Plasma Surface Interactions Research Articles

Diagnostic study of impurity deposition in fusion device by calibration-free laser-induced breakdown spectroscopy

The quantification of impurities in fusion devices is important to improve understanding of all impurity-related issues and their solutions. In this study, the deposition of the impurity layer on the lower hybrid wave gas puffing pipe (LHW-GPP) used in the Experimental Advanced Superconducting Tokamak (EAST) was characterized. The calibration-free laser-induced breakdown spectroscopy (CF-LIBS) was employed to resolve the impurity deposition of the unknown standard by focusing the laser at various spot positions on the sample surface. The impurity elements that appeared in the spectrum of deposition are copper (Cu), tungsten (W), molybdenum (Mo), lithium (Li), silicon (Si), and carbon (C). The laser ablation of impurity showed an uneven deposition pattern. Each laser pulse depicted the different concentrations of the impurity elements. The results obtained from the quantification and the depth profiling showed that the impurity deposition was composed of eroded materials caused by the plasma-surface interactions with the divertors, main wall, limiters, and supporting structure materials. The exponential decrease in impurity signals and constant behavior of substrate intensity were also observed during consecutive laser shots. The study provided the estimation of impurity contents in the fusion plasma.

Heat Transfer on Micro and Nanostructured Rough Surfaces Synthesized by Plasma

The review summarizes recent experimental results of studying heat transfer on rough surfaces synthesized by plasma. The plasma-surface interaction leads to the stochastic clustering of the surface roughness with a high specific area breaking the symmetry of the virgin surface of the initial crystalline materials. Such a surface is qualitatively different from the ordinary Brownian surface. The micro- and nanostructured surface consist of pores, craters, and nanofibers of size from tens of nanometers to tens of microns, which can provide new heat transfer properties related to a violation of the symmetry of the initial materials. In recent years, new results have been obtained in the study of heat transfer during phase change on plasma-modified surfaces in relation to energy, chemical, and cryogenic technologies. The objective of the review is to describe the specific structure of refractory metals after high-temperature plasma irradiation and the potential application of plasma processing of materials in order to create heat exchange surfaces that provide a significant intensification of two-phase heat transfer. Refractory metals with such a highly porous rough surface can be used as plasma-facing components for operation under extreme heat and plasma loads in thermonuclear and nuclear reactors, as catalysts for hydrogen production, as well as in biotechnology and biomedical applications.

A new compact linear beam-plasma discharge simulator BPD-PSI

A new BPD-PSI line-type plasma simulator is introduced.The device features two vacuum units: a target unit and an electron gun unit, connected to each other by a water-cooled electron beam transport channel.The Pierce-type electron gun is used to generate stationary or pulsed electron beam, utilized for beam-plasma discharge generation. The plasma electron density can be varied in the range of n e = 1 × 1016 - 1 × 1018 m-3 with electron temperature in the range of T e = 1-20 eV. The ion flux of 1 × 1021 m-2 s-1 and heat fluxes up to 5 MW m-2 are obtained. A set of utilized plasma and plasma-surface interaction diagnostics are described. Five demonstration experiments are presented: 1) high vs low power beam-plasma operation, 2) beam-plasma discharge with/without presence of an external magnetic field, 3) incident ion flux composition analysis, 4) incident ion energy distribution function measurement, 5) in-situ target surface temperature control and in-vacuo thermal desorption spectroscopy measurement.All of the technical features of the setup described in this paper make it a new tool for studying plasma-surface interactions.

Advances in Magnum-PSI probe diagnosis in support of plasma–surface interaction studies

Magnum-PSI is a linear plasma generator equipped with a superconducting magnet, assuring fusion devices relevant conditions at plasma–surface interface. The plasma column was diagnosed using 64 probes embedded in the target. The cross-sectional distributions of plasma parameters (floating potential, ion saturation current and electron temperature) were measured for hydrogen and deuterium plasmas under various discharge conditions. The radial profile of the floating potential across the plasma column can be described by a reversed Mexican hat-like wavelet, having the most negative potential at the center of the plasma column. The negativity of the floating potential diminishes when the discharge current increases or the magnetic field decreases. The axial gradient of the floating potential is reduced by increasing the magnetic field. The ion saturation current is maximum at the center of the plasma column, increasing with both the discharge current and magnetic field. The ion flux to the target, estimated from Thomson scattering (TS) data, was confirmed by probe measurements. The electron temperature estimated from the ion branch of the probe current–voltage characteristic is few times larger than that obtained from TS. By increasing the gas pressure in the target chamber, the time-dependent ion saturation current measured by probes changes from a constant average current (when the plasma column is attached to the target) to a fluctuating average current with scattered peaks (in a partially detached regime) which vanishes completely in the fully detached regime. With respect to hydrogen, the plasma column is wider in deuterium and is characterized by less negative floating potential distributions.

