Plasma Surface Interactions Research Articles

The upgraded TOMAS device: A toroidal plasma facility for wall conditioning, plasma production, and plasma-surface interaction studies.

The Toroidal Magnetized System device has been significantly upgraded to enable development of various wall conditioning techniques, including methods based on ion and electron cyclotron (IC/EC) range of frequency plasmas, and to complement plasma-wall interaction research in tokamaks and stellarators. The toroidal magnetic field generated by 16 coils can reach its maximum of 125 mT on the toroidal axis. The EC system is operated at 2.45GHz with up to 6 kW forward power. The IC system can couple up to 6 kW in the frequency range of 10 MHz-50MHz. The direct current glow discharge system is based on a graphite anode with a maximum voltage of 1.5 kV and a current of 6A. A load-lock system with a vertical manipulator allows exposure of material samples. A number of diagnostics have been installed: single- and triple-pin Langmuir probes for radial plasma profiles, a time-of-flight neutral particle analyzer capable of detecting neutrals in the energy range of 10 eV-1000eV, and a quadrupole mass spectrometer and video systems for plasma imaging. The majority of systems and diagnostics are controlled by the Siemens SIMATIC S7 system, which also provides safety interlocks.

Characterization of plasma chemistry for an optimized pulse resonance atmospheric-pressure plasma system

The paper deals with monitoring the plasma chemistry of an atmospheric plasma low-frequency AC power supply. The study is based on optical spectroscopy and electrical measurements as diagnostic methods. The work includes details of developing and operating an in-house made, non-thermal, dielectric-free atmospheric plasma system, built for biological and agriculture applications. The experimental results clearly show the cross-dependence of the measured outputs (O3, O2, N2) and a wide range of process parameters (duty cycle, discharge frequency, voltage, etc.). A design-of-experiment approach is taken for phase-resolved optical emission spectroscopy of oxygen and nitrogen spectral lines/bands. The new plasma system allows an increase in the plasma-surface interaction selectivity and reduces plasma-induced damages to the surface.

Perspective of analogy between heat loads and impurity production in L-mode discharges with ICRH in WEST

• RF sheaths due to Ion Cyclotron Resonance Heating (ICRH) on tokamaks. • RF sheaths induce DC potential rectification and increase incident ions energy. • We can be in an ion-dominated regime when magnetically connected to ICRH antennas. • Then heat fluxes and sputtering are carried by ions and an analogy can be relevant. • In ion-dominated regime, infrared and spectroscopic data can become correlated. This study compares experimental observations on plasma surface interactions and heat loads induced by Ion Cyclotron Resonance Heating (ICRH) in WEST medium size tokamak. When powering ICRH, core contamination by metallic impurities systematically results in an increase of the power radiated, often hardening the achievement of long discharges in H-mode at high power. The radiofrequency (RF) sheaths are excited mainly at proximity of ICRF antennas, resulting in an increase of metallic impurity sources. In the presence of RF sheaths, we show that most of the power reaching the wall can become carried by ions. Therefore, both heat loads and material sputtering are the result of ion fluxes on the various plasma facing components, such that some analogy between both phenomena can be made. By combining infrared with spectroscopic measurements, we show experimental evidences from WEST of RF-induced heat fluxes and their correlation with local production of impurity.

Data-driven LPV model predictive control of a cold atmospheric plasma jet for biomaterials processing

Cold atmospheric plasmas (CAPs) are increasingly used for treatment of complex surfaces in biomedical and biomaterials processing applications. However, the multivariable, distributed-parameter, and nonlinear nature of CAP dynamics and plasma–surface interactions, coupled with the sensitivity of plasmas to exogenous disturbances, make their safe, reproducible and effective operation challenging. This paper adopts a data-driven linear parameter-varying (LPV) modeling framework to learn a control-oriented model for predictive control of a kHz-excited atmospheric pressure plasma jet in Helium. A hierarchical model-based control strategy is proposed based on a supervisory LPV-based model predictive controller (LPV–MPC) to regulate the nonlinear thermal effects of plasma on a surface. Real-time control experiments demonstrate the effectiveness of the proposed LPV–MPC strategy for the multivariable control of surface temperature and plasma optical intensity, as well as for controlling the spatial delivery of the cumulative thermal effects of the plasma jet on a surface.

