Kurzbeschreibung
(Deutsch)
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Projektziele
Surface Studies Related to Fusion Reactor Materials
as Projekt umfasst zwei Teilbereiche, die sich in unterschiedlicher Weise mit der Wechselwirkung eines thermonuklearen Fusionsplasmas mit den Wandmaterialien des Vakuumgefässes des jeweiligen Reaktors beschäftigen. Für die Weiterentwicklung der thermonuklearen Fusion zur Energieerzeugung ist diese Plasma-Wand-Wechselwirkung von hoher Bedeutung, weil sie direkt die Reaktion beeinflusst. Die Wirkung kann von schwer zu beherrschenden Instabilitäten im Plasma aufgrund von Verunreini-gungen bis zur Verhinderung der Fusionsreaktion insgesamt reichen. Für eine Verwendung kommen daher Materialien in Frage, die auch unter den extremen Belastungen der Fusionsreaktion nur eine begrenzte Erosion erfahren und somit das Ausmass an Plasmaverunreinigungen gering halten. Im Idealfall ist das Material zusätzlich in der Lage, als Fängermaterial (Getter) solche Verunreinigungen aus dem Restgas abzufangen.
eiden Projektteilen gemeinsam ist der Einsatz der Photoelektronenspektroskopie (PES) als Methode zur chemischen und physikalischen Charakterisierung der Oberflächen von Materialien, die für die Verwendung in der sogenannten "ersten Wand" von Fusionsforschungsreaktoren untersucht werden.
Ex-situ-Experimente befassen sich mit der Oberflächenanalyse von Proben aus der ersten Wand des Fusionsforschungsreaktors TCV (Tokamak à configuration variable) des Centre de recherche en physi-que des plasmas (CRPP). Die erste Wand des Vakuumgefässes ist in diesem Fall zu etwa 95% mit Gra-phitziegeln bedeckt. Während des Betriebes finden routinemässig Boronisierungsentladungen statt, um die Wände zu konditionieren. Proben aus dem Graphit dienen darum dazu, die Qualität der Boro-nisierung zu überprüfen und die Anreicherung von Verunreinigungen im Wandmaterial frühzeitig zu entdecken. Die Planung für 2001 sah zunächst nur die üblichen Analysen von wenigen Testsubstraten vor, die der Boronisierung ausgesetzt werden sollten. Die Entnahme einer grösseren Zahl von Proben aus der ersten Wand des TCV erfolgte gegen Ende des Jahres 2000, so dass für sie noch ein provisori-sches Messprogramm vorgesehen werden konnte.
In-situ-Laborexperimente dienen zur Präparation und Charakterisierung potentieller Materialien für die erste Wand. Da in unserem Labor alle Versuchsschritte einschliesslich umfangreicher Probenpräpa-ration unter Ultrahochvakuum-Bedingungen durchgeführt werden können, ist es möglich, auch reak-tive Gettermaterialien in unreagiertem Zustand zu handhaben. Dies ist eine Grundvoraussetzung, um solche Materialien hinsichtlich ihrer Reaktivität kontrolliert untersuchen zu können. In diesem Bereich war der vorläufige Abschluss der Arbeiten an Systemen mit niedriger Ordnungszahl vorgesehen.
Referenzen
1] Halbjahresbericht dieses Projektes, Juni 2001
2] M. Töwe: Photoelectron spectroscopy studies of carbon based fusion reactor materials, Dissertation Univ. Basel, 2000; die Arbeit wird nach Annahme der endgültigen Version nachgereicht und steht dann auch online zur Verfügung.
