Plasma Surface Interactions Research Articles

Kinetic Monte Carlo simulations of plasma-surface reactions on heterogeneous surfaces

Reactions of atoms and molecules on chamber walls in contact with low temperature plasmas are important in various technological applications. Plasma-surface interactions are complex and relatively poorly understood. Experiments performed over the last decade by several groups prove that interactions of reactive species with relevant plasma-facing materials are characterized by distributions of adsorption energy and reactivity. In this paper, we develop a kinetic Monte Carlo (KMC) model that can effectively handle chemical kinetics on such heterogenous surfaces. Using this model, we analyse published adsorption-desorption kinetics of chlorine molecules and recombination of oxygen atoms on rotating substrates as a test case for the KMC model.

Interaction of long-lived reactive species from cold atmospheric pressure plasma with polymers: Chemical modification by ozone and reactive oxygen-nitrogen species

Atmospheric pressure plasma (APP) sources are able to generate a variety of reactive species that have different effects on materials, such as functionalization, etching, and deposition. In this article, the authors study the effect of long-lived reactive neutral species on polymers using a model plasma-surface interaction system that consists of ultrathin (∼10 nm) polystyrene (PS) films and a surface microdischarge (SMD) reactor operated with various N2/O2 working gas mixtures. The authors characterized and quantified the reactive species generated by SMD using IR and UV absorption, and they found that O3, N2O5, N2O, and HNO3 are the dominant long-lived reactants near the target surface. When exposing PS films to these reactive species, the authors observed material responses including film thickness expansion, surface and bulk oxidation, and surface organic nitrate formation. The quantity of these changes varied with the N2/O2 working gas composition. By correlating material response with gas phase species, the authors find that the chemical modification of PS strongly depends on the density of O3 in the gas phase, which is indicative of an essential role of O3 in the remote APP treatment of polymers. Authors’ results show that O3 causes polymer surface oxidation, participates in the diffusion-reaction process in the polymer bulk, and results in aromatic ring cleavage and the formation of carbonyl groups. In contrast, they did not find a correlation between surface organic nitrate and individual long-lived reactive species mentioned above. This indicates that the organic nitrate formation on polymer surfaces might result from the interaction of multiple species, including O3 and nitrogen containing reactive species. A model for the interphase mass transfer of reactive species from gas to solid was also described.

23rd International Conference on Plasma-Surface Interactions in Controlled Fusion Devices

23rd International Conference on Plasma-Surface Interactions in Controlled Fusion Devices

Ignition and Behavior of Arc Spots on Helium Irradiated Tungsten Under Fusion Relevant Condition

In nuclear fusion reactors, unipolar arcing is a longstanding plasma surface interaction issue. In this paper, short summaries of morphology changes by helium plasma irradiation under fusion relevant conditions and ignition of arcing on the He irradiated surfaces are given. Helium plasma irradiation leads to surface morphology changes, and arcing can be ignited on the surface in response to pulsed heat load accompanied by plasma instabilities. Then, we focused on arcing ignited on large-scale fiberform nanostructures (LFNs) formed with deposition of metallic particles together with helium-ion irradiation. Dusts release from the arc spot and the amount of material erosion are discussed based on the experiments.

Deuterium retention in silicon carbide, SiC ceramic matrix composites, and SiC coated graphite

Abstract Silicon Carbide (SiC) is a low Z material discussed as an alternative to graphite for fusion devices. The retention of hydrogenous species is an important plasma-surface interaction property. Deuterium was implanted into SiC, SiCf/SiC, Cf/C-SiC and SiC coated graphite under various particle energy and substrate temperature conditions. A TDS process was used to characterise the deuterium retention of the implanted specimens. While all SiC materials show elevated retention levels compared to graphite, the differences are limited to about a factor of two over the range of parameters investigated.