Persistent Sputtering Yield Reduction in Plasma-Infused Foams.

Aluminum microfoams are found to exhibit persistent sputtering yield reductions of 40%-80% compared to a flat aluminum surface under 100 to 300eV argon plasma bombardment. An analytical model reveals a strong dependency of the yield on the foam geometry and plasma sheath. For foam pore sizes near or larger than the sheath thickness, the plasma infuses the foam and transitions the plasma-surface interactions from superficial to volumetric phenomena. By defining a plasma infusion parameter, the sputtering behavior of foams is shown to be separated into the plasma-facing and plasma-infused regimes. While plasma infusion leads to a larger effective sputtering area, geometric recapture of ejected particles facilitates an overall reduction in yield. For a given level of plasma infusion, the reductions in normalized yield are more pronounced at lower ion energies since angular sputtering effects enable more effective geometric recapture of sputterants.

Characterization of descriptors in machine learning for data-based sputtering yield prediction

Sputtering of a single-element material surface by monatomic ion impact is one of the simplest and most fundamental phenomena of plasma–surface interaction. Despite its seemingly simple and well-defined nature, its collision cascade dynamics is so complex that no widely applicable formula of the sputtering yield has ever been derived analytically from the first principles. When the first-principles approach to a complex problem fails to unveil its nature, a data-driven approach, or machine learning, may be used to transform the problem into a tractable model. In this study, regression models of sputtering yields of such systems were constructed based on publicly available data derived from a large number of past experiments. The analysis has also identified the descriptors (i.e., physical variables characterizing the surface and incident ion species) on which the sputtering phenomena depend most strongly and presented quantitative evaluation on how sensitively the regression models depend on each descriptor or group of descriptors. Information obtained in this study can facilitate an understanding of the fundamental workings of the sputtering phenomena in the absence of rigorous analytical theory.

Системы in situ диагностики взаимодействия плазмы с поверхностью на токамаке МИФИСТ-1 118

In the Institute for Laser and Plasma Technologies of NRNU MEPhI a compact spherical tokamak MEPhIST (MEPhI-Spherical Tokamak) for educational, demonstration and research purposes is under development and construction. The creation of plasma diagnostics systems involves several stages, determined by the successive complication of the plasma researchtasks, the upgrading of the device and the development of educational and methodological material for laboratory work to be put at the tokamak. Working out in situ methods of plasma-surface interaction analysis is one of the main scientific and technological goals of this tokamak. The complex of diagnostics described in the paper provides complementary information about the processes occurring at plasma with surface contact, is a set of very informative and well-tested diagnostic tools that allow students to obtain visual and reliable information about the processes occurring in the discharge chamber of the tokamak.

Plasma-surface interaction experimental device: PSIEC and its first plasma exposure experiments on bulk tungsten and coatings

Abstract A new laboratory-scale linear plasma device, PSIEC (Plasma-Surface Interaction system under Extreme Conditions), has been designed and constructed at Hefei University of Technology, China, to study plasma-material interactions for fusion reactor application. The PSIEC facility can generate continuous plasmas of hydrogen, deuterium, helium, argon and nitrogen. The electron density of these plasmas ranges from the order of magnitude of 1017 to 1018 /m3 and the electron temperature ranges from 1 to 40 eV. The incident ion energy is adjusted by applying a negative bias up to 110 V to the target. The plasma bombarding flux is varied from the order of magnitude of 1021 to 1022 ions/m2/s. Using the PSIEC facility, first plasma exposure experiments have been conducted on bulk tungsten and coatings. The different microstructure of tungsten have been found to lead to different damage effects under helium plasma exposure. Preliminary results allow us to expect nanocrystalline tungsten to be a promising candidate for plasma-facing material, of which the high concentration of intrinsic defects such as grain boundaries and interfaces offers the possibility of improving their irradiation resistance.