Conceptual Design of a Compact Divertor Heat Load Simulation Device: HIT-PSI

Linear plasma devices have been increasingly applied in investigating plasma–surface interaction (PSI) processes and divertor/scraped-off-layer (D/SOL) physics because of their economy, flexibility, and expandability. However, only a few existing linear plasma devices are able to obtain high heat and particle fluxes. In this work, we report a compact superconducting linear device, with its scientific goals and specific design methods, at Harbin Institute of Technology (HIT), HIT-PSI, capable of implementing an extreme plasma environment with beams of a long discharge pulse, as well as high heat and particle fluxes in the future fusion reactor regime of ITER/CFETR-like parameters. A five-coil integrated superconducting magnet is designed to generate a >2.0 Tesla steady-state magnetic field for confining a long pulse plasma beam with a density of >1020 m−3 produced by a cascaded arc plasma source. With a pump set of 2500 L/s and a water-cooled target system with bias voltage, it is expected to obtain high-density and low-temperature plasma beams with a heat flux of over 10 MW/m2. Subsystems of the platform, including the plasma source, superconducting magnets, vacuum system, and target holder system, are described in detail. In addition, the function and performance of the platform are numerically simulated and represented by SOLPS-ITER code to predict the laboratory simulation results.

(Invited) Surface Chemistry Control in Advanced Plasma Processes

Atomic scale control in fabrication steps for advanced logic and memory manufacture is critical not only for manufacturing viability of widely disparate structures but also functionality of complex devices. Different approaches ranging from the use of “tighter” process control options to leveraging self-limiting cyclic processing have been explored to achieve atomic scale control utilizing seemingly unwieldy plasmas. Process technology is married to plasma as the most capable methods of delivering area selective etch or low temperature selective deposition. A hindrance for advancing the control of plasma modified surfaces is that plasmas modify surfaces resulting in non-ideal states. Current process engineering can accommodate less-than-ideal surfaces; however, even in the best case, managing non-idealities introduces significant complexity. The use of first principles in-situ diagnostics and simulation together provides a path towards increased understanding of plasma-surface interactions and atomic scale control of the surface and sub-surface. Physisorption-mediated plasma processes are useful to illustrate the combination of quantum chemistry and in-situ diagnostics as the etch precursor is delivered to the surface in an unfragmented state. The presentation will include examples of how this system can be used to gain an understanding of the sensitivity of processes to different types of surfaces. It will conclude with forward looking content related to the de-cluttering of plasma processes using new learning concepts.

Influence of plasma species on the early-stage growth kinetics of epitaxial InN grown by plasma-enhanced atomic layer deposition

Plasma-enhanced atomic layer deposition (PEALD) enables the epitaxial growth of ultrathin indium nitride (InN) films at significantly reduced process temperatures and with greater control of layer thickness compared to other growth methods. However, the reliance on plasma-surface interactions increases the complexity of the growth process. A detailed understanding of the relationship between the plasma properties and the growth kinetics is therefore required to guide the tuning of growth parameters. We present an in situ investigation of the early-stage PEALD growth kinetics of epitaxial InN within three different plasma regimes using grazing incidence small-angle x-ray scattering (GISAXS). The GISAXS data are supported by diagnostic studies of the plasma species generation in the inductively coupled plasma source as a function of the relative concentrations of the nitrogen/argon gas mixture used in the growth process. The growth mode is found to be correlated to the production of nitrogen species in the plasma, with high concentrations of the atomic N species promoting Volmer–Weber growth (i.e., island growth) and low concentrations promoting Stranski–Krastanov growth (i.e., layer-plus-island growth). The critical thickness for island formation, island center-to-center distance, and island radius are found to increase with ion flux. Furthermore, the island center-to-center distance and areal density are observed to change only during plasma exposure and to continue changing with exposure even after the methylindium adlayer is believed to have fully reacted with the plasma. Our results demonstrate the potential to control the growth kinetics during PEALD of epitaxial films by intentionally accessing specific regimes of plasma species generation.