Two-way interaction between switching arc and solid surfaces: distribution of ablated contact and nozzle materials

This paper is devoted to two-way plasma-surface interactions by investigating how the plasma arc ablates the nozzle and contacts and how the distribution of ablated materials changes the plasma parameters.For this purpose, a two-dimensional time-dependent model, in axial symmetric coordinates, for an arc at atmospheric pressure burning within a polytetrafluoroethylene nozzle is created. A computational fluid dynamics equations system is solved for plasma velocity, pressure, temperature, magnetic vector potential, and electrical potential. Radiation is modeled based on net emission coefficient and contacts, and nozzle ablation is also considered to better describe the arc formation, contact cooling, and arc temperatures, more precisely. The sublimated materials from contacts and nozzle will be used to calculate the distribution of plasma composition (i.e. ablated mixture ratio). The calculated ratio is used to change the plasma parameters, and data processing techniques are utilized to derive particle distribution and temperature profiles of the arc to investigate its thermo-electrical behavior. The simulation results show good agreement with the measurements obtained in an experimental setup already designed and published. This study provides support to the experimental work and contrariwise. The presence of ablated points on nozzle and contacts, which further modify plasma parameters and therefore the arc voltage are discussed.

Modeling of Synthetic Spectra in Tokamak Edge Plasmas

Atomic processes and plasma surface interactions play a key role in the physics of the edge, divertor and X-point plasmas. Passive spectroscopy is one of the best methods to characterize tokamak edge plasmas. In this work, we report on synthetic spectra calculations for the characterization of electron density in recombining divertor plasma conditions. It is shown that an analysis of the Stark broadening of Balmer lines with moderate principal quantum number provides the electron density with good accuracy.

Influence of a magnetic field on plasma energy transfer to material surfaces in edge-localized mode simulation experiments with QSPA-M

The features of plasma energy transfer to material surfaces during plasma–surface interactions (PSIs) in the presence of a strong magnetic field are investigated within the recently developed quasi-stationary plasma accelerator, QSPA-M. This novel PSI test-bed facility can reproduce edge localized mode (ELM) impacts, both in terms of heat load and particle flux to the surface, and provide plasma transportation in an external magnetic field, which mimics the divertor conditions. Investigations of energy transfer to the material surface have been performed for varied plasma heat load and external magnetic field values. Calorimetry, optical emission spectroscopy and high-speed imaging were applied for PSI characterization. For perpendicular plasma incidence, it has been shown that the transient plasma layer is formed in front of the surface by the stopped head of the plasma stream even for rather small plasma heat loads, which do not result in surface melting. The plasma density in this near-surface layer is much higher than in the impacting stream. It leads to the arisen screening effect for energy transfer to the surface. For B = 0, the thickness of the screening layer is less than 3 cm, but it increases to 15 cm when B = 0.8 T. The shielding effect due to the formation of a dense plasma layer in front of the exposed surface should be favorable for material performance, being important for decreasing the overall erosion of plasma-facing components during a large number of repetitive ELMs.

Design, development and recent experiments of the CIMPLE-PSI device

The CIMPLE-PSI laboratory in India’s northeast corner has been engaged over the last decade in the development of advanced experimental systems for controlled plasma fusion-relevant plasma surface interaction (PSI) studies and material testing. The CIMPLE-PSI experimental device was commissioned recently, where a magnetized collimated helium plasma beam is produced under steady-state conditions inside a linear vacuum chamber, which can be made to interact with remotely placed material targets, under controlled experimental conditions. This paper reports on the design, development of the major sub-systems of this device and the performance of the integrated system. The peak helium ion flux and heat flux were measured as 1024 m−2 s−1 and 5.1 MW m−2, respectively; this confirms that extreme ITER divertor-like parameters are successfully reproduced by this simulation device. Steady-state operation of the electromagnet allowed prolonged operation of the plasma leading to a very high fluence of 0.3 × 1028 m−2, which may be enhanced further in the future. Tungsten samples were exposed in this device under helium plasma that had led to the formation of nanometer-sized tungsten fibre foam structures on the target. The technique of grazing incidence small angle x-ray scattering was successfully utilized to measure the average size of the helium bubbles remaining buried in the exposed sample and its variation with depth from the top of the surface.

