Neutral Ion Research Articles

Simulating vacuum arc initiation by coupling emission, heating and plasma processes

Vacuum arcing poses significant challenges for high-field vacuum devices, underscoring the importance of understanding it for their efficient design. A detailed description of the physical mechanisms involved in vacuum arcing is yet to be achieved, despite extensive research. In this work, we further develop the modeling of the physical processes involved in the initiation of vacuum arcing, starting from field emission and leading to plasma onset. Our model concurrently combines particle-in-cell with Monte Carlo collisions (PIC-MCC) simulations of plasma processes with finite element-based calculations of electron emission and the associated thermal effects. Including the processes of evaporation, impact ionization and direct field ionization allowed us to observe the dynamics of plasma buildup from an initially cold cathode surface. We simulated a static nanotip at various local fields to study the thresholds for thermal runaway and plasma initiation, identifying the significance of various interactions. We found that direct field ionization of neutrals has a significant effect at high fields on the order of 10 GV/m. Furthermore, we find that cathode surface interactions such as evaporation, sputtering and bombardment heating play a major role in the initiation of vacuum arcs. Consequently, the inclusion of these interactions in vacuum arc simulations is imperative.

Holistic managements of textile wastewater through circular, greener and eco-innovative treatment systems developed by minimal to zero liquid discharge

New pragmatic and viable solutions to reduce or prevent discharge and to protect reserves are currently among the top-prioritised research for cleaner, circular, and resource-efficient use of industrial waters. So, the development of eco-sustainable water management is essential for green industrial development that will meet versatile and eco-sensitive regulatory standards, especially in water-intensive industries. Textile wastewater was reclaimed in semi to fully closed loops for minimal to zero liquid discharge. Concentrate-mixed wastewater was steadily treated in a hybrid membrane oxidation reactor at 60–80 % synergistic performances with remarkable UF fluxes of 96.4–820 L/m2h without any sludge discharge. Effluent was purified with 90–100 % removals and 20–80 L/m2h in nanofiltration and reverse osmosis. Due to Fenton-specific operation, more handling by ion exchange and neutralisation required to harvest membrane reuse waters and reactor discharge effluents with guaranteed Fe and pH. All-in-one system simulations indicated that high quality reuse waters are produced by 99.9 % efficiency and 98 and 100 % savings in iron and acid but 20–51 % more oxidant through concentrate recycling and regenerant reuse. It was also revealed that reactor effluents can be released to the sea or conventional biological treatment or can be eco-sustainably exploited for in-situ chemical and ex-situ bio-induced recovery of vivianite. This research demonstrates that how textile wastewater can be managed holistically by liquid discharge approaches from 50 % minimal to 99.9 % zero just in two-step, i.e. pretreatment and pre-concentration, with consumable minimisation and valuable waste recovery through the eco-innovative systems which are developed as circular, greener, and sludge-free compatible with sustainable development goals.

A Comparative Analysis of Gravity Waves in He and Ar Densities in the Martian Thermosphere

AbstractIn this study, we conducted a comparative analysis of gravity waves (GWs) in the Martian upper thermosphere, utilizing He and Ar densities measured by the Neutral Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile EvolutioN spacecraft. Our investigation revealed that GWs are a persistent phenomenon in Martian thermospheric He densities, akin to their presence in heavier species like Ar and CO2. Intriguingly, we observed that local time and seasonal variations in He GW amplitudes exhibit an almost opposite pattern compared to those in Ar. A significant breakthrough was the identification of a temperature‐dependent transition wavelength distinctly demarcating short and long wavelengths. This transition wavelength, which increases with increasing background temperatures, profoundly influences the dominance of GW amplitudes in He and Ar. In the short‐wavelength limit, GW amplitudes in He prevail, while GW amplitudes in Ar take precedence in the long‐wavelength limit. During periods of lower temperatures during midnight and the dawn terminator, transition wavelengths fall within the observed wavelength range enabling the study of dominance of GWs in He and Ar. Conversely, higher temperatures (>180 K) during daytime and the dusk terminator eliminate transition wavelengths within our observational range. Consequently, the amplitudes of GWs in He dominate over those in Ar. Furthermore, under nominal dust conditions, GWs in both He and Ar exhibit an inverse relationship with their respective background densities. During global dust storm, this inverse relationship is maintained for GWs in He, while GWs in Ar display a slight positive correlation with background Ar densities.