Laser ablation molecular isotopic spectroscopy: A novel tool to characterise the distribution of 13C and 12C on graphite after 13CH4 tracer injection in Wendelstein 7-X

Wendelstein 7-X (W7-X) operation with inertially cooled graphite Test Divertor Unit (TDU) was finalised in a series of identical hydrogen plasmas with injection of 13C isotopically marked methane. The break-up of 13CH4, injected through gas inlets at one toroidal position of W7-X, and the subsequent transport and deposition of 13C in the device was studied ex-situ. The TDU was therefore extracted from the vessel and some of the removed TDU elements were examined regarding their 13C distribution in deposited layers on the surface using Laser Ablation Molecular Isotopic Spectroscopy (LAMIS). This study shows the 13C/12C distinction capabilities of LAMIS used to analyse the carbon deposition pattern on Plasma-Facing Components (PFCs) on two of the removed TDU elements. The resulted pattern was compared with complementary results from Nuclear Reaction Analysis (NRA) and simulations of 13C deposition by ERO2.0, a material transport and plasma-surface interaction code. Ultra- short laser pulses of a Nd:YAG laser were used in collinear double-pulse configuration for LAMIS. The first pulse (l1 = 355 nm, t1 = 35 ps, F = 2.3 J/cm 2 ) produced a laser- induced plasma on the surface and 50 ns later, the second laser pulse (l2 = 1064 nm, t2 = 35 ps) was directed into this plasma to enhance the signal. Each pulse pair ablates in total 200 nm of the surface material. The analysis is conducted in 1.5 mbar N2 environment for improved signal-to-noise ratio relative to vacuum conditions. A high throughput custom-made spectrometer in Littrow-arrangement (f = 750 mm, A = 6000 at l = 473 nm, lspan = 14 nm) was used to analyse the C2 Swan bands (d3 ⊓ g - a3 ⊓ u, v = 1 to v’ = 0) of laser-induced plasma emission. • Laser Ablation Molecular Isotopic Spectroscopy is a method for isotopical analysis of materials. • Similar to Laser-Induced Breakdown Spectroscopy with analysis of molecular spectra. • This method has in-situ potential for material deposition studies in fusion devices. • This study shows good agreement with results from Nuclear Reaction Analysis. • General agreement with ERO2.0 simulations were found far from the injection location.

2D imaging of absolute methyl concentrations in nanosecond pulsed plasma by photo-fragmentation laser-induced fluorescence

The methyl radical plays a central role in plasma-assisted hydrocarbon chemistry but is challenging to detect due to its high reactivity and strongly pre-dissociative electronically excited states. We report the development of a photo-fragmentation laser-induced fluorescence (PF-LIF) diagnostic for quantitative 2D imaging of methyl profiles in a plasma. This technique provides temporally and spatially resolved measurements of local methyl distributions, including in near-surface regions that are important for plasma-surface interactions such as plasma-assisted catalysis. The technique relies on photo-dissociation of methyl by the fifth harmonic of a Nd:YAG laser at 212.8 nm to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X(0, 0) band of CH with a second laser at 390 nm. Fluorescence from the overlapping A-X(0, 0), A-X(1, 1), and B-X(0, 1) bands of CH is detected near 430 nm with the A-state populated by collisional B-A electronic energy transfer. This non-resonant detection scheme enables interrogation close to a surface. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photo-dissociation of acetone vapor in a calibration gas mixture. We demonstrate PF-LIF imaging of methyl production in methane-containing nanosecond pulsed plasmas impinging on dielectric surfaces. Absolute calibration of the diagnostic is demonstrated in a diffuse, plane-to-plane discharge. Measured profiles show a relatively uniform distribution of up to 30 ppm of methyl. Relative methyl measurements in a filamentary plane-to-plane discharge and a plasma jet reveal highly localized intense production of methyl. The utility of the PF-LIF technique is further demonstrated by combining methyl measurements with formaldehyde LIF imaging to capture spatiotemporal correlations between methyl and formaldehyde, which is an important intermediate species in plasma-assisted oxidative coupling of methane.