Towards an integrated modeling of the plasma-solid interface

Solids facing a plasma are a common situation in many astrophysical systems and laboratory setups. Moreover, many plasma technology applications rely on the control of the plasma-surface interaction. However, presently often a fundamental understanding of them is missing, so most technological applications are being developed via trial and error. In the majority of plasma simulations surface processes are either neglected or treated via phenomenological parameters such as sticking coefficients, sputter rates or secondary electron emission coefficients. However, those parameters are known only in some cases and with very limited accuracy. Similarly, while surface physics simulations have often studied the impact of single ions or neutrals, so far, the influence of a plasma medium and correlations between successive impacts have not been taken into account. Such an approach cannot have predictive power. In this paper we discuss in some detail the physical processes a the plasma-solid interface which brings us to the necessity of coupled plasma-solid simulations. We briefly summarize relevant theoretical methods from solid state and surface physics that are suitable to contribute to such an approach and identify four methods. The first are mesoscopic simulations such as kinetic Monte Carlo (KMC) and molecular dynamics (MD) that are able to treat complex processes on large scales but neglect electronic effects. The second are quantum kinetic methods based on the quantum Boltzmann equation that give access to a more accurate treatment of surface processes using simplifying models for the solid. The third approach are ab initio simulations of surface process that are based on density functional theory (DFT) and time-dependent DFT. The fourths are nonequilibrium Green functions that able to treat correlation effects in the material and at the interface.

Improved ERO modelling of beryllium erosion at ITER upper first wall panel using JET-ILW and PISCES-B experience

Abstract ERO is a 3D Monte-Carlo impurity transport and plasma-surface interaction code. In 2011 it was applied for the ITER first wall (FW) life time predictions [1] (critical blanket module BM11). After that the same code was significantly improved during its application to existing fusion-relevant plasma devices: the tokamak JET equipped with an ITER-like wall and linear plasma device PISCES-B. This has allowed testing the sputtering data for beryllium (Be) and showing that the “ERO-min” fit based on the large (50%) deuterium (D) surface content is well suitable for plasma-wetted areas (D plasma). The improved procedure for calculating of the effective sputtering yields for each location along the plasma-facing surface using the recently developed semi-analytical sheath approach was validated. The re-evaluation of the effective yields for BM11 following the similar revisit of the JET data has indicated significant increase of erosion and motivated the current re-visit of ERO simulations.

In-operando observation of helium-irradiation ion effects on surface deuterium retention through LiOH bonding in lithium films on tungsten substrates

Abstract The plasma surface interactions within a fusion device are one of the limiting factors to long pulse, power generating operation. A plasma facing component material will require effective heat tolerance, minimal erosion yield, and minimal fuel retention properties. Tungsten (W) has been selected as the divertor material for the International Thermonuclear Experimental Reactor (ITER) due to its high thermal conductivity and high sputter threshold. However, when W is exposed to high particle flux (>1022 ions/m2s) at high surface temperatures (>600 °C), the surface will develop defects such as pits, blisters, and nano-structured tendrils, reducing the beneficial properties of W. To overcome this limitation, a more radiation tolerant thin film material could be used, such as lithium (Li). In addition, the lithium film can protect the plasma from high-Z W sputtered atoms. In multiple tokamak devices, a Li wall coating, has improved the plasma performance by reducing fuel recycling from the walls, stabilizing the edge plasma and decreasing the number of edge localized modes (ELMs). Since ELMs help eject impurities from the core plasma, the complete suppression of ELMs is detrimental. Methods to regulate the frequency of ELMs have been investigated using gas puffs. In this work we report a new method to control the ELM frequency by tuning fuel recycling via the intrinsic helium (He) ions produced as ash from the deuterium (D) – tritium (T) fusion reaction. In Li films, one mechanism to retain D is via a chemical interaction between Li, O (oxygen), and D. Previous work has shown that when He ions are introduced with D ions, in a dual beam irradiation of Li films on W, a reduction in the dynamic surface D retention is observed. To further investigate this phenomenon, 1–2 um films of Li on W were exposed to sequential irradiations of D and He. The He fluence was ≈5% of the D (3.3 × 1020 ions/m2). The energies for the He and D ions were 1000 eV and 250 eV/amu, respectively and samples were exposed at room temperature. The surface chemistry was characterized with x-ray photoelectron spectroscopy (XPS) to determine changes in retention. The XPS scans were conducted in-situ and in-operando for the irradiations. Our results showed a decrease in the surface retention when He follows D ions and little change in the retention when D follows He. This indicates that He breaks the D retention mechanism in Li.