Interactions between atmospheric pressure plasmas and metallic catalyst particles in packed bed reactors

Atmospheric-pressure plasmas sustained in packed bed reactors (PBRs) are being investigated for chemical conversion of gases and pollution control. Metallic catalysts added to the surfaces of the dielectric beads of PBRs can increase the energy efficiency and selectivity of chemical processes by reducing operating temperature and providing additional reaction pathways. In this paper, results from a computational investigation of plasma surface interactions between micron-scale metallic catalysts and humid-air plasmas in PBRs are discussed. We found that high plasma density regions form in the proximity of the metallic catalysts. These higher-density plasma regions were confirmed experimentally using ICCD imaging. The intense plasmas result from geometrical electric field enhancement and redistribution of charges within the conductive particles, leading to further enhancement. The high electric field at the triple points of the catalysts can produce electric field emission of electrons, which provides a pre-ionization source or additional source of electrons. These regions of high electric field and sources of electrons guide discharges towards the catalysts and increases fluxes of excited species, ions, electrons and photons to their surfaces. These fluxes are focused primarily at the triple points between the metal, dielectric and gas. As a result, the catalyst is locally heated, which could lead to further increased rates of thermocatalytic reactions on the surface. Surface roughness of the metal inclusions can lead to additional electric field enhancement, which changes the character of the discharges in the vicinity of the catalysts while reducing breakdown voltage.

PLASMA-SURFACE INTERACTION OF ELECTRIC ARC DISCHARGE BETWEEN COMPOSITE Cu-Cr ELECTRODES

In this work the plasma of electric arc discharges between Cu-Cr composite electrodes, which material were pressured and sintered at different temperatures 750 or 1050°С, are investigated by Optical Emission Spectroscopy (OES). The Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDXS) are used to study the working surface of electrodes, modified by thermal effect of breaking electrical arc in the air atmosphere. The erosion intensity of electrodes’ material due to plasma-surface interaction is estimated in indirect way by determination of metal vapours content in the discharge gap in an assumption of local thermodynamic equilibrium (LTE). The plasma composition is calculated in this case on the base of experimentally determined plasma parameters: temperature and electron density.

Formation and behaviour of plasma spots on the surface of titanium film

Numerous studies have shown that dielectric barrier discharge (DBD) and DBD-like plasma jets interact with a treated surface in a complex manner. Eroded traces after treatment cannot be explained by conventional plasma-surface interaction theory. The mechanisms of a controlled formation of these plasma objects is still unclear. In this work, the authors show that the formation rate and characteristics of eroded traces, treating a titanium surface, can be controlled by process design and the combination of materials used. A thin (0.45 µm) layer of titanium film is deposited onto a glass substrate and is then treated in the effluent of a non-equilibrium atmospheric pressure plasma jet (N-APPJ) operated with argon or krypton flow. Plasma spots with diameters ranging from 100–700 µm are observed using an intensified digital camera on the titanium film surface. These plasma objects are strongly inhomogeneous, forming a core with a very high current density and leave erosion holes with diameters of about 1 µm. By using krypton as a working gas, effective erosion of the titanium substrate can be shown, whereas by using argon no traces are detected. For the latter case, traces can be provoked by deposition of a thin aluminum layer on top of the titanium substrate, by creation of artificial scratches or by an additional swirling flow around the discharge. Based on the experimental results presented in this and previous papers, it is assumed that plasma spots with dense cores are produced by an interaction of micro-vortices within the plasma channel and by the formation of an extremely high axial magnetic field. This assumption is confirmed by destruction of the treated surface material, extraction of paramagnetic atoms and toroidal substrate heating, which is most likely caused by a helical current of the plasma spot.

Spatio-temporal characterization of a pulsed DC atmospheric pressure plasma jet interacting with substrates

Atmospheric pressure plasmas generate a variety of chemically active species in open air, thus providing the unique ability to treat a variety of materials that do not require or are not compatible with vacuum systems. Producing the plasma-surface interaction that leads to a desired change in the substrate is complicated by the codependency between the plasma and the substrate: while the plasma will modify the surface, the surface will also influence the plasma properties. In this work, a pulsed-DC plasma jet produced in helium and impinging upon glass and metal substrates is studied over a range of applied voltage pulse widths extending from 1 to 10 µs. Current–voltage measurements, high speed images, and time-resolved optical emission from three important He and nitrogen excited species are used to examine the evolution of the plasma and its interaction with the surface. At ignition, a streamer is ejected into the open air from the jet exit and eventually collides with the substrate. For a glass substrate, the streamer will hit the surface and form a short-lived plasma across it. This surface plasma is almost completely unaffected by changes in the voltage pulse width. In contrast, when the streamer hits a metal substrate, a surface discharge will form that will last the entirety of the voltage pulse. If the pulse is long enough, a ‘reflected discharge’ will slowly develop that extends from the substrate back towards the outlet of the plasma jet. The emission intensity of the surface discharge closely matches that of the initial streamer, but not the reflected discharge, which suggests different electron kinetics between the two features. The addition of capacitors or resistors between the metal substrate and ground show how differences in substrate electrical properties can account for some of these behaviors. Emission line ratios are used to examine the evolution of electron temperature and the relative importance of Penning processes during the different plasma phases.

