Boron coating of plasma-facing components (PFCs) is widely used in fusion devices to form surface coatings and chemical compounds on the PFCs, thus suppressing plasma contamination by impurities such as oxygen, carbon, and tungsten. Understanding the processes of hydrogenic species retention is crucial to assessing the viability of boron coating techniques in ITER and beyond. In this work, we deposited boronization films on silicon crystal and ITER-grade tungsten samples using a glow discharge boronization plasma of helium (He, 20 %) and diborane (B2D6, 80 %) in the DIII-D tokamak at the heated vessel temperature of 600 K. We performed post-mortem analysis of DIII-D boronization films for thickness utilizing a focused ion beam, deuterium retention utilizing thermal desorption spectroscopy (TDS), and chemical characterization utilizing X-ray photoelectron spectroscopy and Raman spectroscopy. The DIII-D boronization films were exposed to DIII-D tokamak L-, H-mode, and/or PISCES-E linear device D plasmas. We observed a D uptake for B:D = 5:2 in the DIII-D boronization film before D plasma exposures. The observed D retention was mainly in the B-C-D bond with a desorption peak of 1000 K. No B-D bond with a desorption peak around 700 K was found in the TDS spectrum as the DIII-D vessel was heated to 600 K. Exposures to both DIII-D L- and H-mode D plasma, which includes 1-3 % of C ion contamination, on the DIII-D boronization film at similar to 400 K gave a D uptake increase of similar to 20 %. Contrarily, D removal by similar to 30 % was observed after exposing the DIII-D boronization film to a carbon-free PISCES-E D plasma. This implies that future fusion devices should be careful to minimize C impurity contamination to reduce D retention.

Zero-dimensional kinetic modeling of atmospheric pressure Ar-N2-H2 nonthermal plasma was carried out to gain mechanistic insights into plasma-assisted catalytic synthesis of ammonia. Ar dilution is a common technique for tailoring plasma discharge properties and has been shown to enhance NH3 formation when added to N2-H2 plasma. The kinetic model was developed for a coaxial dielectric barrier discharge quartz wool-packed bed reactor operating at near room temperature using a kHz-frequency plasma source. With 30% Ar mixed in a 1:1 N2-H2 plasma at 760 Torr, we find that NH3 production is dominated by Eley-Rideal (E-R) surface reactions, which heavily involve surface NHx species derived from N and H radicals in the gas phase, while the influence of excited N2 molecules is negligible. This is contrary to the commonly proposed mechanism that excited N2 molecules created by Penning excitation of N2 by Ar(4s) and Ar(4p) play a significant role in assisting NH3 formation. Our model shows that the enhanced NH3 formation upon Ar dilution is unlikely due to the interactions between Ar and H species, as excited Ar atoms have a weak effect on H radical formation through H2 dissociation compared to electrons. We find that excited Ar atoms contribute to 28% of the N radical production in the gas phase via N2 dissociation, while the rest are dominated by electron-impact dissociation. Furthermore, Ar species play a negligible role in the product NH3 dissociation. N2 conversion sensitivity analyses were carried out for electron number density (ne) and reduced electric field (E/N), and contributions from Ar to gas-phase N radical production were quantified. The model can provide guidance on potential reasons for observing enhanced NH3 formation upon Ar dilution in N2-H2 plasma beyond changes in the discharge characteristics.

W-SiC composite material is a promising plasma-facing material candidate alternative to pure W due to the low neutron activation, low impurity radiation, and low tritium diffusivity of SiC while leveraging the high erosion resistance of the W armor. Additionally, W and SiC have high thermomechanical compatibility given their similar thermal expansion rates. The present study addresses the synthesis and performance of compositionally graded W-SiC films fabricated by pulsed-DC magnetron sputtering. Compositional gradients were characterized using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS), and crystallographic information was obtained using electron diffraction and X-ray diffraction (XRD). Samples were exposed to L-mode deuterium plasma discharges in the DIII-D tokamak using the Divertor Material Evaluation System (DiMES). Post-mortem characterizations were performed using scanning electron microscopy (SEM) and XRD. Electron diffraction and XRD showed that the compositionally graded W-SiC films were composed of polycrystalline W and amorphous SiC with amorphous W+SiC interlayers. No macroscopic delamination or microstructural changes were observed under mild exposure conditions. This study serves as a preliminary examination of W-SiC compositionally graded composites as a potential candidate divertor material in future tokamak devices.

The micro-trench method is a new technique to measure polar and azimuthal incident ion angles, material erosion, and impurity deposition at plasma-facing surfaces, and has been applied in the DIII-D divertor. This article gives a tutorial of the micro-trench technique consisting of six steps: (1) micro-trench fabrication, (2) pre-exposure measurement of the fabricated micro-trench geometry, (3) tracer material deposition, (4) plasma exposure, (5) post-exposure observation of the tracer material, and (6) post-exposure measurement of the micro-trench geometry. Two criteria need to be satisfied to apply the micro-trench method successfully: (i) uniform impurity deposition on the micro-trench floor, and (ii) erosion dominated by the physical sputtering induced by the impinging ions. When those two criteria are satisfied, post-exposure analysis of the impurity deposition patterns on the micro-trench floor may be used to determine the polar and azimuthal incident ion directions (mean values of the ion angle directions), erosion rate, and impurity deposition rate during plasma exposure without computational interpretation.

