Linear Plasma Device PSI-2 Research Articles

Zeeman effect of isotopes of Kr and Xe investigated at the linear plasma device PSI-2

Laser absorption spectroscopy provides high-resolution spectra of atomic transitions that reveal many often inaccessible features. The line shapes of krypton and xenon measured in magnetized plasmas are strongly affected by the contribution of the odd-numbered isotopes 83Kr, 129Xe and 131Xe due to their hyperfine structure, creating more challenging spectra in comparison to even-numbered ones. The lines originating from metastable levels of krypton and xenon with J = 2 (Kr I 760.4 nm) and J = 0 (Kr I 785.7 nm, Xe I 764.4 nm) were measured and analyzed in the linear plasma device PSI-2 in the field range of 22.5 mT–90 mT. Evaluating the Hamiltonian, including hyperfine and Zeeman interaction terms for these magnetic field strengths, unveils a deviation from the linear energy shift of the sublevels as a function of the magnetic field and from constant relative intensities that the weak field formulas provide. We prove that modeling the transitions in Xe using the weak field approximation, frequently used in magnetized plasma, becomes inadequate at ≈50 mT. In particular, the spectra of the 131Xe isotope show pronounced deviations from the weak field results. For krypton, however, the situation is less critical compared to xenon due to the low natural abundance of the odd-numbered isotope.

Space-resolved line shape model for sputtered atoms of finite-size targets

High-resolution emission spectroscopy provides valuable information on the physical sputtering process during plasma-wall interaction. Up to now, analyzing the observed spectral lines during sputtering did not account for the finite size of the targets. It becomes crucial if the size of the target becomes comparable with the distance the sputtered atoms travel before emitting the photons. So, for example, the generally used standard emission model based on an infinite target or the point source approximation breaks for observations using two lines of sight: parallel and perpendicular to the normal of the target. It is impossible to achieve consistent results for energy and angular distribution of sputtered atoms. The new space-resolved emission model for finite-size targets developed in this work removes this gap. It incorporates the space-velocity transformation for the distribution function and includes the finite lifetime of excited states. The model was validated using emission spectra of sputtered atoms from a polycrystalline tungsten sample bombarded by monoenergetic Ar+ with kinetic energies of 100 eV to 140 eV at normal incidence in the linear plasma device PSI-2. Using the new model enables the simultaneous fitting of the line shapes of sputtered tungsten for both observation angles. The optimization process is performed using the standard Thompson distribution by separating the energy-dependent parameter and the angular distribution.

Application of a hyperspectral camera for in situ plasma-material interaction studies at the linear plasma device PSI-2.

A hyperspectral camera (HSC-type Specim IQ) has been applied at the linear plasma device PSI-2 under steady-state conditions. The camera has the capacity of hyperspectral imaging (HSI) with the dimension of a data array 512 × 512 × 204 (x, y, λ) covering the spectral span from 400 to 1000nm with moderate average spectral resolution (FWHM ∼7nm). After radiometric calibration and background/continuum emission subtraction, two main applications of the camera, (i) plasma diagnostics in helium (He) plasmas and (ii) plasma-material interaction studies with tungsten (W) targets in neon (Ne) plasmas, have been carried out. The measurements were complemented by a movable Langmuir double probe system (LP) measuring electron temperature (Te) and electron density (ne) in radial direction r and a fiber-coupled cross-dispersion spectrometer with high spectral resolution (Spectrelle) recording neutral He, W, and Ne emission lines over the full plasma column. (i) Two-dimensional (2D) imaging of Te and ne radial profiles in axial direction z of the He plasma column were for the first time obtained by the regression analysis of Te and ne (from LP) and six He I line ratios (from HSC). The spatially resolved plasma parameters covered in these studies range between Te ∼ 0.8-13.4eV and ne ∼ 0.2 × 1018-3.9 × 1018m-3 and permit a reconstruction of the plasma conditions in PSI-2 in 2D without LP perturbation. (ii) W sputtering was studied in situ in Ne plasmas exposing W target samples (negatively biased at 100V) under perpendicular Ne plasma impact. Simultaneously, the 2D distributions of W (W I line at 429.5nm) in front of the target and the 2D Ne plasma distribution (Ne I line at 703.2nm) were recorded with complete spectral separation as confirmed by the Spectrelle spectrometer. This permits the simultaneous measurement of the neutral W penetration and its angular distribution induced in the sputtering process and of the impinging plasma distribution. The HSI technique offers, despite a few technical drawbacks, such as the moderate spectral resolution and poor time resolution, a new possibility to distinguish multiple emission lines from plasma and impurities and complements the portfolio of existing Optical Emission Spectroscopy techniques, providing a good compromise regarding spectral, spatial, and temporal resolution.

