Impurity Species Research Articles

Experimental research on the TCV tokamak

Abstract Tokamak à configuration variable (TCV), recently celebrating 30 years of near-continual operation, continues in its missions to advance outstanding key physics and operational scenario issues for ITER and the design of future power plants such as DEMO. The main machine heating systems and operational changes are first described. Then follow five sections: plasma scenarios. ITER Base-Line (IBL) discharges, triangularity studies together with X3 heating and N2 seeding. Edge localised mode suppression, with a high radiation region near the X-point is reported with N2 injection with and without divertor baffles in a snowflake configuration. Negative triangularity (NT) discharges attained record, albeit transient, β N ∼ 3 with lower turbulence, higher low-Z impurity transport, vertical stability and density limits and core transport better than the IBL. Positive triangularity L-Mode linear and saturated ohmic confinement confinement saturation, often-correlated with intrinsic toroidal rotation reversals, was probed for D, H and He working gases. H-mode confinement and pedestal studies were extended to low collisionality with electron cyclotron heating obtaining steady state electron iternal transport barrier with neutral beam heating (NBH), and NBH driven H-mode configurations with off-axis co-electron cyclotron current drive. Fast particle physics. The physics of disruptions, runaway electrons and fast ions (FIs) was developed using near-full current conversion at disruption with recombination thresholds characterised for impurity species (Ne, Ar, Kr). Different flushing gases (D2, H2) and pathways to trigger a benign disruption were explored. The 55 kV NBH II generated a rich Alfvénic spectrum modulating the FI fas ion loss detector signal. NT configurations showed less toroidal Alfvén excitation activity preferentially affecting higher FI pitch angles. Scrape-off layer and edge physics. gas puff imaging systems characterised turbulent plasma ejection for several advanced divertor configurations, including NT. Combined diagnostic array divertor state analysis in detachment conditions was compared to modelling revealing an importance for molecular processes. Divertor physics. Internal gas baffles diversified to include shorter/longer structures on the high and/or low field side to probe compressive efficiency. Divertor studies concentrated upon mitigating target power, facilitating detachment and increasing the radiated power fraction employing alternative divertor geometries, optimised X-point radiator regimes and long-legged configurations. Smaller-than-expected improvements with total flux expansion were better modelled when including parallel flows. Peak outer target heat flux reduction was achieved (>50%) for high flux-expansion geometries, maintaining core performance (H 98 > 1). A reduction in target heat loads and facilitated detachment access at lower core densities is reported. Real-time control. TCV’s real-time control upgrades employed MIMO gas injector control of stable, robust, partial detachment and plasma β feedback control avoiding neoclassical tearing modes with plasma confinement changes. Machine-learning enhancements include trajectory tracking disruption proximity and avoidance as well as a first-of-its-kind reinforcement learning-based controller for the plasma equilibrium trained entirely on a free-boundary simulator. Finally, a short description of TCV’s immediate future plans will be given.

Thermodynamic simulation of stepwise precipitation of NH4VO3 and NaHCO3 from carbonating Na3VO4 solution

Thermodynamic simulation was conducted to design a new process of stepwise precipitating NH4VO3 and NaHCO3 from regulating the CO2 carbonation of Na3VO4 solution. Firstly, a new V(V) speciation model for the aqueous solution containing vanadate and carbonate is established by using the Bromley−Zemaitis activity coefficient model. Subsequently, thermodynamic equilibrium calculations are conducted to clarify the behavior of vanadium, carbon, sodium, and impurity species in atmospheric or high-pressure carbonation. To ensure the purity and recovery of vanadium products, Na3VO4 solution is initially carbonated to the pH of 9.3−9.4, followed by precipitating NH4VO3 by adding (NH4)2CO3. After vanadium precipitation, the solution is deeply carbonated to the final pH of 7.3−7.5 to precipitate NaHCO3, and the remaining solution is recycled to dissolve Na3VO4 crystals. Finally, verification experiments demonstrate that 99.1% of vanadium and 91.4% of sodium in the solution are recovered in the form of NH4VO3 and NaHCO3, respectively.