Fundamental mechanisms governing the erosion and prompt re-deposition of tungsten impurities in tokamak divertors are identified and analyzed to inform the lifetime of tungsten plasma-facing components in ITER and other future devices. Various experiments conducted at DIII-D to benchmark predictive models are presented, leveraging the DiMES removable sample exposure probe capability and the Metal Rings Campaign, in which toroidally symmetric rows of tungsten-coated tiles were installed in the DIII-D divertor. In tokamak divertors, the width of the electric sheath is of the order of the main ion Larmor radius, and a vast majority of sputtered tungsten impurities are typically ionized within the sheath. Therefore, W prompt redeposition is mainly governed by the ratio of the characteristic ionization mean-free path of neutral tungsten to the width of the sheath. In-situ monitoring of the prompt redeposition of tungsten impurities in divertors is demonstrated via the use of WII/WI line ratios and the ionizations/photon (S/XB) method in L-mode discharges. Even with this relatively limited set of emission measurements, net erosion measurements were found to be a consistent upper bound to an analytic scaling based on the ratio of the W ionization length, λ iz , and the width of the magnetic sheath rather than the ratio of λ iz and the W + gyro-radius. In the far-scrape-off layer (SOL) of the ITER divertor, however, it is calculated that the measurement of photon emissions associated with the ionization of tungsten impurities up to W 5 + may be required. Finally, W deposition patterns on DiMES collector probes, interpreted via DIVIMP-WallDYN modelling, reveal the key roles of progressive W erosion/re-deposition staps and E × B drifts in regulating long-range high-Z material migration.

A commercially available electron cyclotron resonance (ECR) plasma source (GenII Plasma Source, tectra GmbH) is widely used for surface processing. This plasma source is compatible with ultrahigh vacuum systems, and its working pressure is relatively low, around 10-6-10-4 Torr even without differential pumping. Here, we report ion flux concentration ratios for each ion species in an ion beam from this source, as measured by a mass/energy analyzer that is a combination of a quadrupole mass spectrometer, an electrostatic energy analyzer, and focusing ion optics. The examined beams were those arising from plasmas produced from feed gases of H2, D2, N2, O2, Ar, and dry air over a range of input power and working pressures. H2(D2) plasmas are widely used for nuclear fusion applications and, hence, the ion concentration ratios of H+, H2+, and H3+ reported here will be useful information for research that applies this plasma source to well-controlled plasma-material interaction studies. Ion energy distributions, stability of operation, and impurity concentrations were also assessed for each of the plasma species investigated.

Experiments have been conducted in the DIII-D tokamak to explore the in-situ growth of silicon-rich layers as a potential technique for real-time replenishment of surface coatings on plasma-facing components (PFCs) during steady-state long-pulse reactor operation. Silicon (Si) pellets of 1 mm diameter were injected into low- and high-confinement (L-mode and H-mode) plasma discharges with densities ranging from $3.9-7.5\times10^{19}$ m$^{-3}$ and input powers ranging from $5.5-9$ MW. The small Si pellets were delivered with the impurity granule injector (IGI) at frequencies ranging from 4-16 Hz corresponding to mass flow rates of $5-19$ mg/s ($1-4.2\times10^{20}$ Si/s) at cumulative amounts of up to 34 mg of Si per five-second discharge. Graphite samples were exposed to the scrape-off layer and private flux region plasmas through the divertor material evaluation system (DiMES) to evaluate the Si deposition on the divertor targets. The Si II emission at the sample correlates with silicon injection and suggests net surface Si-deposition in measurable amounts. Post-mortem analysis showed Si-rich coatings containing silicon oxides, of which SiO$_2$ is the dominant component. No evidence of SiC was found, which is attributed to low divertor surface temperatures. The in-situ and ex-situ analysis found that Si-rich coatings of at least $0.4-1.2$ nm thickness have been deposited at $0.4-0.7$ nm/s. The technique is estimated to coat a surface area of at least 0.94 m$^2$ on the outer divertor. These results demonstrate the potential of using real-time material injection to form Si-enriched layers on divertor PFCs during reactor operation.