Modeling the structure of the dayside Venusian ionosphere: Impacts of protonation and Coulomb interaction

The CO$_2$-dominated thick atmosphere of Venus coexists with an ionosphere that is mainly formed, on the dayside, via the ionization of atmospheric neutrals by solar extreme ultraviolet and soft X-ray photons. Despite extensive modeling efforts that have reproduced the electron distribution reasonably well, we note two main shortcomings with respect to prior studies. The effects of protonation and Coulomb interaction are crucial to unveiling the structure and composition of the Venusian ionosphere. We evaluate the role of protonated species on the structure of the dayside Venusian ionosphere for the first time. We also evaluate the role of ion-ion Coulomb collisions, which are neglected in many existing models. Focusing on the solar minimum condition for which the effect of protonation is expected to be more prominent, we constructed a detailed one-dimensional photochemical model for the dayside Venusian ionosphere, incorporating more than 50 ion and neutral species (of which 17 are protonated species), along with the most thorough chemical network to date. We included both ion-neutral and ion-ion Coulomb collisions. Photoelectron impact processes were implemented with a two-stream kinetic model. Our model reproduces the observed electron distribution reasonably well. The model indicates that protonation tends to diverge the ionization flow into more channels via a series of proton transfer reactions along the direction of low to high proton affinities for parent neutrals. In addition, the distribution of O$_2^+$ is enhanced by protonation by a factor of nearly 2 at high altitudes, where it is efficiently produced via the reaction between O and OH$^+$. We find that Coulomb collisions influence the topside Venusian ionosphere not only directly by suppressing ion diffusion, but also indirectly by modifying ion chemistry. Two ion groups can be distinguished in terms of the effects of Coulomb collisions: one group preferentially produced at high altitudes and accumulated in the topside ionosphere, which is to be compared with another group that is preferentially produced at low altitudes and, instead, depleted in the topside ionosphere. Both protonation and Coulomb collisions have appreciable impacts on the topside Venusian ionosphere, which account for many of the significant differences in the model ion distribution between this study and early calculations.

Pushing nano-aerosol measurements towards a new decade – technical note on the Airmodus particle size magnifier 2.0

Abstract. Accurate measurement of the size distribution of sub-10 nm aerosol particles is still a challenge. Here we introduce a novel version of the Airmodus particle size magnifier (PSM 2.0), which is a condensation-particle-counter-based instrument with a sizing range of 1–12 nm. The extended size range compared to the earlier PSM version enables the direct detection of forming clusters and particles as well as the study of their growth processes without the challenges related to particle charging. It also gives an overlap between the activation size distribution measurements with the PSM and mobility size distribution measurements with conventional mobility particle sizers. We compared the performance of PSM 2.0 to that of a mobility particle size spectrometer, the original A10 particle size magnifier, and a Neutral cluster and Air Ion Spectrometer (NAIS) during field measurements. Also, calibration results were compared against the A10 instrument. The results show that PSM 2.0 is able to activate sub-2 nm clusters and that the concentration and size distribution between 2–12 nm compare well, especially with the NAIS.

Cold Ion-Molecule Reactions in the Extreme Environment of a Coulomb Crystal.

Coulomb crystals provide a unique environment in which to study ion-neutral gas-phase reactions. In these cold, trapped ensembles, we are able to study the kinetics and dynamics of small molecular systems. These measurements have connections to chemistry in the Interstellar Medium (ISM) and planetary atmospheres. This Feature Article will describe recent work in our laboratory that uses Coulomb crystals to study translationally cold, ion-neutral reactions. We provide a description of how the various affordances of our experimental system allow for detailed studies of the reaction mechanisms and the corresponding products. In particular, we will describe quantum-state resolved reactions, isomer-dependent reactions, and reactions with a rarely studied, astrophysically relevant ion, CCl+.

