Investigation of the He content in W layers, deposited by Ar-He magnetron discharges, by LIDS, LIBS and TDS measurements

Fuel accumulation in plasma-facing and structural materials used in fusion devices is highly important both for radiation safety and for the assessment of the impact of gas recycling on plasma operation. Co-deposition, that is simultaneous deposition of previously eroded plasma facing materials and plasma particles, is considered one of the primary sources of hydrogen fuel accumulation in tokamaks [1]. Helium (He) quantity in future reactors will be about the same as that of fuel particles due to deuterium-tritium (D-T) fusion reactions. It is well known that radiation/exposure causes much more material damage than hydrogen atoms such as He bubbles and nano-structures, known as “fuzz” [2]. As laboratory experiments have demonstrated [3], He is trapped in tungsten (W) samples by ion and plasma implantation. Therefore, it is important to investigate how He is accumulated in co-deposited or re-deposited layers observed in fusion devices. As reported in the literature [4] W-He co-deposits, which simulate the re-deposited layers, can be produced in the laboratory by exploiting magnetron sputtering technology. In this contribution, the laboratory production of reference coatings mimicking tokamak W-He deposits and the characterization of their morphology and He content are presented. The effect of the different process parameters on the properties of the coatings is also addressed. W-He films were deposited by magnetron sputtering with variations of pressure ranging from 1-5 Pa. The morphology of the coatings was investigated by Scanning Electron Microscopy (SEM) whereas the He content within the layer was measured by Laser Induced Desorption Spectroscopy (LIDS), Laser-Induced Breakdown Spectroscopy (LIBS) and Thermal Desorption Spectroscopy (TDS). He TDS spectra exhibit a broad desorption peak in the 500-600 K range and another one, significantly increased, at ~960 K. To quantify He content in the samples, a calibration procedure that takes into account the conductance and pumping speed of the device has been performed allowing the determination of the sensitivities of the mass spectrometers used as detectors for TDS and LIDS.