Neoclassical transport computations in non-isothermal tokamak plasmas

Neoclassical transport in a non-isothermal plasma in which each plasma species has different equilibrium temperature is investigated by solving the drift kinetic equation with a Fokker–Planck (FP) collision operator in a circular tokamak model. Since it is known that a linearized FP operator does not have a self-adjoint property in a non-isothermal plasma, approximate models are developed for comparison to intend to have the self-adjoint property in the non-isothermal case. To achieve this, we set a common temperature that the system should reach after a long time, and the individual temperature of each particle species is expressed by a parameter to measure a shift of the individual temperature from the common one. Then, both the Vlasov part and the collision term of the kinetic equation are expanded around the common temperature, taking the temperature shift parameter up to the first order. It is found that the lowest order collision term of expansion preserves the self-adjointness while the first-order, nonlinear FP term does not. A large difference of the ion heat neoclassical transport is found in comparison between the developed models with and without the self-adjointness and the original FP collision term in the non-isothermal plasma, especially in a strong temperature equilibration regime, showing that a contribution of the collision term without the self-adjointness seems to be significant. Furthermore, when an impurity species is included, the result is complicated where the usual enhancement in the main ion particle transport coefficient, due to the impurity effect, is rather suppressed with the increase in the ion heat transport coefficients by the non-isothermal effects.

TECXY simulations of the power exhaust in the multi-impurity plasma of DTT reactor

Reduction of the heat load to plasma-facing components is a crucial problem for future fusion reactors like Divertor Test Tokamak (DTT). Mitigation of the power load via increased plasma radiation with the use of puffed ions of impurities would be one way to mitigate power in the scrape-off layer. This paper presents a numerical investigation of the impact of seeded impurities on the radiation pattern and the power load to the divertor plates of the high-field DTT reactor in the single null (SN) configuration. The simulations have been done with the use of the TECXY code, which solves multi-species plasma transport equations for multiple impurity species simultaneously and all associated ionization stages in a two-dimensional poloidal geometry. TECXY represents the model of plasma transport in the scrape-off layer region by a classical set of transport equations of multi-species plasma derived by Braginskii. The paper aims to compare the mitigation capabilities of neon and argon impurities seeded in the plasma of the DTT device and to obtain a significant energy flux reduction to the target plate at the smallest possible impurity concentration. Performed investigations showed the effects of neon and argon impurities seeding separately for constant electron density at the separatrix. It has been found that the decrease in electron temperature on the divertor plates up to 3 eV at the outer and the inner divertor plates and the peak power load below 15 MW m−2 at the outer divertor plate can be achieved with argon seeding and much lower impurity concentration than that in the case of neon impurity seeding. Studies have shown the complexity of the effect of neon and argon impurities on the boundary plasma. It was found that the reduction of temperature and the power on both divertor plates was the most effective for the high upstream plasma densities. The results show also that diffusive perpendicular transport strongly affects impurity radiation and thus plasma condition at divertor plates.

Development of double-walled bellows for ITER VUV spectrometer systems

This study describes the design and development of double-walled bellows used in ITER's vacuum ultra-violet (VUV) spectrometer systems. These bellows were designed to accommodate port plug displacement resulting from the thermal expansion of the ITER vacuum vessel and other interface loads. The three VUV spectrometer systems under development—the VUV core survey spectrometer, divertor-VUV spectrometer, and VUV-edge spectrometer—are intended to measure the VUV spectrum to monitor impurity species in the plasma. While the ex-vessel components of ITER VUV spectrometers in the interspace and port cell areas, are fixed to the tokamak building, the closure plate of the port plugs, to which the sight pipes are attached, is expected to move by several centimeters both vertically and radially owing to the vacuum vessel's thermal expansion. As a result, the structural integrity of the system requires a flexible structure that can compensate for displacement caused by thermal expansion. To address this need, two types of double-walled bellows were developed, gimbal bellows for lateral displacement and axial bellows for axial displacement. These were developed in accordance with the ASME SEC.VIII code and EJMA standard to meet ITER's safety and vacuum requirements. A double wall is mandated for vacuum bellows due to their vulnerability in terms of vacuum safety. The structural integrity and functionality of the designed bellows were confirmed through functional tests on the manufactured prototype.