Plasma-surface interaction: dielectric and metallic targets and their influence on the electric field profile in a kHz AC-driven He plasma jet

Plasma catalysis, biomedical applications or atomic layer deposition at atmospheric pressure all make use of non-thermal plasmas in contact with a wide variety of surfaces. As the presence of a target (substrate) has been shown to modify the plasma in addition to the plasma modifying the target, it is reasonable to describe and study the plasma-surface as one system. This work shows how the presence of dielectric and metallic targets influences a kHz AC-driven discharge in a He plasma jet. Next to bringing the absolute values of the axial electric field along the plume of the jet, the presence of the surface has been shown to significantly elongate both the plume and the electric field profile. In addition, when a dielectric target is placed closer than the maximum length of the freely expanding jet, the electric field profile is enhanced only in the vicinity of the dielectric, typically between 0.3 and 2 mm above the target surface. The maximum measured relative increase is 31%, for 1000 SCCM flow with the target at 7 mm distance, when the electric field increased from 14.1 kV cm−1 for the freely expanding jet to 32.6 kV cm−1 when the jet was impinging on glass. Finally, a grounded metallic target enhances the electric field compared to the glass target only within a very thin layer just above the surface, typically about 0.2 mm. The highest measured electric field was 40.1 kV cm−1 at a grounded metallic target 12 mm away from the nozzle, for 1000 SCCM of helium flow. The discussion on the effects of the flow on the electric field profile are supported by the visualization of the flow. The discussion brings, among other, the comparison of properties between the 30 kHz AC-driven system and the 5 kHz pulsed jet.

RF sheath modeling of experimentally observed plasma surface interactions with the JET ITER-Like Antenna

Waves in the Ion Cyclotron Range of Frequencies (ICRF) enhance local Plasma-Surface Interactions (PSI) near the wave launchers and magnetically-connected objects via Radio-Frequency (RF) sheath rec ...

CO2 Activation on TiO2-Supported Cu5 and Ni5 Nanoclusters: Effect of Plasma-Induced Surface Charging

Surface charging is an often overlooked factor in many plasma–surface interactions and in particular in plasma catalysis. In this study, we investigate the effect of excess electrons induced by a p...

Response of a semiliquid epoxy film to a DC plasma

The investigation on plasma-surface interaction was extended from solid and liquid to a semiliquid surface in this study. A semiliquid epoxy film on a steel plate under a local electric field was found to be attracted and repelled alternatingly by a negatively charged electrode tip, exhibiting an interesting oscillation behavior during polymerization, and eventually forming a compression region in the cured film. A model involving electrostatic forces and streamer discharge processes was proposed to interpret the semiliquid film oscillation and compression region formation.