Bubble Formation in ITER-Grade Tungsten after Exposure to Stationary D/He Plasma and ELM-like Thermal Shocks

Plasma-facing materials (PFMs) in the ITER divertor will be exposed to severe conditions, including exposure to transient heat loads from edge-localized modes (ELMs) and to plasma particles and neutrons. Tungsten is the material chosen as PFM for the ITER divertor. In previous tests, bubble formation in ITER-grade tungsten was detected when exposed to fusion relevant conditions. For this study, ITER-grade tungsten was exposed to simultaneous ELM-like transient heat loads and D/He (6%) plasma in the linear plasma device PSI-2. Bubble formation was then investigated via SEM micrographs and FIB cuts. It was found that for exposure to 100.000 laser pulses of 0.6 GWm−2 absorbed power density (Pabs), only small bubbles in the nanometer range were formed close to the surface. After increasing Pabs to 0.8 and 1.0 GWm−2, the size of the bubbles went up to about 1 µm in size and were deeper below the surface. Increasing the plasma fluence had an even larger effect, more than doubling bubble density and increasing bubble size to up to 2 µm in diameter. When using deuterium-only plasma, the samples showed no bubble formation and reduced cracking, showing such bubble formation is caused by exposure to helium plasma.

Hydrogen isotope analysis in W-tiles using fs-LIBS

Laser-Induced Breakdown Spectroscopy (LIBS) is a promising technology for in-situ analysis of Plasma-Facing Components in magnetic confinement fusion facilities. It is of major interest to monitor the hydrogen isotope retention i.e. tritium and deuterium over many operation hours to guarantee safety and availability of the future reactor. In our studies we use ultraviolet femtosecond laser pulses to analyze tungsten (W) tiles that were exposed to a deuterium plasma in the linear plasma device PSI-2, which mimics conditions at the first wall. A high-resolution spectrometer is used to detect the Balmer-alpha transition of the surface from implanted hydrogen isotopes (H and D). We use Calibration Free CF-LIBS to quantify the amount of deuterium stored in W. This proof-of-principle study shows the applicability of femtosecond lasers for the detection of low deuterium concentration as present in first wall material of prevailing fusion experiments.

Synergistic and separate effects of plasma and transient heat loads on the microstructure and physical properties of ITER-grade tungsten

Once ITER commences full power operation, the ITER divertor will be exposed to high thermal and particle loads. Tungsten was chosen as plasma facing material for the ITER divertor. It is, therefore, of the utmost importance to understand the behavior of ITER-grade tungsten under conditions similar to those it will have to withstand inside the reactor. In this study, ITER-grade tungsten samples were exposed to stationary D/He(6%) plasma and ELM-like transient heat loads in the linear plasma device PSI-2. The effects of each kind of load was first studied separately, and the synergistic effects obtained when exposed to both loads simultaneously were then analyzed. Additionally, the hardness of a recrystallized tungsten sample after exposure to simultaneous loads was tested via nanoindentation. The results indicate that hydrogen and helium embrittlement worsens the cracking behavior of the material when exposed to the simultaneous loads compared to only heat loads. Additionally, bubbles of up to 1 μm are formed under the surface due to the synergistic effects at the highest heat load. The nanoindentation tests showed that plasma and heat loads increase the hardness of the material by 39%, but only plasma loads appeared to have no effect on it.