Kinetic Modeling of Spacecraft Surfaces in a Plume Backflow Region

Plasma-surface interactions caused by electric propulsion devices are an important spacecraft aspect of the design that is difficult to measure in ground-based facilities. The negatively biased solar panel surfaces attract the slow-moving charge exchange (CEX) ions generated inside an ion core plume, which can cause surface sputtering on the protective coatings of the solar panels. We use a fully kinetic particle-in-cell direct simulation Monte Carlo (PIC-DSMC) approach that models both electron and ion trajectories to allow us to fully characterize the plasma sheath formed near these surfaces and to understand how the plasma sheath affects the trajectories of CEX ions, their incident energies and angles, and surface sputtering rates. We find that, outside the plasma core, the ion and electron distribution functions are highly non-Maxwellian, and the assumption of electron temperatures is questionable. We introduce a novel floating potential ground boundary condition that enables us to emulate the spacecraft ground for a high range of plasma number densities and surface charging conditions. Finally, we estimate the erosion of the surface using the kinetic results and surface yield empirical relations.

Measurements of the plasma parameters at the edge of the FT-2 tokamak and comparison with the gyro-kinetic model

Diagnostics of a plasma edge is an extremely important field of tokamak physics because this region determines the plasma-surface interaction. In the paper the plasma density, electron temperature and radial electric field at the plasma edge of the FT-2 tokamak are analyzed using the results of probe measurements. The measurements were performed in the experiments conducted with a plasma pinch being in a symmetrical vertical position and were compared with the results derived by the ELMFIRE code simulations to correct the model calculations. The experimental observations are shown to be in a reasonable agreement with the gyro-kinetic model predictions.

RF-heating system of helicon-type for PLM-2 plasma device: design parameters

The PLM-2 linear magnetic plasma device is for steady-state stationary hours-long plasma confinement with parameters providing the relevant fusion reactor plasma loads on materials. The machine is designed both for fundamental studies of plasma-surface interactions under high heat and particle fluxes, and as a high-heat flux facility for the tests of plasma-facing components under realistic plasma conditions. In the PLM-2 plasma device, a plasma heating ICR system will be created using a helicon antenna. Production of plasma by using helicon waves is known as helicon discharge, and the plasma produced by using the helicon discharge has a totally distinct character where the enhancement of the plasma parameters takes place at particular conditions. Helicon discharges produce higher density plasma at a comparatively lower given input power than any other RF or DC discharges, and also the plasma is least contaminated due to the presence of the antenna out-side the chamber.

Boron transport simulation using the ERO2.0 code for real-time wall conditioning in the large helical device

Abstract The three-dimensional Monte-Carlo impurity transport and plasma surface interaction code ERO2.0 is applied to a full-torus model for the Large Helical Device (LHD). In order to find an optimum experimental condition for effective real-time wall conditioning (boronization) using an Impurity Powder Dropper (IPD), the toroidal and poloidal distribution of the boron flux density on the divertor components and the vacuum vessel are surveyed in various experimental conditions. The source profile of the neutral boron atoms originated from boron powders supplied from the IPD is calculated using the DUSTT code in background plasmas provided by the EMC3-EIRENE code. The simulations using ERO2.0 predict that higher plasma density operation is inappropriate for the effective wall conditioning because of the toroidally localized boron flux density in a closed helical divertor region. The ERO2.0 simulations have successfully revealed an optimum experimental condition for the wall conditioning with the toroidally uniform boron flux density in the closed helical divertor region.

