Monitoring of tritium and impurities in the first wall of fusion devices using a LIBS based diagnostic

Abstract

Laser-induced breakdown spectroscopy (LIBS) is one of the most promising methods for quantitative in-situ determination of fuel retention in plasma-facing components (PFCs) of magnetically confined fusion devices like ITER and JET. In this article, the current state of understanding in LIBS development for fusion applications will be presented, based on a complete review of existing results and complemented with newly obtained data. The work has been performed as part of a research programme, set up in the EUROfusion Consortium, to address the main requirements for ITER: (a) quantification of fuel from relevant surfaces with high sensitivity, (b) the technical demonstration to perform LIBS with a remote handling system and (c) accurate detection of fuel at ambient pressures relevant for ITER. For the first goal, the elemental composition of ITER-like deposits and proxies to them, including deuterium (D) or helium (He) containing W–Be, W, W–Al and Be–O–C coatings, was successfully determined with a typical depth resolution ranging from 50 up to 250 nm per laser pulse. Deuterium was used as a substitute for tritium (T) and in the LIBS experiments deuterium surface densities below 1016 D/cm2 could be measured with an accuracy of ∼30%, confirming the required high sensitivity for fuel-retention investigations. The performance of different LIBS configurations was explored, comprising LIBS systems based on single pulse (pulse durations: ps–ns) and double pulse lasers with different pulse durations. For the second goal, a remote handling application was demonstrated inside the Frascati-Tokamak-Upgrade (FTU), where a compact, remotely controlled LIBS system was mounted on a multipurpose deployer providing an in-vessel retention monitor system. During a shutdown phase, LIBS was performed at atmospheric pressure, for measuring the composition and fuel content of different area of the stainless-steel FTU first wall, and the titanium zirconium molybdenum alloy tiles of the toroidal limiter. These achievements underline the capability of a LIBS-based retention monitor, which complies with the requirements for JET and ITER operating in DT with a beryllium wall and a tungsten divertor. Concerning the capabilities of LIBS at pressure conditions relevant for ITER, quantitative determination of the composition of PFC materials at ambient pressures up to 100 mbar of N2, the D content could be determined with an accuracy of 25%, while for atmospheric pressure conditions, an accuracy of about 50% was found when using single-pulse lasers. To improve the LIBS performance in atmospheric pressure conditions, a novel approach is proposed for quantitative determination of the retained T and the D/T ratio. This scenario is based on measuring the LIBS plume emission at two different time delays after each laser pulse. On virtue of application of a double pulse LIBS system, for LIBS application at N2 atmospheric pressure the distinguishability of the spectra from H isotopes could be significantly improved, but further systematic research is required.

Developing solid-surface plasma facing components for pilot plants and reactors with replenishable wall claddings and continuous surface conditioning. Part B: required research in present tokamaks

“…A promising approach to measuring surface changes in a fusion device with temporal and spatial resolution is to use in situ surface analysis methods, for example, laser-based ones [69][70][71][72][73][74][75]:…”

Latest results of Eurofusion plasma-facing components research in the areas of power loading, material erosion and fuel retention

“…One example is the quantification of the T and D retention in the first wall material of fusion devices with in-situ methods (without removing samples from the facility). A dedicated task utilising laser-induced breakdown spectroscopy (LIBS) in PSI-2 [75] and MAGNUM-PSI [76] as well as LIBS/laser-induced ablation with quadrupole mass spectroscopy (LIA-QMS) in laboratory arrangements was executed successfully for this purpose [77][78][79]. The set of laser-based experiments demonstrated the capability to resolve hydrogen isotope as well as quantify the fuel retention in different types of ITER-like codeposits, containing Be and W as well as other impurities found in JET samples.…”

Quantification of hydrogen isotopes by CF-LIBS in a W-based material (WZr) at atmospheric pressure: from ns towards ps

“…So far, research in these areas is largely supported by post-mortem analysis of wall tiles, but the final goal is to develop analytical methods for measuring, during shutdowns, in situ the deposition of material and retention of plasma fuel on the walls of fusion devices, without removal of plasma-facing components (PFCs). To date, laser techniques are among the most promising methods, and laser-induced breakdown spectroscopy (LIBS) is becoming an ideal candidate for online and in situ monitoring plasma-facing materials (PFMs) in current and next-generation nuclear fusion devices [2][3][4]. With LIBS, all elements with the emission lines in the classical UV-NIR range can be analyzed, and the isotope species H and D can also be identified clearly [4,5].…”

“…To date, laser techniques are among the most promising methods, and laser-induced breakdown spectroscopy (LIBS) is becoming an ideal candidate for online and in situ monitoring plasma-facing materials (PFMs) in current and next-generation nuclear fusion devices [2][3][4]. With LIBS, all elements with the emission lines in the classical UV-NIR range can be analyzed, and the isotope species H and D can also be identified clearly [4,5]. Enlarging the spectrum down to the VUV offers detection of the extended degree of ionization (e.g., W II-III) [6,7] and some other fusion-relevant light elements (e.g., B, C) [6].…”