In situ study of short-term retention of deuterium in tungsten during and after plasma exposure in PSI-2

In situ monitoring of the hydrogenic fuel inventory in plasma-facing materials (PFMs) is a key issue for the sustainable and safe operation of a magnetically confined fusion device. A laser induced breakdown spectroscopy system has been applied on the linear plasma device PSI-2 to determine the short-term retention of deuterium (D) in the most promising PFM-tungsten (W). Measurements of the fuel content were performed (a) during plasma exposure, providing the dynamic D content in W and (b) after plasma exposure, determining the outgassing of D from W in vacuum. The in situ measurement of D content in W is in the time range of about 100 minutes before plasma termination (during plasma exposure) to 240 minutes after plasma termination (after plasma exposure). The measured surface D content in W drops to 17%–37% of the initial amount (measured before the plasma is switched off), showing a reduction factor of ca. 3–6 within the first 240 minutes.

Zeeman-resolved TDLAS using metastable levels of Ar in the weakly magnetized plasma of the linear plasma device PSI-2

Tunable diode laser absorption spectroscopy was applied at the linear plasma device PSI-2 to measure the magnetic field, temperature of argon and density of metastable species in a low density gas discharge. The measurements on the two metastable levels of Ar were performed by scanning the plasma column of PSI-2 at different radii. The obtained magnetic field using the lines at 763 and 772 nm (Ar) was found to be systematically lower (by 5% to 17%) than the calculated vacuum field. Part of the deviation arises from the line integration of the absorption signal. The radial gradient of the magnetic field strength combined with the radial metastable density determines the magnitude of this contribution (2%–3%). The temperature of the neutral gas was found to be essentially constant within the discharge chamber. The gas temperature rises with increasing cathode current and magnetic field due to an increase in the plasma density and, consequently, an increase in the energy transferred to the neutral gas by collisions with the charged particles. The density of the 4 s metastable level with J = 2 was found to be 8–9 times higher than that of the level with J = 0 similarly to observations by others in non-magnetized plasmas. To understand this trend a simple collisional-radiative model for the metastable argon 4s J = 2 level was developed. Depending on the treatment of the 4p levels it predicts a lower and an upper limit of the metastable density. The experimental values are within the limits predicted by the model indicating that the complex kinetics of the excitation and deexcitation collisional-radiative processes lead to this deviation from the statistical equilibrium.

The impact of surface morphology on the erosion of metallic surfaces – Modelling with the 3D Monte-Carlo code ERO2.0

• The micro-scale model for simulation of the surface roughness effect on the erosion of PFCs and transport of sputtered material implemented previously into the 3D Monte-Carlo code ERO2.0 was applied for modelling of sputtering and deposition in the JET-ILW divertor conditions (smooth Tile 5 and rough Tile 6). • Micro-scale simulations have been conducted using two different assumptions: with and without tracing of incident plasma/impurity ions near the rough surface. Tracing of incident ions near the surface forms more realistic shadowing patterns than the case when this pattern is calculated based on magnetic field lines. • Simulations without tracing of incident ions have shown that surface roughness leads to a reduction of the effective sputtering yield by up to 50% depending on surface parameters. • Simulations including tracing of incident ions near the surface have confirmed decrease of the net erosion from the rough surface of the Tile 6 in comparison to the smoother surface of the Tile 5 by up to 50%. The roughness of metallic surfaces has a vital impact on the erosion of plasma-facing materials. Roughness determines the effective sputtering yield Y eff of the facing material. The angular/energy distribution of sputtered particles, and the spatial erosion and deposition distribution. The model for simulation the effect of the surface roughness was earlier implemented into the 3D Monte-Carlo code ERO2.0 and validated using results of ion beam experiments and experiments in the linear plasma device PSI-2. In the present study the developed ERO2.0 surface morphology model was applied to the JET-ILW tungsten (W) divertor consisting of smooth bulk W and W-coated CFC components. Influence of the surface roughness on the W erosion as well as on the transport of sputtered material in conditions of inclined magnetic field was investigated. Simulation results are in a good agreement with existing experimental findings.

