This work reports on the investigation of Eurofer-97 erosion behaviour when exposed to the deuterium plasma of the linear device GyM. The erosion dependence of Eurofer-97 on the deuterium ion fluence, 1025 m-2, and temperature of the samples, T = 600 K and 990 K, was addressed. A bias voltage of -200 V was applied to GyM sample holder during the experiments. Samples were deeply characterised by: profilometry, scanning electron microscopy, atomic force microscopy, energy-dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry, Rutherford backscattering spectroscopy and particle-induced X-ray emission. The behaviour of Eurofer-97 erosion rate with the ion fluence strictly depends upon temperature. At 600 K, it was 0.14 nm/s after 4.7 × 1024 m-2, then decreased, reaching a steady state value of 0.01 nm/s from 8.0 × 1024 m-2. At 990 K instead, the erosion rate was roughly constant around 0.019 nm/s for 1025 m-2. The value at 2.35 × 1025 m-2 was slightly lower. The erosion rate at 990 K was greater than that at 600 K for every ion fluence. Microscopy and surface analysis techniques showed that Eurofer-97 erosion rate dependence on at 600 K was primarily determined by the preferential sputtering of iron and other mid-Z elements of the alloy, leading to a surface rich in W and Ta difficult to be sputtered. The erosion behaviour at 990 K was dominated by the morphology dynamics, instead. The different properties of the morphology developed at the two temperatures can explain the higher erosion rate at 990 K for all the ion fluences.

GyM is a linear plasma device operating at Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Milan, with the original aim of studying basic plasma physics, such as turbulent processes. Since 2014, GyM experimental program has been mainly focused on the issue of plasma-material interaction (PMI) for magnetic confinement nuclear fusion applications. GyM consists of a stainless steel vacuum chamber (radius and length of 0.125 m and 2.11 m), a pumping system, a gas injection system, 10 magnetic field coils and two magnetron sources at 2.45 GHz, capable of delivering a total microwave power up to 4.5 kW. Highly reproducible steady-state plasmas of different gas species, at a maximum working pressure of ~10-1 Pa, can be obtained by electron cyclotron resonance heating in the resonance layer at 87.5 mT. Plasmas of GyM have electron and ion temperature <=15 eV and ~0.1 eV, respectively. The electron density is in the range of 1015-1017 m-3 and the ion flux is <=5 × 1020 ions?m-2s-1. Main plasma diagnostics of GyM comprise Langmuir probes, an optical emission spectrometer, a mass spectrometer and a fast camera system equipped with an image intensifier unit. For the purpose of investigating the topic of PMI, GyM is provided with two sample exposure systems. Both are biasable at a negative bias voltage down to -400 V, to tune the energy of the impinging ions. One of them is also equipped with a heating lamp and can reach and sustain a temperature of 990 K for several hours, thus allowing to study the role of sample temperature during the plasma-material interaction. This contribution presents the layout of GyM, the diagnostics, the sample exposure systems and the typical plasma parameters. A brief overview of the main PMI activities carried out so far and a description of future machine upgrades are also given.

Plasma-material interaction (PMI) in tokamaks determines the life-time of first-wall (FW) components. Due to PMI, FW materials are eroded and transported within the device. Erosion is strongly influenced by the original morphology of the component, due to particle redeposition on near surface structures and to the changing of impact angle distributions, which results in an alteration of the sputtering effects. The Monte-Carlo impurity transport code ERO2.0 is capable of modelling the erosion of non-trivial surface morphologies due to plasma irradiation. The surface morphology module was validated against experimental data with satisfactory agreement. In this work, we further progress in the validation of the ERO2.0 capabilities by modelling both numerically generated surfaces as well as real surfaces, generated using atomic force microscopy (AFM) measurements of reference tungsten samples. The former are used to validate ERO2.0 against one of the morphology evolution models present in literature, in order to outline the conditions for reliable code solutions. Modifications induced in AFM-generated surfaces after argon and helium plasma irradiation are compared, showing a similar post-exposure morphology, mostly dominated by surface smoothing. Finally, the ERO2.0 morphology retrieved after He plasma exposure is compared to experimentally-available scanning electron microscopy and AFM measurements of the same surface morphology exposed in the linear plasma device GyM, showing the need for further improvements of the code, while a good agreement between experimental and simulated erosion rate is observed.

