Philippe Mertens

Study of the feasibility of applying laser-induced breakdown spectroscopy for in-situ characterization of deposited layers in fusion devices

This paper presents a feasibility study of laser-induced breakdown spectroscopy (LIBS) for the development of an in-situ diagnostic for the characterization of deposition layers on plasma-facing components in fusion devices. Preferentially, LIBS would be applied in the presence of a toroidal magnetic field and under high vacuum conditions. The impact of the laser-energy densities on the laser-induced plasma parameters and correspondingly on the number of emitted photons and on the reproducibility of the LIBS method has been studied in laboratory experiments and in TEXTOR on fine-grain graphite (EK98) as well as on bulk W samples coated with carbon and metallic-containing deposits. The effect of magnetic fields and of ambient pressures in the range from 2×10–4 Pa to 10 Pa on the carbon plasma plume produced by the LIBS technique has been studied on TEXTOR between plasma pulses. The possibility of applying this method to ITER is discussed.

Approaches to Analyze Structural Issues of the European DEMO Toroidal Field Coil System at an Early Design Stage

Because of its ambitious goals, the DEMO project faces many technical challenges. The mechanical performance of the superconducting toroidal field coil (TFC) system is mostly determined by the high electromagnetic forces. A structural evaluation of the TFC of the EU DEMO 2014/2015 baseline configurations is presented. The emphasis is focused on the parametric study with the use of simplified numerical modeling. Physical interpretation of the numerical results makes the system mechanical behavior transparent and directly leads to the design recommendations. An express approach is proposed to reconstruct the detailed conductor stress state from the homogenized winding. This allows for sorting out the WP designs with respect to their robustness against acceptance criteria while giving a way for their improvement. An analysis workflow for the TFC system stress analysis is proposed. The main features of a semianalytical procedure allowing for the coil predimensioning prior to the 3-D numerical simulations are highlighted. It features the structural optimization of the layered winding, thus defining the minimum space required for the coil winding.

Optical Coatings as Mirrors for Optical Diagnostics

The aim of this work was to provide a comprehensive insight concerning coated films which might be used for first mirrors in ITER. The influence of the mirror crystallite size has been addressed as well as the coating techniques to provide nanocrystalline films. Tests of coated mirrors both in laboratories and in tokamaks are reviewed. For the tokamak tests a wide angle camera system has been installed in JET-ILW which is composed of a mirror box with 3 stainless steel mirrors coated with rhodium viewing the torus through a conically shaped aperture. The system delivered the required image quality for plasma monitoring and wall protection. No or insignificant degradation of the optical transmittance has been observed during the experimental campaign in 2014 with about 3000 plasma pulses in different magnetic field configurations.

In situ measurements of the spectral reflectance of metallic mirrors at the H

The efficient and reliable control and monitoring of the quality of the optical properties of mirrors is an open problem in laboratory plasmas. Until now, the measurement of the reflectance of the first mirrors was based on the methods that require additional light calibration sources. We propose a new technique based on the ratio of the red- and blue-shifted emission signals of the reflected hydrogen atoms which enables the in situ measurement of the spectral reflectance of metallic mirrors in low-density Ar-H or Ar-D plasmas. The spectral reflectance coefficients were measured for C, Al, Ag, Fe, Pd, Ti, Sn, Rh, Mo, and W mirrors installed in the linear magnetized plasma device PSI-2 operating in the pressure range of 0.01-0.1 Pa. The results are obtained for the Hα line using the emission of fast atoms induced by excitation of H atoms through Ar at a plasma-solid interface by applying a negative potential U = -80, …, -220 V to the mirror. The agreement between the measured and theoretical data of reflectance is found to be within 10% for the investigated materials (except for C). The spectra also allow us to efficiently determine the material of the mirror.