Unveiling the Chemical Composition of Sulfur-Fumigated Herbs: A Triple Synthesis Approach Using UHPLC-LTQ-Orbitrap MS-A Case Study on Steroidal Saponins in Ophiopogonis Radix.

Ophiopogonis Radix (OR) is a traditional Chinese medicine. In recent years, in order to achieve the purpose of drying, bleaching, sterilizing and being antiseptic, improving appearance, and easy storage, people often use sulfur fumigation for its processing. However, changes in the chemical composition of medicinal herbs caused by sulfur fumigation can lead to the transformation and loss of potent substances. Therefore, the development of methods to rapidly reveal the chemical transformation of medicinal herbs induced by sulfur fumigation can guarantee the safe clinical use of medicines. In this study, a combined full scan-parent ions list-dynamic exclusion acquisition-diagnostic product ions analysis strategy based on UHPLC-LTQ-Orbitrap MS was proposed for the analysis of steroidal saponins and their transformed components in sulfur-fumigated Ophiopogonis Radix (SF-OR). Based on precise mass measurements, chromatographic behavior, neutral loss ions, and diagnostic product ions, 286 constituents were screened and identified from SF-OR, including 191 steroidal saponins and 95 sulfur-containing derivatives (sulfates or sulfites). The results indicated that the established strategy was a valuable and effective analytical tool for comprehensively characterizing the material basis of SF-OR, and also provided a basis for potential chemical changes in other sulfur-fumigated herbs.

Ultra‐Fast Kelvin Wave Packets in Mars' Atmosphere and Their Interactions With Tides as Viewed by MAVEN/NGIMS and MRO/MCS

AbstractA key element of successful aerobraking operations at Mars is accurate thermospheric density predictions. Evidence suggests that much of the longitude variability in Mars' aerobraking region is associated with atmospheric tides, and the day‐to‐day variability is connected with tidal modulation by longer‐period global‐scale waves. Specifically, ultra‐fast Kelvin waves (UFKWs) and their modulation of the tidal spectrum play a key role in coupling Mars' lower (<∼80 km) and middle (∼80–100 km) atmosphere with the aerobraking region above. In this study, over 5 years of Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer CO2 density observations are employed to reveal prominent, frequent, and persistent 2.5‐ to 4.5‐day UFKW packets in the whole Martian middle and upper thermosphere (ca. 150–200 km), and large secondary waves arising from their nonlinear interactions with the tidal spectrum. Detailed analyses focusing on a prominent ∼2.5‐day UFKW event in late 2015 demonstrate primary and secondary wave amplitudes growing twofold with altitude from ∼7%–14% near 150 km to ∼12%–25% near 200 km and their combined effects to account for ∼60%–80% of the altitude‐longitudinal variability of Mars' thermospheric density. Concurrent temperature measurements from Mars Reconnaissance Orbiter Mars Climate Sounder reveal consistent wave signatures near 80 km altitude suggesting propagation of both primary and secondary waves from the lower atmosphere. This study demonstrates that UFKWs and secondary waves from UFKW‐tide interactions are sources of significant altitude‐longitude variability in the Mars' aerobraking region that should be accounted for when analyzing satellite observations and nonlinear models.

Photon Acceleration by Superluminal Ionization Fronts

This paper explores the use of superluminal ionization fronts to accelerate and amplify electromagnetic radiation. These fronts are defined as optical boundaries between two regions of a gas, the neutral region and the plasma region, characterized by two different values of the refractive index. For that reason, the front velocity is not necessarily related to the motion of material particles, such as neutral atoms, ions and electrons, which can stay at rest. The fronts can therefore become superluminal without violating causality. In recent years, different experimental configurations, such as the flying focus, showed that it is possible to create superluminal fronts in the laboratory. These fronts can easily be described theoretically in a special reference frame, called the time frame, which is used here. In this frame, superluminal fronts reduce to time refraction, a process that is symmetrical to the well-known optical refraction. It is shown that propagation through such fronts can lead to considerable frequency shifts and energy amplification of probe laser beams. This could eventually be used to develop new sources of tunable radiation.

