TY - JOUR
T1 - Appearance and analysis of a reflecting coating damage
AU - ASDEX Upgrade Team3
AU - Schubert, Martin
AU - Elgeti, Stefan
AU - Hunger, Katja
AU - Grigore, Eduard
AU - Ruset, Cristian
AU - Neu, Rudolf
AU - Stober, Jörg
AU - Vorbrugg, Stefan
AU - Zammuto, Irene
N1 - Publisher Copyright:
© The Authors, published by EDP Sciences.
PY - 2024/11/15
Y1 - 2024/11/15
N2 - Eight reflecting gratings are installed into the plasma facing wall of ASDEX Upgrade (AUG) in order to provide a controlled second pass through the plasma centre in 140 GHz heating scenarios with reduced single pass absorption. Four of these gratings are machined out of W1.4901 steel and coated with tungsten to increase the reflectivity. During plasma operation three of them worked very well, only one showed a strong correlation between the launcher On-time and an unusual increase in plasma radiation. After completion of the 2022 experimental campaign, this tile was carefully inspected. Traces of local melting were visible and the tile was examined with a scanning electron microscope to determine the surface material composition. The image of backscattered electrons revealed that tungsten is missing locally and along some of the ridges of the complex topology of this grating. Within these areas, the steel surface started to melt, which is in accordance with the assumption, that an intact tungsten coating indeed prevents the steel from melting. The damaged tile is currently being replaced and we have implemented two measures in order to prevent such damage on the new tile. The first measure is to consequently finish all machining steps before the coating procedure. This is because a mechanical damage of the coating before the installation could not be ruled out. The second measure is to control and minimize the surface roughness after machining and before the coating procedure. It turned out that the roughness was up to 3 microns in the past, which seems to be too high for the desired quality of this particular coating. We have tested and developed an electropolishing procedure to decrease the surface roughness to the order of 1 micron and keep the grating topology as precise as possible.
AB - Eight reflecting gratings are installed into the plasma facing wall of ASDEX Upgrade (AUG) in order to provide a controlled second pass through the plasma centre in 140 GHz heating scenarios with reduced single pass absorption. Four of these gratings are machined out of W1.4901 steel and coated with tungsten to increase the reflectivity. During plasma operation three of them worked very well, only one showed a strong correlation between the launcher On-time and an unusual increase in plasma radiation. After completion of the 2022 experimental campaign, this tile was carefully inspected. Traces of local melting were visible and the tile was examined with a scanning electron microscope to determine the surface material composition. The image of backscattered electrons revealed that tungsten is missing locally and along some of the ridges of the complex topology of this grating. Within these areas, the steel surface started to melt, which is in accordance with the assumption, that an intact tungsten coating indeed prevents the steel from melting. The damaged tile is currently being replaced and we have implemented two measures in order to prevent such damage on the new tile. The first measure is to consequently finish all machining steps before the coating procedure. This is because a mechanical damage of the coating before the installation could not be ruled out. The second measure is to control and minimize the surface roughness after machining and before the coating procedure. It turned out that the roughness was up to 3 microns in the past, which seems to be too high for the desired quality of this particular coating. We have tested and developed an electropolishing procedure to decrease the surface roughness to the order of 1 micron and keep the grating topology as precise as possible.
UR - http://www.scopus.com/inward/record.url?scp=85212691710&partnerID=8YFLogxK
U2 - 10.1051/epjconf/202431302002
DO - 10.1051/epjconf/202431302002
M3 - Conference article
AN - SCOPUS:85212691710
SN - 2101-6275
VL - 313
JO - EPJ Web of Conferences
JF - EPJ Web of Conferences
M1 - 02002
T2 - 22nd Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating, EC 2024
Y2 - 22 April 2024 through 26 April 2024
ER -