Influence of neon seeding on the deuterium retention and surface modification of ITER-like forged tungsten

In order to investigate the effect of neon seeding on deuterium retention and surface modification of ITER-like forged tungsten with grains elongated perpendicular to the surface, pure and neon-seeded deuterium plasma exposures were performed in the linear plasma device PSI-2. The ion percentage of neon in the mixed plasma was around 10%. The sample temperature and deuterium ion fluence were kept at 450 K and 1 × 1026 m−2, respectively. The incident ion energy was 40 eV, just reaching the tungsten sputtering threshold for neon. Surface observations show that neon seeding leads to pronounced cracking and erosion of blister caps and a significant reduction of small blisters (<0.8 μm). This is possibly attributed to surface sputtering and the inhibitory effect of neon-induced defects on blister nucleation by hindering dislocation movements. On the other hand, neon seeding increases the total deuterium retention by ∼70% according to the thermal desorption spectroscopy (TDS) measurements. A low-temperature desorption peak (∼660 K) which should be mainly ascribed to deuterium de-trapping from dislocations is observed to be broadened and shifts towards higher temperature in the case of neon seeding, suggesting enhanced inward diffusion and trapping of deuterium. An additional deuterium release peak appears at a higher temperature (∼803 K), indicating the creation of new deuterium-traps induced by neon. A comparison of the results of nuclear reaction analysis and TDS reveals that neon seeding not only increases deuterium retention in the surface region (<7.2 μm) but also in the bulk (>7.2 μm). We ascribe the enhanced deuterium retention to the increased deuterium-traps in the top-surface induced by Ne irradiation and the vertical grain boundary configuration of the forged W promoting the inward diffusion of deuterium.

Performance of Eurofer97 under deuterium plasma exposure with seeded impurities at elevated temperature

Eurofer97, P92 and Fe samples were exposed simultaneously in the linear plasma device PSI-2 to deuterium plasma and with addition of He, Ar or Kr at elevated temperature in the range of 800–950 K. Samples were exposed to plasma with an incident ion energy of 60–80 eV and an incident ion fluence of 5 × 1025 m−2. Surface morphology investigation of exposed samples reveals formation of fuzz-like structures on the surface steels when exposed to D and D+He plasma. Tungsten enrichment on the surface of the steels was observed after all exposures, but the average tungsten concentration varied from 4 to 18 at.%, depending on plasma composition. The highest tungsten enrichment on the surface was observed for samples exposed to D+He plasma. Addition of He to the plasma increased D retention, which was attributed to near-surface bubble formation, confirmed by transmission electron microscopy studies.

On the plasma suitability of WCrY smart alloys—the effect of mixed D+Ar/He plasmas

Tungsten-chromium-yttrium (WCrY) smart alloys are foreseen as first wall materials for future fusion devices such as DEMO. While suppressing W oxidation during accidental conditions, they should behave like pure W during plasma operation due to preferential sputtering of the lighter alloying elements Cr and Y causing W enrichment at the surface. This paper reports on the results of the simultaneous exposure of WCrY and pure W reference samples to mixed D + 1%Ar+5 %He plasma in the linear plasma device PSI-2. Further, a comparison with exposures to pure D and D + 1 %Ar plasma is made. At incident ion energies of 120 eV, exposure to pure D plasma results in a W-enriched alloy surface due to the preferential sputtering of Cr and Y, while the addition of Ar leads to enhanced erosion for W and WCrY and reduces the W enrichment in smart alloys. With the addition of He to the plasma, erosion of WCrY is enhanced compared to that of pure W. To investigate the plasma impact on the oxidation behaviour, plasma-exposed and reference samples were oxidised in controlled dry oxygen-containing atmosphere at . The sample geometry has a great impact on the oxidation behaviour. Yet, it can be shown that the good oxidation-suppressing properties of WCrY smart alloys are preserved during plasma exposure.

Light-reflection-induced changes in the line shape of sputtered atoms

The erosion from plasma-facing components has to be monitored in many kind of laboratory and fusion plasmas. For this purpose, spectroscopy is an essential tool. Under certain conditions the particle flux can be calculated from the absolute line intensities of the sputtered material using so-called S/XB values. The impact of light reflection on the emission induced by sputtered particles at the mirror-grade polished surface of tungsten (W) and aluminum (Al) was investigated in a low-density (ne ≈ 2 × 1012 cm−3) and low-temperature (Te ≈ ) argon (Ar) plasma in the linear plasma device PSI-2 using high-resolution spectroscopy. Using the line shape affected by Doppler shift we show that the light reflection has a considerable impact on the number of measured photons and has to be taken into account for calculating particle fluxes. The Al target was sputtered by Ar ions at the incident ion energy of . The measured profile of the Al I line () was compared with a Doppler-shifted emission model based on the Thompson energy distribution function. In this new model, the instrumental broadening and the impact of the Zeeman effect were also taken into account. The parameter for the high-energy fall-off n of the energy distribution function (), the surface binding energy Eb and the surface reflectance were derived by comparing the experimental and the synthetic spectrum. The W target was sputtered by Ar ions at incident ion energies in the range of –. The influence of the ion impact energy on the energy distribution of the sputtered particles was demonstrated.