Investigation on plasma ionization process of a micro-cathode arc thruster

Micro-cathode arc thrusters (μCATs) are space propulsion options suitable for microspacecraft, and have recently attracted much attention because of their low electrical power requirements and simple, compact propellant systems. The plasma ionization process, however, is not currently well understood. In this study, a μCAT prototype was designed with the use of different specific materials for each component, enabling the study of the ionization process from the emissive light of excited and ionized states from different elements. ICCD emission spectroscopy and scanning monochromator measurements are conducted inside the electrode channel of a μCAT with a coaxial configuration. The excited state atomic spectral results and ionization state spectral results give us a comprehensive understanding of the behavior of neutral particles and ions during the discharge process of the μCAT. The ionization sequence follows in the order of C+, Al+, Ti+, Cu+, and Ti2+. From the duration of the spectral lines, C I has the shortest duration (16.9 μs), which shows that the ablation of the conductive film is mainly concentrated during the first half of the discharge, while anode and cathode ionization occur over the entire discharge duration. Additionally, Al+ ionization is observed during the discharge process, showing that part of the discharge energy is expended in ionizing the thruster outer shell, resulting in efficiency loss. An ionization ratio of k is proposed in this work to represent the relative ionization of the conductive film and cathode material, aiding us in determining the optimal operation conditions for lifetime extension.

Energy Transfer in Mixed Lanthanides Complexes: Toward High‐Performance Pressure Sensors Based on the Luminescence Intensity Ratio

AbstractIn this study, a reversible pressure‐sensing material with high sensitivity is presented. Mixed β‐diketonate complexes of Tb3+ and Eu3+ [(Ln)(acac)3phen] are synthesized with phenanthroline as an ancillary ligand. The organic ligands provide the antenna effect to make the Ln3+ complex excitable at 405 nm. Eu3+ emission results from efficient energy transfer (ET) from Tb3+. Under 405 nm excitation, the emission intensity of Tb3+ decreases whereas the emission intensity of Eu3+ increases with pressure, making this complex a potential pressure sensor based on luminescent‐intensity‐ratio (LIR) up to 700 MPa. This study then discusses the application of this Tb3+/Eu3+ complex for pressure sensing depending on measurement conditions. The addition of the optically neutral ion Y3+ to the system can reduce the impact of pressure‐induced structural defects on the emission, thus improving the reversibility of the LIR variation as a function of pressure. Therefore, a self‐calibrating, reliable, and reversible pressure‐sensing material is proposed here, with remarkable pressure sensitivity compared to a peak shift‐based pressure sensor.

Active generation and control of radial electric field by local neutral beamlets injection in tokamaks

The radial electric field plays an important role in plasma confinement in tokamaks and can be generated through neutral beam injection. In this study, we propose a model for calculating the radial electric field resulting from tangential local neutral beamlet injection, aiming to externally control and improve plasma confinement. The Neutral beamlet ion and Energetic particles Orbit mover and Electric field solver code has been developed to analyze this issue, and its simulation results have been validated against results from other codes as well as measurements from correlation reflectometers. The charge separation is primarily caused by the redistribution and loss of beam ions due to magnetic gradient and curvature drift as well as collision effects, and it is maintained through continuous beamlet injection. The electric field is calculated using Poisson’s equation, taking into account both classical and neoclassical polarization effects. The results demonstrate that despite the high losses and low heating efficiency associated with localized beamlets, they are capable of generating a significant radial electric field characterized by a steep gradient. This presents opportunities for external control of the electric field, potentially enhancing plasma confinement.