Efficient plasma-catalysis coupling for CH4 and CO2 transformation in a fluidized bed reactor: Comparison with a fixed bed reactor

The transformation of methane and carbon dioxide by coupling plasma and catalysis was investigated using a fluidized bed reactor and the results, in terms of reactant conversion and yields in products, were compared with those obtained in a fixed bed reactor. A series of alumina, including a commercial sample and various meso-macro materials synthesized in the laboratory, was tested in this study. Their surface areas varied from 260 to 312 m2 g−1 depending on their calcination temperature. A correlation between reactant conversion and surface area of alumina was highlighted for the plasma-fluidized bed, the best conversions being reached with the alumina presenting the highest surface area. CH4 conversion increased from 8.5 to 12.1% for S = 260 and 312 m2 g−1 respectively and the CO2 conversion from 3.4 to 6.2% for a deposited power of 4 W, in an excess of CO2. This correlation was not corroborated for the fixed bed reactor. It proves that an efficient coupling of plasma and catalysis can be expected as soon as solid particle are moving in the gas flow, enhancing the plasma-surface interaction.

Impact of liquid metal surface on plasma-surface interaction in experiments with lithium and tin capillary porous systems

Abstract The lithium and tin capillary-porous systems (CPSs) were tested with steady-state plasma in the PLM plasma device which is the divertor simulator with plasma parameters relevant to divertor and SOL plasma of tokamaks. The CPS consists of tin/lithium tile fixed between two molybdenum meshs constructed in the module faced to plasma. Steady-state plasma load of 0.1 – 1 MW/m2 on the CPS during more than 200 min was achieved in experiments on PLM which is a modeling far scrapeoff- layer and far zone of divertor plasma of a large tokamak. The heating of the CPS was controlled remotely including biasing technique which allows to regulate evaporated metal influx to plasma. After exposure, the materials of the tin and lithium CPSs were inspected and analyzed with optic and scanning electron micriscopy. Experiments have demonstrated sustainability of the tin and lithium CPSs to the high heat steady state plasma load expected in a large scale tokamak. The effect of evaporated lithium and tin on the plasma transport/radiation was studied with spectroscopy to evaluate changes of plasma properties and plasma-surface interaction.

Recrystallization suppression through dispersion-strengthening of tungsten

Tungsten is the material of choice for the divertor region of future nuclear fusion reactors, an environment that will expose plasma-facing components (e.g. divertor, etc...) to high temperatures and transient high heat flux events. Under these conditions, recrystallization and grain growth of tungsten can occur, leading to undesirable microstructural and mechanical property changes. Therefore, there is a need to raise the recrystallization temperature of tungsten and limit the kinetics of the recrystallization and grain growth processes. In this work, we examine the effect of different types (TiC vs. TaC vs. ZrC) and different concentrations (1.1 vs. 5 vs. 10 wt.%) of dispersed second phase particles in a tungsten matrix on the high temperature performance. The addition of second-phase particles effectively increases the temperature of and time for recrystallization and slow grain growth; however, the addition of a high weight fraction of particles alters the surface chemistry, which may impact subsequent plasma-surface interactions. These results show that the addition of small concentrations of dispersed particles can be effectively employed in tungsten to raise the upper operating temperature limit for tungsten in a fusion reactor.

Spectroscopic characterization of composite lithium materials irradiated with high-temperature plasma

High-temperature plasma irradiation of materials leads to significant modification of surface structure, growth of deposited composite films and surface layers with induced self-similar granularity on the scale from macroscales to nanoscale due to strong plasma-surface interaction. The aim of this study was to characterize lithium materials irradiated with high-temperature plasma in the T-10 tokamak and PLM device. The reactivity of lithium leads to reactions with impurities in the plasma and on the vessel. Post-mortem analyses by the X-ray photoemission spectroscopy and X-ray analysis have been used to identify deposits composition and morphology. Lithium carbonate composites have been detected by analysis demonstrating materials mixing and evidence of plasma-induced structure. New structures with the high specific surface area of hierarchical granular are registered. The reference industrial powder of lithium carbonate irradiated with steady-state plasma in the PLM device has acquired a new similar structure demonstrating universal influence of plasma on the structure of irradiated materials.