Interaction of an unwetted liquid Li-based capillary porous system with high-density plasma

This study examined the effects of plasma irradiation on an unwetted liquid lithium-based capillary porous system (Li-CPS). The Li-CPS was irradiated with high-density Ar plasma using a linear plasma device at Sichuan University for Plasma Surface Interaction. The high-speed camera, Langmuir probe, and multi-channel spectrometer were used to characterize the effects of plasma irradiation. Upon Ar plasma irradiation, liquid Li drops were formed on the surface of the unwetted Li-CPS. Immediately after this irradiation, the drops fractured and were ejected into the plasma within ∼20 ms scale, which is not observed before to the best of our knowledge. Related results showed that the ejection behavior of Li could effectively cool electron temperature and reduce incident heat flux by ∼30% and correspondingly matrix temperature ∼150 °C, revealing an enhanced vapor shielding effect. The involved internal mechanism and physical processes deserve further investigations.

Effect of small amount of CF4 addition on He and Ar atmospheric pressure plasma jet and influence of relative permittivity of downstream material on plasma–surface interaction

CF4 is an important source of fluorine groups in atmospheric pressure plasma jet (APPJ). In order to obtain reactive fluorine species under atmospheric pressure, noble gas (Ar or He) and CF4 are usually mixed and used as the working gas of APPJ. In this paper, the differences in the discharge dynamics on He/CF4 APPJ and Ar/CF4 APPJ are investigated experimentally. Meanwhile, combined with simulation, the effects of downstream targets with different relative permittivity on the radial propagation range of the plasma plume and the distribution of F-containing reactive species are studied. It is discovered that the addition of a small amount of CF4 (20 sccm) will increase the intensity of He/CF4 APPJ due to the contribution of Penning ionization of metastable He with CF4. Differently, the addition of CF4 will continuously lead to a significant decrease in the intensity of Ar/CF4 APPJ. The radial propagation range of He/CF4 APPJ on the target surface decreases with the increase in the relative permittivity of the downstream target. The smaller relative permittivity inhibits the axial propagation speed of APPJ, but it increases the radial propagation range of reactive species. The larger relative permittivity promotes the production of F-containing reactive species and their flux intensity on the target surface.

Pulse-width-dependent dynamic charge deposition on quartz dielectrics interacted with atmospheric pressure He plasma jet

Surface processing with atmospheric pressure plasma jet (APPJ) is attracting attention from both academia and industry for its capability of flexible plasma release onto targets. It is significant to understand the characteristics and mechanism of interfacial phenomenon in plasma surface interaction. This paper reports the fundamental surface charging phenomenon on quartz dielectrics by a He APPJ excited by μs-width pulses with duty ratios from 0.5% to 75%. The targeted quartz plate is attached to an electro-optic crystal grounded in the back for charge measurement. The evolution of surface charges is detected with a reflective-type optical measurement platform based on Pockels effect and the fast charge inversion on 2D-FTA. Results show that ionization waves and quartz dielectric interaction generates a round positive charge deposition, whose lifetime greatly depends on the pulse widths. Under a fixed pulse width, higher voltage magnitudes encourage more charges with a higher deposition rate. An interesting polarity reversal of surface charging occurs when the pulse width increases to over half-cycle, with a reduced positive charge on pulse-on and more negative charges on pulse-off. Efforts are devoted to exploring the mechanisms of charge maintenance on pulse-on and the polarity reversal under longer pulses of several hundred microseconds. These results reveal that the unchanged charge density in dark channel is due to the much lower rate of charge decay compared with the fast ionization wave propagation. The abrupt decrease at the falling edge results from the charge neutralization in the negative conductive channel formed by the secondary discharge. By measuring the initial evolution during the foremost 1000 periods, we prove that the accumulative difference in dissipated charges between pulse-on and pulse-off periods is the main cause of the occurrence of polarity reversal under long pulses. This study provides a more compatible tool for in-depth insight into the charging process of plasma interacting with multiple dielectrics, and exhibits a potential strategy of regulating surface charging by adjusting pulse parameters, which are of interest to the plasma community.

Cleaning of nitrogen-containing carbon contamination by atmospheric pressure plasma jet