The Role of Surface Oxygen in Eliminating Fluorine Impurities and Relithiation Towards Direct Cathode Recycling

Recycling materials back to supply chain not only mitigates the shortage of critical materials such as the metals of Li, Ni, Co, etc, but also protects global environment. Direct recycling of lithium nickel manganese cobalt oxides (NMC) mainly relies on a relithiation process based on purified spent NMC cathode materials. How to efficiently get rid of chemically bonded surface impurities on spent NMC and obtain a clean surface is critical for relithiation. One of the unavoidable impurities is fluorine, which exists in the form of either carbon fluoride (CFx) or metal fluoride (M-F) and can be eliminated by either thermal treatment or hydrothermal procedure under alkaline environments depending on the state of the F.1,2 Generally the hydrothermal method needs relative high temperature (> 200 °C ) and long reaction time (> 16 hours) due to the slow kinetics. A redox mediator such as H2O2, ethanol or ethylene glycol has been reported to be effective to lower the temperature and pressure for relithiation of recycled NMC111 and NMC622, which was pretreated with N-Methyl-2-pyrrolidone solvent.3 However the mechanism is still not fully understood.This work combines material characterizations and electrochemistry and density functional theory (DFT) calculations to study the role of surface oxygen in eliminating fluorine species on recycled NMC532 cathode material. The recycled NMC532 powders were separated from graphite by froth flotation method and there were significant amount of fluorine impurity species on the surface confirmed by XPS. Initial trial of the addition of small amount of H2O2 additive to LiOH aqueous solution during the hydrothermal process was able to decrease the reaction time significantly to fully eliminate both CFx and M-F species compared to the reaction without H2O2. Further calcination steps with the addition of 5% excessive lithium source using LiOH successfully recovered the recycled NMC532 and the half coin cell using the rejuvenated cathode materials shows a discharge capacity of 154 mAh/g at 0.2 C within 2.7 to 4.2 V voltage range. DFT calculations was conducted on a NMC (012) surface and both lithium hydroxide and sodium hydroxide were selected to understand the role of OH- group and surface oxygen species. The kinetics study was conducted with transition state theory4 and the activation energy barriers were calculated by climbing image nudged elastic band (cNEB) method.5,6 The removal of surface F- takes place with the displacement of F- with OH- (*-(M)F(s) +XOH(s) à XF(s) + *-(M)OH(s), X= Li, Na). F- displacement reactions require more than one basic molecule per F- removal with the formation of either LiF or NaF, which is soluble in water and can be removed by a filtration process. Depending on the surface ‘foreign’ oxygen species, the reaction can either go with a hydroxide pathway or a peroxide pathway, with the latter involving the formation of a peroxide intermediate with a ‘foreign’ oxygen atom leading to much smaller barrier energy compared to that in the hydroxide pathway. On a delithiated surface, the barrier energy for F- removal is predicted to be higher than pristine surfaces as the delithiated surface is less reactive to bind ‘foreign’ oxygen. This is consistent with the experimental results in which the additive of H2O2 can provide ‘foreign’ oxygen atoms and favors peroxide pathway leading to shorter time to remove surface fluorine. Additionally, the reaction with NaOH has a lower barrier than that with LiOH for the removal of fluoride species. Future work will investigate alternative additives as well as fluorine surface-bulk transfer which potentially happen if small amount of fluorine residues exist during calcination step.

Implementation of an impurity diagnostic suite on the Pegasus-III experiment.

A suite of diagnostics used to assess impurity content and dynamics has been updated, upgraded, and installed on the Pegasus-III Experiment. Typical plasma parameters during local helicity injection start-up are τshot ∼ 10 ms, ne ∼ 1 × 1019 m-3, and Te ∼ 50eV. The deployed diagnostics are compatible with this modest temperature and density regime and provide species identification, source localization, and estimation of radiation losses. Impurity species are determined by recording time-evolving, single line-of-sight spectra at 1.25 kfps using a SPRED (Survey, Poor Resolution, Extended Domain) vacuum ultraviolet spectrometer. SPRED is equipped with 450 g/mm grating, giving a spectral resolution of 0.33nm and a spectral range from ∼10 to 110nm, useful to identify light impurity species in this temperature and density range. An absolutely calibrated spectrometer that collects light from the plasma at Rtan = 15.9cm and Δt ≥ 2ms is used as a visible survey spectrometer and for continuum measurements. The radiated power from the plasma is estimated with a photodiode-based diagnostic. Two 16-channel absolute extreme ultraviolet diode arrays are placed behind pinhole apertures, resulting in 32 lines of sight at Z = 0, with a spatial resolution of 2-3cm and a time response of 60kHz. A photometrically calibrated collinear Dα/near infrared filtered photodiode-based system measures the Dα emission and around 1040nm. All these instruments have been designed to suppress electromagnetic interference from megawatt-class switching power supplies.