3] Jahresbericht dieses Projektes, Dezember 2000
Autor und Koautoren : P. Oelhafen, M. Töwe
Beauftragte Institution : Institut für Physik der Universität Basel
Adresse : Klingelbergstrasse 82, CH-4056 Basel
Telefon, E-mail, Internetadresse : +41-(0)61-267 37 13, peter.oelhafen@unibas.ch ; http://www.physik.unibas.ch/dept/pages/de/personnel/oelhafen.htm
BFE Projekt-/Vertrag-Nummer : 16873/56366
Dauer des Projekts : 01. April 1996 - 30. Juni 2006
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Kurzbeschreibung
(Englisch)
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Nuclear fusion is going to enter a new era. The next international reactor, ITER, will reach performances never attained until now. To control and understand these performances, knowledge of the plasma parameters is of highest interest. Several diagnostic techniques will be used in order to achieve this aim. However, due to the severe conditions, direct viewing of the plasma will not be possible. That is why metallic mirrors will be used as plasma facing components or as light transmitters through a labyrinth. Depending on the distance between the mirror and the plasma, mirrors will have to withstand different damaging mechanisms. Indeed, mirrors close to the plasma will suffer from erosion whereas deposition of impurities (carbon, boron ¿) will occur on mirrors placed in the labyrinths (remote from the plasma). High-Z refractory materials such as molybdenum and tungsten may have sufficiently long lifetime when erosion is the dominant mechanism. Reactivity of these metals towards impurities found in the pla sma has to be investigated because modification of the surface chemistry may have an influence on the optical properties. We have used surface analysis techniques to study the reactivity of molybdenum towards oxygen and carbon. First results about reactivity of tungsten towards oxygen are also presented. Results have shown a great influence of the surface composition on the optical properties. Simultaneously, we have continued and developed our collaborations with groups from different tokamaks (TCV, Textor, JET, and Tore Supra). Indeed, we made optical characterizations on metallic mirrors exposed in Textor and Tore Supra; we are involved in the first mirror test in JET and we plan to expose a new set of samples in TCV.
Surface Studies Related to Fusion Reactor
Materials responsible: Dr. P. Reinke, Dipl. Chem. M. Töwe, Prof. Dr. P. Oelhafen
This project is concerned with the preparation and characterization of materials which are of interest with respect to the application in the first wall of Tokamak-type fusion reactors. In a thermonuclear fusion device, materials have to meet - among others - the following requirements: Where transport of eroded material to the plasma centre is possible, all elements involved should have low atomic numbers Z in order to minimize radiative energy loss. A thermally stable and inert structural component is to be combined with a more reactive species as oxygen getter. The removal of oxygen contaminations from the residual gas in the reactor improves the performance of the fusion plasma. While most of the work within this project is conducted on a laboratory scale in an ultrahigh-vacuum apparatus at the Universität Basel, some of the samples investigated originate from the Tokamak à Configuration Variable (TCV) at the Centre de Recherches en Physique des Plasmas (CRPP) at Lausanne. These samples are taken fro m first wall materials and from layers deposited during boronization procedures. Analysis of these samples is particularly important for the characterization of the boronization.
Materials and Methods:
uring the course of this project a variety of materials has been investigated, beginning with combinations of boron and carbon, including boron and B4C and a large number of films from various precursor compounds. In parallel, measurements on samples from boronization procedures in tokamaks were performed.
In an attempt to further enhance oxygen gettering capability while lowering Z, studies continued with combinations of lithium and amorphous hydrogenated carbon, a-C:H. These experiments have later been shifted towards hydrogen-free materials in order to establish the metal-carbon interaction in more detail. This is especially important for the option of adding lithium to existing graphite first walls.
According to the variety of materials, different deposition methods have been applied, including sputter deposition, plasma activated chemical vapour deposition (PACVD, both rf and dc), ion beam deposition and evaporation/sublimation techniques.
All these methods can be applied under ultrahigh-vacuum (UHV) conditions and samples can be transferred to the spectrometer chamber without breaking the vacuum. This is an essential prerequisite for the application of our surface sensitive analytical techniques (photoelectron spectroscopy with ultraviolet light and x-rays (UPS, XPS), electron energy loss spectroscopy (EELS)). These measurements are supplemented by optical and microscopic methods.
The in situ analysis with UPS and XPS allows to determine the respective material's surface's elemental composition and its electronic properties. In particular, it is possible to monitor the reactivity towards reactive species such as oxygen molecules and ions, hydrogen and hydrocarbon species and others by repeated exposure and measurement cycles. Our methods then yield the elemental composition of the material's surface after exposure to such species.
Future:
More recent research in Tokamaks has shown that high Z materials may well be employed in certain regions of a reactor because ionic particles eroded from these materials are redeposited close to their origin while improved plasma control allows to prevent neutral particles from unwanted transport to the plasma centre.
