Book/Report

FZJ-2018-04318

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Reflektometrie mit Lichtleitern zur direkten Beobachtung des Wachstums dünner Schichten in TEXTOR

Weschenfelder, F. (Corresponding author)FZJ*

1993
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag Jülich

Please use a persistent id in citations: http://hdl.handle.net/2128/19391

Report No.: Juel-2793

Abstract: In future fusion reactors the riet erosion of wall material will determine the lifetime of the first wall in a critical way. lt is a balance between primary erosion andredeposition depending on the local plasma parameters. Only the control of these prozesses will enable a long time operation. In situ techniques to measure erosion and deposition are highly desired in order to irnprove the understanding and modeiling. So far, only ex-situ measurements on wall components are reported which had been exposed in the scrape-off-layer (SQL) to many tokamak discharges, making the detailed understanding of the observations difficult. A new technique has now been developed which is based on the interference of Iight on the thin serni-transparent layers of the redeposited material. The film (a-C:H, a-C/B:H) formed by co-deposition of carbon, boron and hydrogen. lt is depositited on a glass substrate. The end face of a two armed fibre optic bundle is attached to the backside of the substrate. The first arm of the bundle guides monochrornatic light to the substrate, where its interference at the two Interfaces glass/film and film/vacuum causesa certain reflected intensity. This is guided back by the second arm of the fibre optic bundle and detected by a photodiode. The reflected intensity is related to the actual film thickness which can grow up to more than 10 nm per discharge. Because of the difference in the refractive indices (glass: 1.5, a-C:B/H: 1.7-2.2), the reflected intensity is high (12-28 %). The observation of the reflected intensity from the backside of the target is independent of erratic target motions which would highly disturb interferometry or ellipsometry made from die front side over long observation distances (meters) and through windows as it will be unavoidable in fusion machines. The method has been tested in laboratory experiments including the characterization of die optical constants of a-C:H and a-C/B:H by ellipsometry. The thickness resolution is in the nanometer Tange. The results show that the method is applicable in TEXTOR. For the first application, the Stockholm-TEXTOR collector probe system has been equipped with a fibre optic system and quartz targets located inside a graphite housing befind a slit. The. quartz plates are exposed in die SOL of TEXTOR with there surfaces oriented perpendicular to the toroidal field direction. They can be changed after an exposure for further analysis. First exposures were made discharge by discharge observing the reflected intensities up to higher orders of interference due to the incremental growth of die deposit (up to ~ 500nm). Analysis of the extremal values provide an estimation of the optical constants and the calculation of the reflected intensity depending on thickness. By comparison, changes in the growth rates were determined wich typical values of 2-3nm/s a few centimeters in the SOL. First results show that the deposition rates in die SOL of TEXTOR decrease in ohmic discharges with increasing central plasma density suggesting that the sputter erosion at the limiters is the major source for the collected carbon. If trimethylboron is used instead of Deuterium (D$_{2}$) as fuelling gas the rates increase at higher densities. This is due to die carbon additionally introduced by the B(CD$_{3}$)$_{3}$ molecules. lt shows that the wall elements can be covered whit a protecting deposit during the tokamak discharge itself without a signifficant change in the central plasma parameters.

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Contributing Institute(s):

1. Publikationen vor 2000 (PRE-2000)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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