Visible core spectroscopy at Wendelstein 7-X.

This paper presents an overview of recent hardware extensions and data analysis developments to the Wendelstein 7-X visible core spectroscopy systems. These include upgrades to prepare the in-vessel components for long-pulse operation, nine additional spectrometers, a new line of sight array for passive spectroscopy, and a coherence imaging charge exchange spectroscopy diagnostic. Progress in data analysis includes ion temperatures and densities from multiple impurity species, a statistical comparison with x-ray crystal spectrometer measurements, neutral density measurements from thermal passive Balmer-alpha emission, and a Bayesian analysis of active hydrogen emission, which is able to infer electron density and main ion temperature profiles.

Impurity transport study based on measurement of visible wavelength high-n charge exchange transitions at W7-X

A recently installed high-speed charge exchange diagnostic at the W7-X stellarator has been used to identify several high-n Rydberg emission lines near 500 nm following impurity injections. The wavelengths of observed high-n Rydberg transitions are independent of the impurity species and originate from ions with ionization states ranging from 14+ to 45+ suggesting that this approach can be applied to a variety of heavy impurities. Moreover, little to no passive signal is observed since the high-n energy levels are unlikely to be populated by electron impact excitation. The combination of the newly developed diagnostic and the observation of high-n Rydberg states provides spatially resolved, high-speed measurements of multiple charge states which are analyzed in a Bayesian inference framework to determine both impurity diffusion and convection profiles. Measurements from the 2023 experimental campaign conclusively show high diffusion and an inward pinch in the core, well above predictions by neoclassical theory.

Highly Luminescent TCNQ in Melamine.

Optical properties of molecules change drastically as a result of interactions with surrounding environments as observed in solutions, clusters, and aggregates. Here, we make 7,7,8,8-tetracyanoquinodimethane (TCNQ) highly luminescent by encapsulating it in crystalline melamine. Colored single crystals are synthesized by slow evaporation of aqueous tetrahydrofuran solutions of melamine and TCNQ. Single-crystal X-ray diffraction reveals the lattice structure of pure melamine, meaning that the color is of impurities. Both mass spectrometry and UV-vis spectroscopy combined with density-functional theory calculations elucidate that the impurity species are neutral TCNQ and its oxidation product, dicyano-p-toluoyl cyanide anion (DCTC-), whose concentrations in a melamine crystal can be controlled by adjusting the molar ratio between melamine and TCNQ in the precursor solution. Fluorescence excitation-emission wavelength mappings on the precursor solutions illustrate dominant emissions from DCTC- while the emission from TCNQ is quenched by the resonance energy transfer to DCTC-. On the contrary, TCNQ in crystalline melamine is a bright fluorophore whose emission wavelength centered at 450 nm with internal quantum yields as high as 19% and slow fluorescence lifetimes of about 2 ns. The method of encapsulating molecules into transparent melamine would make many other molecules fluorescent in solids.

Study of impurity C transport and plasma rotation in negative triangularity on the TCV tokamak

Carbon impurity transport is studied in the TCV tokamak using a charge exchange recombination diagnostic. TCVs flexible shaping capabilities were exploited to extend previous impurity transport studies to negative triangularity (δ < 0). A practical way of studying light impurity transport (like C, TCVs main impurity species due to graphite tiled walls) is to investigate the correlations between the impurity ion gradients that, in this study, highlighted significant differences between positive (PT) and negative δ (NT) plasma configurations. δ scans ( −0.6<δ<+0.6 ) were performed in limited configurations, but displayed little correlation between C temperature, rotation and density gradients for positive δ. This stiff response for δ > 0 changes for negative δ, where the evolution of ∇vtor was accompanied by variations of ∇nC over a range of negative δ, showing that transport, in NT, is affected by velocity gradients. Similar δ scans were performed with additional NBH (Neutral Beam Heating), with power steps ranging from 0.25 MW to 1.25 MW, highlighting increased momentum confinement in negative δ. Finally, the evolution of intrinsic plasma toroidal rotation across linear to saturated ohmic confinement regime (LOC/SOC) transitions was explored at δ < 0, expanding previous studies performed in TCV for δ> 0 (Bagnato et al 2023 Nucl. Fusion 63 056006). Toroidal rotation reversal was not observed for δ < 0, despite clear LOC/SOC transitions, confirming that these two phenomena occur concomitantly only in a restricted number of cases and under specific conditions.