The magnetic pre-sheath (MPS) length, L MPS, is a critical parameter to define the sheath potential, which controls the ion trajectory of low-Z species (D, T, He, and C), as well as the prompt re-deposition of high-Z species. To determine L MPS, we fabricated micro-trenches (30 × 30 × 4 μm) via focused ion beam milling on a silicon surface and exposed them to L-mode deuterium plasmas in DIII-D via the divertor material evaluation system (DiMES) removable sample exposure probe. The areal distribution of impurity depositions, mainly consisting of carbon, was measured by energy-dispersive x-ray spectroscopy (EDS) to reveal the deuterium ion shadowing effect on the trench floors. The carbon deposition profiles showed that the erosion was maximized for the azimuthal direction of φ = −40° (referenced to the toroidal magnetic field direction) as well as the polar angle of θ = 80°. A Monte Carlo equation-of-motion (EOM) model, based on a collisionless MPS, was used to calculate the azimuthal and polar deuterium ion angle distributions (IADs) at the surface for a range of L MPS = k × ρ i, where ρ i is the ion gyro radius and k = 0.5–4. Then, gross erosion profiles were calculated by a Monte Carlo micro-patterning and roughness (MPR) code for ion sputtering using as input the calculated azimuthal and polar IADs for each value of k. Good agreement with the experimental C deposition profiles was obtained for the case k = 2.5–3.5. This result is consistent with a previous kinetic modeling prediction of k ∼ 3, as well as previous analytical investigations that predicted the L MPS to be several ion gyro radii. A validation of theoretical sheath models supports its applicability to ITER and pilot plant divertors to successfully predict plasma–materials interactions.

This work presents a computational study of the retention, reflection, and sputtering processes at amorphous and crystalline lithium surfaces by the impact of low energy (5-100 eV) hydrogen and deuterium atoms and D-2 molecules for a range of incident angles of 0 degrees (normal to the surface) to 85 degrees. Classical molecular dynamics simulations were performed with the reactive bond-order force field (ReaxFF) potentials. Effects of the temperature of the surface slab were also considered. The extent of retention, and the energy and angular distributions of reflected and sputtered atoms were determined. Comparison of the results of these simulations with available experimental data on the sputtering rate for Li atoms is in good agreement for incident angles of 0 degrees, and the simulation results predict significant increase in the sputtering probabilities for incident angles larger than 30 degrees. (C) 2022 Elsevier B.V. All rights reserved.

We report carbon impurity ion incident angles and deposition rates, along with silicon erosion rates, from measurements of micro-engineered trenches on a silicon surface exposed to L-mode deuterium plasmas at the DIII-D divertor. Post exposure ex-situ analysis determined elemental maps and concentrations, carbon deposition thicknesses, and erosion of silicon surfaces. Carbon deposition profiles on the trench floor showed carbon ion shadowing that was consistent with ERO calculations of average carbon ion angle distributions (IADs) for both polar and azimuthal angles. Measured silicon net erosion rates negatively correlated with the deposited carbon concentration at different locations. Differential erosion of surfaces on two different ion-downstream trench slope structures suggested that carbon deposition rate is affected by the carbon ion incident angle and significantly suppressed the surface erosion. The results suggest the C impurity ion incident angles, determined by the IADs and surface morphology, strongly affect erosion rates as well as the main ion (D, T, He) incident angles.

Ammonia synthesis at 533 K and atmospheric pressure was investigated in a coaxial dielectric barrier discharge (DBD) plasma reactor without packing and with porous gamma-Al2O3, 5 wt % Ru/gamma-Al2O3, or 5 wt % Co/gamma-Al2O3 catalyst particles. Gas-phase species were monitored in situ using an electron impact molecular-beam mass spectrometer (EI-MBMS). Gas-phase species NNH and N2H2 were first identified under common conditions of plasma-assisted ammonia synthesis and were present at levels comparable to that of NH3 in the plasma discharge. Concentrations of NNH, N2H2, and NH in a reactor packed with gamma-Al2O3 or other particles were lower than those observed in an empty reactor, while the concentration of NH3 increased. These observations point to the importance of NNH and N2H2 in plasma-assisted surface reactions in ammonia synthesis. Reaction pathways of direct adsorption of gas-phase NNH and N2H2 on solid surfaces and subsequent reactions were proposed. This study demonstrated that in situ identification of gas-phase species via EI-MBMS provides a powerful approach to study the kinetics of plasma-assisted catalysis.

We report the first detailed experimental verification of the polar deuterium ion impact angle distribution (IAD) on the DIII-D divertor surface in L-mode plasmas using micro-engineered trenches in samples mounted on the DiMES probe. These trenches were fabricated via focused ion beam (FIB) milling of a silicon surface partially coated with aluminum. The sample surfaces were exposed to eight repeat L-mode deuterium discharges (30 s total exposure time). The samples were examined by scanning electron microscopy (SEM), which revealed changes on the trench floor due to material deposition and evidence for shadowing of the incident deuterium ions by the trench walls. The areal distribution of carbon and aluminum deposition was measured by energy-dispersive X-ray spectroscopy (EDS). One-dimensional profiles of this deposition are in agreement with net erosion profiles calculated from a Monte Carlo micro-patterning and roughness (MPR) code for ion sputtering using as input the polar and azimuthal deuterium IADs reported previously (Chrobak et al., Nucl. Fusion 58, 106019 (2018)). The deposition profiles verified the characteristic shape of the polar IADs, which have a broad maximum from 79° to 86°, over the experimental range of 68°–83°, where 0° is the surface normal direction.