Atmospheric pressure plasma jet (APPJ) was used to clean nitrogen-containing carbon films (C–N) fabricated by plasma-assisted chemical vapor deposition method employing the plasma surface interaction linear device at Sichuan University (SCU-PSI). The properties of the contaminated films on the surface of pristine and He-plasma pre-irradiated tungsten matrix, such as morphology, crystalline structure, element composition and chemical structure were characterized by scanning electron microscopy, grazing incidence x-ray diffraction and x-ray photoelectron spectroscopy. The experimental results revealed that the removal of C–N film with a thickness of tens of microns can be realized through APPJ cleaning regardless of the morphology of the substrates. Similar removal rates of 16.82 and 13.78 μm min−1 were obtained for C–N films deposited on a smooth pristine W surface and rough fuzz-covered W surface, respectively. This is a remarkable improvement in comparison to the traditional cleaning method. However, slight surface oxidation was found after APPJ cleaning, but the degree of oxidation was acceptable with an oxidation depth increase of only 3.15 nm. Optical emission spectroscopy analysis and mass spectrometry analysis showed that C–N contamination was mainly removed through chemical reaction with reactive oxygen species during APPJ treatment using air as the working gas. These results make APPJ cleaning a potentially effective method for the rapid removal of C–N films from the wall surfaces of fusion devices.

On the electron sheath theory and its applications in plasma–surface interactions

In this work, an improved understanding of electron sheath theory is provided using both fluid and kinetic approaches while elaborating on their implications for plasma–surface interactions. A fluid model is proposed considering the electron presheath structure, avoiding the singularity in electron sheath Child–Langmuir law which overestimates the sheath potential. Subsequently, a kinetic model of electron sheath is established, showing considerably different sheath profiles in respect to the fluid model due to non-Maxwellian electron velocity distribution function and finite ion temperature. The kinetic model is then further generalized and involves a more realistic truncated ion velocity distribution function. It is demonstrated that such a distribution function yields a super-thermal electron sheath whose entering velocity at the sheath edge is greater than the Bohm criterion prediction. Furthermore, an attempt is made to describe the electron presheath–sheath coupling within the kinetic framework, showing a necessary compromise between a realistic sheath entrance and the inclusion of kinetic effects. Finally, the secondary electron emissions induced by sheath-accelerated plasma electrons in an electron sheath are analysed and the influence of backscattering is discussed.

Designing N-Graphene Nanowalls Via Plasma-Enabled Nitrogen Incorporation and Substitution

Surface engineering of graphene-based materials with nitrogen is a potential technique for tailoring the electrical and electronic features of the nanostructures. Surface engineered graphene can be fabricated by adsorbing foreign molecules into the graphene lattice or substitutional doping. Considering different chemical and physical doping techniques, plasma-assisted surface treatment is one of the fastest approaches to design nitrogen-incorporated graphene (N-graphene) with different nitrogen bonding configurations. However, it is still a great challenge to create N-graphene with controlled concentrations and desired configurations of nitrogen. Several studies demonstrated that the structural defects in graphene have a critical role in incorporating N-atoms, which has been well-explained by DFT theories. However, there is a lack of experimental studies to confirm the DFT theories explained for the influence of structural defects for nitrogen incorporation in graphene. In this study, we have demonstrated a systematic study on designing N-graphene with controlled concentration and configuration of nitrogen by plasma post-treatment on vertically oriented graphene nanowalls (GNWs). A low-pressure inductively coupled radio-frequency plasma is used for surface engineering with nitrogen gas as the reactive gas. The structural and morphological analysis describes a remarkable difference in the plasma surface interaction, nitrogen concentration, and nitrogen configurations in GNWs concerning the plasma treatment conditions. As the result of plasma treatment, the creation of structural defects and their healing upon prolonged treatment is determined with several spectroscopic and microscopic techniques. These results, along with the other measurements, revealed that the configurations of the nitrogen groups in graphene are highly influenced by the structural defects formed during the processes. Also, the electrical conductivity of the N-GNWs is strongly influenced by the concentration and different configurations of C-N bonding. These findings demonstrate the potential of plasma to tune the graphene lattice by controlled surface engineering in a fast and facile green approach, which can be fruitful for several future electronics and energy devices.

Arc hopping dynamics induced by interfacial negative differential resistance.

Pattern formation in plasma-solid interaction represents a great research challenge in many applications from plasma etching to surface treatment, whereby plasma attachments on electrodes (arc roots) are constricted to self-organized spots. Gliding arc discharge in a Jacob's Ladder, exhibiting hopping dynamics, provides a unique window to probe the nature of pattern formation in plasma-surface interactions. In this work, we find that the existence of negative differential resistance (NDR) across the sheath is responsible for the observed hopping pattern. Due to NDR, the current density and potential drop behave as activator and inhibitor, the dynamic interactions of which govern the surface current density re-distribution and the formation of structured spots. In gliding arc discharges, new arc roots can form separately in front of the existing root(s), which happens periodically to constitute the stepwise hopping. From the instability phase-diagram analysis, the phenomenon that arc attachments tend to constrict itself spontaneously in the NDR regime is well explained. Furthermore, we demonstrate via a comprehensive magnetohydrodynamics (MHD) computation that the existence of a sheath NDR can successfully reproduce the arc hopping as observed in experiments. Therefore, this work uncovers the essential role of sheath NDR in the plasma-solid surface pattern formation and opens up a hitherto unexplored area of research for manipulating the plasma-solid interactions.