Dynamics and thermochemistry of the negatively charged clusters in a 2-hydroxyethylhydrazinium nitrate ionic liquid system.

The formation and fragmentation of negatively charged 2-hydroxyethylhydrazinium nitrate ([HOCH2CH2NH2NH2]+NO3-, HEHN) ionic liquid clusters were examined using a guided-ion beam tandem mass spectrometer furnished with collision-induced dissociation of selected ions with Xe atoms. Measurements included the compositions of cluster ions formed in the ionization source, and the dissociation products, cross sections, and 0 K threshold energies for individually selected cluster ions. To identify the structures of the main cluster ion series [(HEHN)n(HNO3)0-1NO3]- formed, molecular dynamics simulations were employed to create initial geometry guesses, followed by optimization at the ωB97XD/6-31+G(d,p) level of theory, from which global minimum structures were identified for reaction thermodynamics analyses. A comparison was made between the cluster formation and fragmentation in the negatively charged 2-hydroxyethylhydrazinium nitrate with those in the positive mode (reported by W. Zhou et al., Phys. Chem. Chem. Phys., 2023, 25, 17370). In both modes, the cluster ions were predominantly composed of m/z below 350; loss of a neutral 2-hydroxyethylhydrazinium nitrate ion pair represents the most important cluster fragmentation pathway, followed by intra-ion pair proton transfer-mediated 2-hydroxyethylhydrazine and HNO3 elimination; and all clusters started to dissociate at threshold energies less than 1.5 eV. The overwhelming similarities in the formation and fragmentation chemistry of positively vs. negatively charged 2-hydroxyethylhydrazinium nitrate clusters may be attributed to their inherent ionic nature and high electric conductivities.

HCNH+ abundance in cold dense clouds based on the first hyperfine resolved rate coefficients

The protonated form of hydrogen cyanide, HCNH+, holds significant importance in astrochemistry, serving as an intermediate species in ion-neutral reactions occurring in the cold molecular clouds. Although it plays a crucial role in the chemistry of HCN and HNC, the excitation rate coefficients of this molecular cation by the dominant interstellar colliders have not been thoroughly investigated, leading to limitations in the radiative transfer models used to derive its abundance. In this work, we present the first hyperfine-resolved excitation rate coefficients for HCNH+ induced by collisions with both He and H2 at low temperatures, addressing a crucial requirement for precise modeling of HCNH+ abundance in typical cold dense molecular clouds. Using non-local thermodynamic equilibrium (non-LTE) radiative transfer calculations, we reproduced the 1 → 0 and 2 → 1 observational spectra of HCNH+ fairly well and derived updated molecular column densities. For the TMC-1 molecular cloud, the new HCNH+ abundance is twice as large as suggested by previous LTE modeling, whereas the column density of this molecular cation is improved only by 10% in the case of the L483 proto-star. The factor of two in the case of TMC-1 most likely arises from an error in the early analysis of observational spectra rather than an effect of the LTE assumption, given that the HCNH+ lines are predominantly thermalized at densities higher than 2 × 104 cm−3. For multiline studies of clouds of moderate densities, we strongly recommend using the collisional rate coefficients reported in this work.