Low-damage nitrogen incorporation in graphene films by nitrogen plasma treatment: Effect of airborne contaminants

Graphene films grown on copper by chemical vapor deposition were exposed to the late afterglow of a reduced-pressure N2 plasma sustained by microwave electromagnetic fields. X-ray photoelectron and Raman spectroscopies reveal extremely high incorporation of plasma-generated N atoms into the graphene film (N/C = 29%) while maintaining an unprecedentedly low-damage generation (D:G = 0.35–0.45) compared to the literature (0.5–2.5). The incorporation dynamics between graphene on copper and graphene on copper oxide are also compared and discussed. After transfer on SiO2/Si substrate, the N/C content decrease to only 6%. This reveals that a large part of the N atoms are weakly bonded to the graphene surface. Most of the nitrogen incorporation seems linked to the functionalization of weakly bonded hydrocarbons initially adsorbed from air exposure or carbon-nitrogen structures arising from plasma-surface interactions.

Diagnostic setup for the divertor manipulator at wendelstein 7-X

The investigation of plasma wall interactions in terms of erosion processes, fuel retention and new plasma facing components exhibits a scientific gap as discharges in tokamaks and stellarators last only seconds up to a minute. Currently, linear plasma machines are bridging this gap, while Wendelstein 7-X (W7-X) will provide steady-state discharges of up to 30 min in its second operation phase (OP2), in which a divertor manipulator is envisaged to expose samples to reactor-relevant plasma fluxes and fluences. With a design following the limiter lock system at TEXTOR and the DIM-II at ASDEX Upgrade (AUG), the divertor manipulator at W7-X aims to study plasma surface interactions ex-situ by daily/weekly probe exchange and in-situ by embedded diagnostics and an observation system.The design, layout and capabilities of the embedded and observing diagnostics determine the accessible parameters for the in-situ investigation of the exposed probe materials, ranging from plasma edge measurements in the manipulator shaft to intersecting the magnetic island structure on the manipulator head. A versatile laser and observation system adds active surface analysis techniques. To prevent local overheating and excess thermal stress, essential diagnostics for temperature control are identified and combined with first considerations of the manipulator head shaping, based on the field line tracing tool of the W7-X web-service.

Metallic impurity sources behavior during ICRH in EAST

High-Z impurities production is often observed during Ion Cyclotron Range of Frequencies (ICRF) waves’ injection, which are likely due to RF sheath formation on solid materials. Based on Extreme UV (EUV) spectrometry data, this study in EAST consists in extracting information on plasma-surface interactions during ICRH. Intensities of the different spectral lines normalized to line-integrated plasma density, exhibit different parametric dependencies over scans of ICRH and LH power and for different toroidal phasing between straps. Materials close and magnetically-connected to an active antenna tend to show better correlation with ICRF parameters – such as power and feeding scheme – than those which are far away or not connected. ICRF phasing did not have influence on impurity, probably due to the absence of feedback control. It is further shown that the plasma content in impurities from divertor region better correlates with the total injected power rather than with ICRF power.

Surface structurization and control of CuS particle size by discharge mode of inductively coupled plasma and vapor-phase sulfurization

Copper sulfide (CuS) nanoparticle films with different nanoparticle sizes were fabricated using an inductively coupled plasma (ICP) and a vapor-phase sulfurization method. First, the ICP is applied to a thin copper film to obtain a copper nanoparticle film, based on the plasma–surface interactions through the ion bombardment on the film. The fabrication of the Cu nanoparticles revealed that their size and spatial distribution depend on the discharge mode of the ICP and plasma irradiation time. From the measurements of the plasma density, optical emission spectrum, and ion flux energy distribution function, it was found that the inductive mode of the ICP, compared to the capacitive mode of the ICP, is efficient at fabricating the uniform nanoparticle film due to the optimal plasma potential and high ion flux with narrow ion energy distribution. The Cu nanoparticles are then transformed to CuS nanoparticles through vapor-phase sulfurization. For the hydrophobicity application of the CuS nanoparticle film, the contact angles of the CuS nanoparticle films were measured and compared with those of other thin films, such as SiO2 and bulk CuS. The contact angle of the CuS nanoparticle film, fabricated through the plasma–surface interactions and sulfurization, was significantly higher than that of other thin films owing to the hydrophobic surface of the CuS with the nanoparticle structure.