDEX Upgrade. Tungsten samples were exposed to H-mode discharges at the outer divertor target plate using the divertor manipulator II (DIM-II) system (Herrmann et al 2015 Fusion Eng. Des. 98–9 1496–9). Designed as near replicas of the geometries used also in separate experiments on the JET tokamak (Coenen et al 2015 J. Nucl. Mater. 463 78–84; Coenen et al 2015 Nucl. Fusion 55 023010; Matthews et al 2016 Phys. Scr. T167 7), the samples featured a misaligned leading edge and a sloped ridge respectively. Both structures protrude above the default target plate surface thus receiving an increased fraction of the parallel power flux. Transient melting by ELMs was induced by moving the outer strike point to the sample location. The temporal evolution of the measured current flow from the samples to vessel potential confirmed transient melting. Current magnitude and dependency from surface temperature provided strong evidence for thermionic electron emission as main origin of the replacement current driving the melt motion. The different melt patterns observed after exposures at the two sample geometries support the thermionic electron emission model used in the MEMOS melt motion code, which assumes a strong decrease of the thermionic net current at shallow magnetic field to surface angles (Pitts et al 2017 Nucl. Mater. Energy 12 60–74). Post exposure ex situ analysis of the retrieved samples show recrystallization of tungsten at the exposed surface areas to a depth of up to several mm. The melt layer transport to less exposed surface areas leads to ratcheting pile up of re-solidified debris with zonal growth extending from the already enlarged grains at the surface.