Electron extraction mechanism of magnet array microwave discharge neutralizer

Microwave discharge neutralizer is an important part of microwave discharge ion thruster system, which plays a vital role in maintaining potential balance between spacecraft and neutralizing ion beam. Its electron extraction property directly affects the operation condition of ion thruster system. In order to break through the power limit of miniature microwave discharge ion thruster, a magnet array microwave discharge ion thruster system is designed and tested. In the experiment on finalizing magnetic field structure of magnet array microwave discharge neutralizer, an interesting phenomenon is found that the <i>I</i>-<i>V</i> curves of electron current, after rotating the magnetic array orientation, are very different. Defining forward direction of magnet array can normally extract electrons, then backward direction of magnet array can hardly extract electrons. Because the diameter of discharge chamber is only 10 mm, it is too small to perform Langmuir probe diagnosis. And thus, an integrative particle-in-cell method is used to simulate the neutralizer operation processes of two different magnetic field structures, and for the sake of accuracy, real vacuum permittivity is used. The simulation results show good consistence with experimental phenomenon. In the initial discharge process, it is found that the magnetic field gradient leads to different plasma distributions; in electron extraction process, it is found that the potential distribution near the orifice determines the electron extraction property of the neutralizer. Through comparing the plasma parameter distributions under different magnetic field structures and operating voltages, an assumption that the ion is an important factor in electron extraction process is proposed. Then, a simulation that ions disappear artificially outside the orifice is conducted, and the simulation results show that electrons cannot be effectively extracted without ions near the orifice. According to the simulation and experiment results, two necessary conditions are summarized for electron extraction of the neutralizer. The first condition is magnetic field structure: the magnetic field gradient should point towards the orifice to guide plasma migration towards the orifice, the second one is potential distribution: there should be enough ions to lift the potential near the orifice for reducing or breaking the potential well. These two conditions can help understand the electron extraction mechanism of microwave discharge neutralizer and provide theoretical reference for optimizing the performance of neutralizer in future.

Calculation of negative ion beam loss for two region arc plasma negative ion source

Korea Atomic Energy Research Institute (KAERI) is currently developing a Two Region Arc Plasma (TRAP) negative ion source for neutral beam injection (NBI) systems of a fusion tokamaks which is characterized by two distinct regions with different plasma temperatures without the use of external filter field magnets. The source has recently been designed, manufactured, and installed for testing. Plasma discharge characterization experiments of the TRAP ion source are presently in progress, with planned beam extraction experiments. The beam extractor of the TRAP ion source is capable of accelerating hydrogen ions up to 30 keV and comprises four grids with slit-type aperture structure. Understanding the negative ion beam loss in the extractor is crucial for expecting potential issue within the beam extractor and comprehending the plasma discharge characteristics through negative ion beam. Using a 3-D Monte Carlo program (molflow+), we calculated the vacuum pressure distribution within the TRAP ion source experimental device, and based on these results, we calculated the loss of negative ion beams within the beam extractor and the neutralization efficiency from the beam exit to the target, an effect arising from residual gases. The computational results reveal that approximately 20 ∼ 50 % of the negative ion beam is lost within the beam extractor, and a reduction of more than 60 % in negative current occurs from the beam exit grid to the target within the test chamber due to neutralization of negative ions.

Optical emission spectroscopy of vanadium cathodic arc plasma at different nitrogen pressure

Optical emission spectroscopy studies of vanadium plasma in a cathodic-arc discharge in a nitrogen atmosphere have been carried out. Spectral lines of neutral atoms and ions of the cathode material V, V1+, and V2+, and nitrogen N2 and N2+ were observed in the discharge plasma. Analysis and comparison of the intensity of vanadium and nitrogen spectral lines as a function of nitrogen pressure showed that in vacuum excited ions V2+ and V+ are registered, with increasing pressure, the lines V+*, N2*, and N2+* are observed, and at pressures above 0.5 Pa, the neutral vanadium lines are additionally registered. The electron temperature of Te decreases from 5.9 to 3–4 eV with increasing pressure. Studies of cross-sectional scanning electron microscopy images of VN coatings deposited at different nitrogen pressures have shown that a dense, homogeneous, fine-grained microstructure is formed in the coating when the number of neutral V in the plasma is low, while in the presence of a large number of neutrals, the coating structure changes to a dense structure with columnar growth.