ERO2.0 modelling of the effects of surface roughness on molybdenum erosion and redeposition in the PSI-2 linear plasma device

The surface morphology of plasma-facing components (PFCs) and its evolution during plasma irradiation has been shown to have a significant effect on the erosion and subsequent transport of sputtered particles in plasma. This in turn can influence the resulting lifetime of PFCs. A model for treatment of the effect of surface roughness on the erosion of PFCs has recently been incorporated into the three-dimensional Monte Carlo code ERO2.0. First simulations have confirmed a significant influence of the assumed surface roughness (for both regular and stochastic numerically constructed samples) on both the effective sputtering yields Yeff and the effective angular distributions of sputtered particles. In this study, a series of experiments at the linear plasma device PSI-2 are conducted to test the effect of surface roughness on the sputtering parameters. Graphite samples prepared with a 100 nm molybdenum layer with various surface roughness characteristic sizes (Ra = 110 nm, 280 nm, 600 nm and Ra < 20 nm) were exposed to a helium plasma in the PSI-2 linear plasma device at a magnetic field B = 0.1 T. These PSI-2 experiments were simulated using ERO2.0 with a surface morphology model. Simulations are able to reproduce the experimentally observed significant suppression of erosion for higher Ra values.

Synergistic effects of particle and transient heat loads on ITER-grade tungsten

ITER’s divertor will have to sustain high particle and heat loads. Tungsten has been chosen as plasma facing material for the entirety of ITER’s divertor. Therefore, it is important to study the behavior of ITER-grade tungsten under fusion relevant loads. In this work, tungsten samples with two different microstructures were studied, one with needle-like grains transversal to the surface, and the other with larger, isotropic grains obtained after recrystallization. The samples were exposed to a steady state plasma and a pulsed laser in the linear plasma device PSI-2 to simulate the conditions of the divertor in a fusion reactor. The results show that the transversal samples have a higher damage threshold, but at higher power density and pulse numbers, the damage observed, as well as the surface roughness, is similar. The recrystallized samples displayed a shallower crater depth, which might be an important factor to be considered when selecting a material for ITER and should be studied further.

Materials development for new high heat-flux component mock-ups for DEMO

Material issues pose a significant challenge for future fusion reactors like DEMO. When using materials in a fusion environment a highly integrated approach is required. Damage resilience, power exhaust, as well as oxidation resistance during accidental air ingress are driving issues when deciding for new materials. Neutron induced effects e.g. transmutation adding to embrittlement are crucial to material performance. Here advanced materials such as tungsten fibre-reinforced tungsten Wf/W and fibre-reinforced copper Wf/Cu composites could allow the step towards a fusion reactor. Recent developments in the area Wf/W mark a possible path towards a component mock-up early enough for utilisation in DEMO. High heat-flux tests show that having short fibres at the exposed surface leads to their selective erosion and melting. Initial tests in the linear plasma device PSI-2 confirm this behaviour.

Surface roughness effect on Mo physical sputtering and re-deposition in the linear plasma device PSI-2 predicted by ERO2.0

Abstract Surface morphology and its evolution during the plasma irradiation is known to have a large influence on the erosion and resulting lifetime of plasma-facing components as well as tritium retention. For instance, surface roughness can affect physical sputtering, re-deposition, as well as angular distributions of the sputtered species. In this study the effect of surface roughness is implemented into the 3D Monte-Carlo code ERO2.0. First modelling results for molybdenum (Mo) irradiated with deuterium (D) in the conditions foreseen for the planned experiments at the linear plasma device PSI-2 are presented. Using the constructed examples of surfaces with various (regular and fractal) roughness types it is shown that the effective sputtering yield decreases for rough surfaces in comparison to smooth ones. The angular distribution of particles escaping from the rough surface collimates with the increase of the surface structure's aspect ratio. Moreover, the modelling predicts flattening of the surface during the plasma irradiation due to the preferable re-deposition in the “valleys” and sputtering of the peak tops.