Revisiting the numerical stability/accuracy conditions of explicit PIC/MCC simulations of low-temperature gas discharges

Particle-in-cell (PIC) with Monte Carlo collisions is a fully kinetic, particle based numerical simulation method with increasing popularity in the field of low temperature gas discharge physics. Already in its simplest form (electrostatic, one-dimensional geometry, and explicit time integration), it can properly describe a wide variety of complex, non-local, non-linear phenomena in electrical gas discharges at the microscopic level with high accuracy. However, being a numerical model working with discretized temporal and (partially) spatial coordinates, its stable and accurate operation largely depends on the choice of several model parameters. Starting from four selected base cases of capacitively coupled, radio frequency driven argon discharges, representing low and intermediate pressure and voltage situations, we discuss the effect of the variation of a set of simulation parameters on the plasma density distribution and the electron energy probability function. The simulation parameters include the temporal and spatial resolution, the PIC superparticle weight factor, as well as the electron reflection and the ion-induced electron emission coefficients, characterizing plasma–surface interactions.

Analytical expressions for thermophysical properties of solid and liquid beryllium relevant for fusion applications

The status of the literature is reviewed for thermophysical properties of pure polycrystalline solid and liquid beryllium which constitute input for the modeling of intense plasma–surface interaction phenomena that are important for fusion applications (thermal analysis, vapor shielding, melt motion, arcing, dust generation, dust transport). Reliable experimental data are analyzed for the latent heats, specific isobaric heat capacity, electrical resistivity, thermal conductivity, mass density, vapor pressure, work function, total hemispherical emissivity and absolute thermoelectric power from the room temperature up to the normal boiling point of beryllium as well as for the surface tension and the dynamic viscosity across the liquid state. Analytical expressions are recommended for the temperature dependence of these thermophysical properties, which involve high temperature extrapolations given the absence of extended liquid beryllium measurements.

Experimental investigations of plasma dynamics in the hysteresis regime of reactive RF sputter processes

Reactive radio frequency (RF) sputter processes are highly relevant for thin film deposition, but there is no complete understanding of the fundamentals of their operation. While the Berg model describes the hysteresis regime considering the oxygen coverage of the boundary surfaces, a complete fundamental understanding of the plasma–surface interactions and their effects on the discharge is still missing. In this work, we provide such fundamental insights based on an extensive experimental analysis of the physics in the hysteresis regime of magnetized reactive sputter processes, where the same reactive gas admixture can lead to different discharge characteristics depending on the previous state of the plasma. A variety of plasma and surface diagnostics is used to reveal these insights. A low pressure capacitively coupled RF discharge (CCP, 13.56 MHz) with a magnetron-like magnetic field topology adjacent to the target is operated in argon gas with a variable admixture of O2. The applied RF power, the gas flows/pumping speed, as well as the neutral gas pressure are changed systematically to understand the effects of these external control parameters on the hysteresis regime. The magnetic asymmetry effect is found to play an important role, since an axially non-uniform magnetic field is used to realize a local electron confinement at the target. Similar to process control in applications, the DC self-bias is measured to stabilize the surface composition using a feedback controller with the oxygen gas flow as the manipulated variable.

Data-driven surrogate modeling of hPIC ion energy-angle distributions for high-dimensional sensitivity analysis of plasma parameters' uncertainty

We present a data-driven strategy for effective construction of a surrogate model in high-dimensional parameter space for the ion energy-angle distribution (IEAD) output of hPIC simulations of plasma-surface interactions. The methodology is based on a bin-by-bin least-squares fitting of the IEAD in the parameter space. The fitting is performed in a transformed coordinate system to normalize the IEAD, and it employs sparse grids for sampling the parameter space to overcome sampling challenges in high dimensions. The surrogate model is significantly cheaper computationally than direct hPIC simulations yet maintains high fidelity to them, providing a fast emulator for hPIC simulations. Sensitivity analysis based on the surrogate model is utilized to characterize the dependence of the ion impact angle and energy moments on the physical parameters.1