Effects of strike point location on the divertor particle and energy flux decay widths on EAST by experiment and SOLPS modeling

The new lower tungsten divertor of EAST uses a right-angle shape consisted by horizontal and vertical targets, which has the capacity of increasing the divertor closure. The strike point (SP) sweeping experiment is carried out to (1) avoid long-term deposition of particle and heat flux at the same location, thus protecting the target, (2) study the dependence of power control capability on the SP location. The particle and energy flux densities to the target depends strongly on their decay width. Therefore, it is important to know how the SP location influences the outer target (OT) particle parallel λjs ,OT and OT parallel heat flux decay widths λq ,OT. In this work, SOLPS-ITER simulations combined with SP sweeping experiment are applied to study this issue. Four cases, which are taken from different time during SP sweeping (including both horizontal and vertical divertor) in L-mode experiment with high heating power, are selected for investigation. The simulation result is in satisfactory agreement with experiment data, suggesting the simulation is valid. The results indicate that the SP location can affect neutral particles accumulation and ionization positions, thus affecting λjs ,OT and λq ,OT. (1) When SP is located in horizontal target, the higher neutral particle ionization in common flux region leads to wider λjs ,OT than those of vertical target. (2) When SP is located on horizontal target, the divertor power radiation is higher than that of vertical target, resulting in wider λq ,OT. (3) Increasing upstream plasma density can effectively broaden λq ,OT, while λjs ,OT remains almost unchanged. This study improves the understanding of the influence of divertor shape on λjs ,OT and λq ,OT, and can be applied to heat flux control during long-pulse high-power discharges on EAST.

Self-consistent multi-component simulation of plasma turbulence and neutrals in detached conditions

Simulations of high-density deuterium plasmas in a lower single-null magnetic configuration based on a TCV discharge are presented. We evolve the dynamics of three charged species (electrons, D+ and D2+ ), interacting with two neutrals species ( D and D2 ) through ionization, charge-exchange, recombination and molecular dissociation processes. The plasma is modelled by using the drift-reduced fluid Braginskii equations, while the neutral dynamics is described by a kinetic model. To control the divertor conditions, a D2 puffing is used and the effect of increasing the puffing strength is investigated. The increase in fuelling leads to an increase of density in the scrape-off layer and a decrease of the plasma temperature. At the same time, the particle and heat fluxes to the divertor target decrease and the detachment of the inner target is observed. The analysis of particle and transport balance in the divertor volume shows that the decrease of the particle flux is caused by a decrease of the local neutral ionization together with a decrease of the parallel velocity, caused by the lower plasma temperature and the increase in momentum losses. The relative importance of the different collision terms is assessed, showing the crucial role of molecular interactions, as they are responsible for increasing the atomic neutral density and temperature, since most of the D neutrals are produced by molecular activated recombination and D2 dissociation. The presence of strong electric fields in high-density plasmas is also shown, revealing the role of the E × B drift in setting the asymmetry between the divertor targets. Simulation results are in agreement with experimental observations of increased density decay length, attributed to a decrease of parallel transport, together with an increase of plasma blob size and radial velocity.

Electrical energy input efficiency limitations in CO2‐to‐CO electrolysis and attempts for improvement

AbstractElectrochemical CO2 reduction is a potentially up‐coming technology to convert anthropogenic emitted CO2 into chemical feedstock. Due to alkaline operating conditions of CO2‐electrolyis in gas diffusion electrodes, exothermal hydroxide ion neutralization with the excess of supplied CO2 leads to unavoidable electricity‐to‐heat conversion at the cathode, therefore limiting electrical energy input efficiency. The decomposition reaction of carbonates by protons from water oxidation completes the inherent CO2 transport at the anode. In this work, different production routes to CO are thermodynamically examined and experimentally validated. Using formic acid as an intermediate towards CO the electrical energy input efficiency can rise to 71% on a thermodynamical basis. Additionally, the possibility of altering the mechanism of CO2 reduction under acidic conditions is investigated, which would lead to even larger electrical energy input efficiencies. The concept was investigated by pH series measurements (pH = 0–6) at 50 mA/cm2 where Pb derived from Pb3O4 was used as a CO2 reduction catalyst. The reduction to formic acid under acidic bulk electrolyte pH is stable at FEHCOOH = 70% down to pH ≈ 1, while HER is becoming dominant below. Even under such acidic bulk electrolyte conditions no change in reduction mechanism could be forced, which is reflected in invariant cell voltages in the model experiment.