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Reflectivity measurements of metallic mirrors exposed in the JET tokamak

Mr. de Temmerman from University of Basel and his colleagues analysed the reflectivity of metallic mirrors exposed in the JET. For the measurements he used two spectrometers (getSpec-2048 and getSpec-NIR-1.7), a tungsten halogen light source (getLight-Hal-S) and an integrating sphere (getSphere-50).

Nuclear fusion is one of the interesting possible options for base energy in future. However, up to now, none of the existing experimental machines has shown a sufficient energy production rate. Recently the decision to built the International Thermonuclear Experimental Reactor (ITER) in France was taken. The main aim of ITER is to demonstrate the scientific and technologic feasibility of energy production by fusion reaction.
ITER is also an experimental device, a large set of diagnostic systems (about 40 individual measurement systems) will be installed on the machine to control the plasma performances and to understand the underlying physics. The high level of radiation expected in ITER will lead to enhanced absorption in refractive components such as optical windows. Therefore, metallic mirrors are planned to be used as plasma viewing components in all optical plasma diagnostics systems. Mirrors will transmit the plasma radiations from the vacuum vessel to the detectors located metres away, behind the biological shield. Therefore they need to maintain their initial reflectivity as long as possible despite the very harsh conditions the surface will experience (intense level of radiation, neutron bombardment, sputtering by high energy particles…). To decide which material will be used for the mirrors in ITER, experiments in nowadays tokamaks are needed.
The JET tokamak (Joint European Torus, located in UK) being the world’s largest tokamak, a comprehensive "First Mirror Test" was initiated to address this question. The fact that some plasma facing components are made of beryllium coated carbon raised the needs to develop a specific measurement setup compatible with handling of material contaminated by beryllium and tritium (low level).

Inside view of the JET vacuum vessel in which samples are placed at various locations

Experimental Setup:
The system has been designed to meet the following requirements: Such equipment must satisfy the following requirements: (i) the use of an integrating sphere which allows the measurement of both total and diffuse reflectivity; (ii) wavelength range 350-1700 nm to cover the visible and near-infrared; (iii) physical separation of the integrating sphere and spectrometer to minimize the size of the glove box which must be constructed to enclose the contaminated mirrors.

Schematic drawing of the system

The following items have chosen as the best compromise between safety and budgetary restrictions and the requirements for measurement range and accuracy:
- getSpec-2048 optical spectrometer with a 2048 pixel CCD (charge coupled device) detector and a 200 Ám entrance slit (350-1100 nm) ;
- getSpec NIR-1.7 256 PC optical spectrometer with a peltier cooled 256-element InGaAs diode array detector (900-1700 nm);
- 50 mm diameter integrating sphere coated with PTFE (PolyTetra-Fluoro Ethylene, i.e. Teflon«). A gloss trap coated with a black absorbing material is used to exclude specular reflection and measure only the diffuse reflectivity;
- 400 Ám core diameter optical fibres.

The equipment has calibrated, using samples whose reflectivity was measured both by a UV-Vis-NIR spectrophotometer equipped with an integrating sphere and by spectroscopic ellipsometry. Results, presented in figure 3, are found to be in good agreement over the whole wavelength range.

Comparison between a total reflectivity measurement made on a JET molybdenum sample
with the setup installed at JET and with the spectrophotometer in Basel (Varian Cary 5)

A picture of the final setup is shown in Fig. 4. Only the measurement units (i.e. the integrating sphere and transmission sample holder) are installed in the glove box. The connection to the spectrometers and the light source is made by means of optical fibers.

Final set-up of the equipment with the measurement units
(integrating sphere, transmission holder) installed in a glove box

Last change 08/15/2007 01:33 PM
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