Nitrogen (N) seeding is routinely applied in tokamaks with tungsten (W) walls to control the power exhaust toward the divertor. Open questions, concerning the interaction of N with W, are the influence of ion energy and W temperature on retention of implanted N and the erosion by deuterium (D) of the tungsten nitride being formed. Moreover, the extremely high particle fluxes in ITER and DEMO will erode the W tiles and the sputtered atoms will re-deposit forming W-based layers with a different behaviour toward the interaction with N seeded D plasmas. In this work, W films with different morphology and structure were exposed to the N seeded D plasma of the linear device GyM, in order to address all these issues. The experiments were performed at the fixed N2/D2 partial pressure ratio of ~4% keeping the total pressure constant at 5.3×10-4 mbar. The exposure conditions were: (i) sample temperature of ~850 K, (ii) particle fluxes of 2-2.2×1020 ions?m-2?s-1 and (iii) particle energies up to ~320 eV. W columnar films (c-W) with properties close to those of virgin W coatings deposited on the tiles of JET Iter-Like Wall and ASDEX Upgrade and W amorphous films (a-W) resembling nanostructured W-based deposits found in present-day tokamaks and expected in ITER and DEMO, were considered. W columnar and amorphous coatings were produced by means of magnetron sputtering and pulsed laser deposition, respectively. The specimens were characterised by profilometry, X-ray depth-profiling photoelectron spectroscopy, optical microscopy, scanning electron microscopy, atomic force microscopy and X-ray diffraction. The main evidence is that the behaviour of the W films upon D+N plasma exposure in GyM strictly depends on their morphology and nanostructure. For all the films, a surface N-enriched layer, which is thermally stable and does not decompose at least up to ~850 K, is observed. Moreover, blisters are not present on the surface of the samples. The c-W coatings erode faster than the a-W ones and have a higher nitrogen retention and diffusivity. The mechanisms behind these results are here discussed together with their possible implications from the point of view of the topic of plasma-wall interaction in tokamaks.

The electron temperature (Te) and electron density (ne) are important parameters for characterizing the plasma status in the Scrape-Off-Layer (SOL) and the divertor of magnetically confined fusion devices and investigating its physical and chemical properties in different operating conditions [1]. These parameters are usually derived locally using Langmuir Probes (LP), which are intrusive techniques. Optical diagnostics, such as Optical Emission Spectroscopy (OES), utilizing plasma radiation, offer a non-invasive complementary technique providing extremely powerful insights when supported by an accurate atomic modelling [2, 3]. This work focuses on the implementation of the OES technique to analyze an argon plasma in the linear device GyM and to derive an estimate of the line-of-sight averaged Te and ne.

Iron-tungsten (Fe-W) mixed films were exposed to the low flux deuterium plasma of GyM in order to study the behavior of the sputtering yield with the ion fluence and temperature of the samples. From literature, it is known that an increase of the former lowers the Fe-W layers' sputtering yield as a consequence of the preferential sputtering of Fe leading to an enrichment in W of the outermost layers. An opposite trend was instead found for the latter probably due to the inter-diffusion of Fe and W (effective from 200 degrees C) resulting in the suppression of the W enrichment. Moreover, from 500 degrees C, also W segregation to the surface occurs. What is missing from literature is a systematic investigation of the role of temperature on W enrichment. In this work, dedicated experiments in GyM were carried out to fill this gap. After exposure, W enrichment was evaluated by Rutherford Backscattering Spectrometry (RBS) and inferred from measuring the eroded thickness of the samples using RBS and profilometer. Concerning the Fe-W sputtering yield as a function of fluence, it decreases by a factor of similar to 3 between the lowest (3.0 x 10(22) D+ m(-2)) and the highest fluence (9.0 x 10(23) D+ m(-2)) values considered. The other main result is that, at the lowest fluence, the exposure at room temperature leads to an erosion of the Fe-W samples more pronounced than that associated to the exposure at 500 degrees C. (C) 2017 The Authors. Published by Elsevier Ltd.