Diffusion model of the impact of helium and argon impurities on deuterium retention in tungsten

The influence of helium and argon impurities on the deuterium retention in tungsten is investigated by a numerical diffusion model, which treats diffusing depth profiles for deuterium and helium or argon in tungsten, taking into account the suggested effects of helium or argon. With helium, a helium nanobubble layer builds up at the surface of the sample, with depths higher than the penetration depth of the incident helium and deuterium ions. The nanobubbles form a porous network, which allows the release of trapped deuterium by surface recombination and diffusion through the pores to the surface. For argon, only a shallow layer of argon-induced defects exists, which also act as trapping sites for deuterium. A number of experiments with tungsten samples were conducted at the linear plasma device PSI-2 in support of the model. Helium and argon were admixed to deuterium plasma in ratios of up to 8% for otherwise similar exposure conditions. In addition, a variation of ion fluences was performed for investigation of the onset and evolution of the effects of impurities. The model shows that the influence on the deuterium retention both for helium nanobubbles as well as for argon-induced defects depends strongly on the ratio between the thickness of the helium- or argon-affected layer and the penetration depth of deuterium ions.

An upgraded LIBS system on linear plasma device PSI-2 for in situ diagnostics of plasma-facing materials

Laser induced breakdown spectroscopy (LIBS) is used to detect and monitor the D and H content on the W surface due to its capability of fast direct in situ measurement in extreme environment (e.g., vacuum, magnetic field, long distance, complex geometry). A new LIBS system has been set up on the linear plasma device PSI-2 to perform in situ, real-time and remote monitoring of deuterium retention on the target surface. The new system is an upgraded version of the previous LIBS system [1] and is improved in several ways. The laser profile is improved and better focused into the chamber, the emission light collecting optics are more efficient. In addition, the timing and triggering system is updated and two different spectrometers are optimised for blue and red regime on the spectrum respectively. One of the spectrometers with Littrow configuration is dedicated to the detection of hydrogen and deuterium Balmer-alpha lines. A number of key parameters including laser energy, the (de)focusing of the laser beam, the delay time of image intensifier (MCP) are investigated and improved for the new system. The measurement by the new LIBS system is then compared to thermal desorption spectroscopy (TDS) and nuclear reaction analysis (NRA) results. The overall LIBS sensitivity and measurement accuracy are greatly improved, the time evolution of D, H content on W samples is studied before, during and after plasma-off point in the scales of seconds, minutes and hours.

Impact of Kr and Ar seeding on D retention in ferritic-martensitic steels after high-fluence plasma exposure

Total deuterium (D) retention from the bulk material of reduced-activation ferritic-martensitic (RAFM) steel Eurofer’97 (EU’97) and commercial ferritic-martensitic grade P92 was traced experimentally by means of thermal desorption spectroscopy (TDS) and linked to the role of krypton (Kr) and argon (Ar) seeding during high-fluence plasma exposure. The influence of impurity seeding on the steel microstructure was determined using scanning electron microscopy (SEM) and extended with focused ion beam (FIB) cross-sectioning. D capture at depths in the µm range was measured by nuclear reaction analysis (NRA). Plasma exposure of the steel samples occurred at 470 K with ion energy of 30–40 eV in the linear plasma device PSI-2 with up to 10% simultaneous impurity admixture in plasma. D inventory achieved values in the 1020 D/m2 range after plasma exposure with high fluences of up to 1 × 1026 D+/m2. In pure and mixed plasmas, the majority of D was trapped in the steel bulk. The Kr and Ar seeding of D plasma resulted in the population of multiple already existing D trapping sites. A similarity of D desorption spectra suggests that D trapping in P92 follows the same mechanism as in EU’97. Kr and Ar seeding mostly contributed to the surface sputtering of the steel samples, yielding less D retention due to the material loss.