In combination with the increasing thermal loads, this makes the application of both low Z and high Z materials - each in a certain region - a perspective for the next step device ITER-FEAT. Our own research has already started with respective experiments on Si/C and Mo/C materials.
Publications:
P. Reinke, D. Rudmann, and P. Oelhafen, Temperature dependence of silicon carbide interface formation: A photoelectron spectroscopy study, Phys. Rev. B 61 (2000) 16967-16971.
P. Reinke and P. Oelhafen, The molybdenum-carbon interface: formation and electronic structure of the carbide layer, Surface Science 468 (2000) 203-215.
M. Töwe, P. Reinke, and P. Oelhafen, Alkali-metal containing amorphous carbon: reactivity and electronic structure, in Amorphous and Nanostructured Carbon, edited by J.P. Sullivan, J. Robertson, O. Zhou, T.B. Allen, and B.F. Coll (Mater. Res. Soc. Proc. 593, Warrendale, PA,2000) pp.167-172.
Töwe, P. Reinke, and P. Oelhafen, Reactivity of lithium-containing amorphous carbon (a-C) films, to be published in J. Nucl. Mater. (Proceedings of the 14th Int. Conf. on Plasma Surface Interactions, Rosenheim (Germany) May 22-26, 2000).
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Schlussbericht
(Englisch)
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In the course of the ongoing project, our research was focused on the investigation of the erosion and deposition mechanisms affecting the optical reflectivity of potential materials for ITER first mirrors both by active collaborations with groups from different Tokamak sites and by simulation in laboratory experiments in Basel. At Forschungszentrum Jülich (Germany) studies were initiated by exposing polycrystalline molybdenum and tungsten mirrors in the erosion and deposition dominated zones in the scrape-off layer (SOL) plasma of TEXTOR. Recently, a comparative test of single crystalline and polycrystalline mirrors under erosion conditions showed the superiority of single crystalline mirrors in terms of homogeneous sputtering resulting in a very low deterioration of optical reflectivity. Such mirrors may have a sufficient lifetime under conditions where erosion is the dominating damaging effect.
Long-term exposure of mirrors made of different materials was performed in Tore-Supra (CEA Cadarache, France). The mirrors were installed on the first wall and open to the plasma, being submitted to bombardment of charge exchange neutrals during the Tokamak discharges and to helium ions during the conditioning glow discharge. It was found that a major part of mirror erosion is due to glow discharge, putting the stress on the necessity in ITER to protect the mirrors during conditioning discharges.
At CRPP-EPFL, mirror samples prepared from different materials (Mo, W, Si) have been installed on a specially designed manipulator that allows their insertion in the divertor floor region of TCV. A separate pumping system allows the mirrors to be easily inserted and retrieved following exposure without any requirement for a vacuum vessel vent. Samples are recessed behind the front surface of the graphite divertor floor tiles to avoid direct plasma impact. Under identical exposure conditions the mirror substrate can strongly influence the deposit thickness found on the sample: the carbon layer thickness on a Si sample is found to be five times higher than on a Mo substrate. This substrate dependent deposition efficiency may be of importance for the mirror material choice in ITER.
At JET, a comprehensive mirror test has been initiated within the framework of tritium retention studies. Mirrors from stainless steel and molybdenum are being exposed at various locations in the divertor (outer, inner and base) and on the first wall. Mirrors are exposed in the vicinity of quartz microbalances allowing monitoring on a shot by shot basis the erosion and deposition experienced by the mirrors. Besides its involvement in the experiment, our group was responsible for the design and installation of a spectrophotometer compatible with beryllium handling. This system will be used for optical measurements on mirrors exposed during the 2006 campaign.
In the laboratory in Basel, mirrors made from polycrystalline copper and stainless steel were exposed to low temperature deuterium plasma with controlled partial pressure of methane in the gas mixture. Under similar conditions markedly different erosion and deposition patterns are observed on different materials, showing the influence of the material itself on the erosion and deposition mechanisms.
Auftragnehmer/Contractant/Contraente/Contractor:
Institut für Physik der Universität Basel
Autorschaft/Auteurs/Autori/Authors:
De Temmerman,G.
Oelhafen,P.
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