A thermodynamic perspective on electrode poisoning in solid oxide fuel cells

A critical challenge to the commercialization of clean and high-efficiency solid oxide fuel cell (SOFC) technology is the insufficient stack lifespan caused by a variety of degradation mechanisms, which are associated with cell components and chemical feedstocks. Cell components related degradation refers to thermal/chemical/electrochemical deterioration of cell materials under operating conditions, whereas the latter regards impurities in feedstocks of oxidant (air) and reductant (fuel). This article provides a thermodynamic perspective on the understanding of the impurities-induced degradation mechanisms in SOFCs. The discussion focuses on using thermodynamic analysis to elucidate poisoning mechanisms in cathodes by impurity species such as Cr, CO2, H2O, and SO2 and in the anode by species such as S (or H2S), SiO2, and P2 (or PH3). The author hopes the presented fundamental insights can provide a theoretical foundation for searching for better technical solutions to address the critical degradation challenges.

Impurities in 4H silicon carbide: Site preference, lattice distortion, solubility, and charge transition levels

4H Silicon carbide (SiC) is widely recognized as one of the most advanced wide bandgap semiconductors used in the production of high-efficiency power electronic devices. Impurities play a crucial role in achieving the desired electrical properties in 4H-SiC, yet the understanding of impurities in this material remains limited. In this study, first-principles formation-energy calculations were employed to establish a comprehensive database of formation-energy diagrams for impurities in 4H-SiC. This database includes valuable information on site preference, lattice distortion, solubility, and charge transition levels (CTLs) of the impurities. The site preference for each impurity is closely related to factors such as the Fermi energy, chemical potential, and the impurity species itself. To assess the lattice distortion caused by each impurity, a comparison was made between the volume changes before and after doping. Moreover, the solubility of each impurity was determined using the detailed balance theory, thereby enabling a direct measure of the maximum impurity concentration achievable in the material. Based on the CTLs, the impurities in 4H-SiC were classified into four categories: n-type impurities, p-type impurities, amphoteric impurities, and non-electroactive impurities. This comprehensive property database for impurities in 4H-SiC provides valuable insights for tailoring the material properties through controlled doping, thereby ultimately leading to enhanced performance of power electronic devices.

Vacuum-Ultraviolet spectrometer system for impurity emission measurement on a Compact Torus Injection System of EAST

Recently, a compact torus injection (CTI) system was developed for central fueling on experimental advanced superconducting tokamak (EAST). As impurity generated by the interaction between CTI plasma and the electrode material can dilute the fuel particle, it is important to measure impurity content in CTI and evaluate the effect of impurity on the plasma before the application of CTI to EAST. A vacuum-ultraviolet (VUV) spectrometer that utilizes a concave holographic grating with 1200 groove mm-1 was developed and installed on EAST-CTI for impurity emission measurement and transport study. The mainly parts of the spectrometer are an entrance slit, a concave holographic grating with 1200 groove mm-1 and a charge coupled device (CCD). The spectrometer is designed to image the spectra of 50–460 nm by turning the grating. Preliminary experimental results were obtained from the recent EAST-CTI campaign. Impurity line spectra from 50 to 460 nm wavelength range were measured and identified for several impurity species, such as iron, copper, chromium, oxygen, carbon and fluorine. For example, spectra in the intervals of 150 nm to 300 nm contained strong Fe lines. Helium spectra are also measured in the form of He I and He II spectral lines as helium is the main component of the plasma. The results show that the VUV spectrometer is capable of measuring impurity radiations on EAST-CTI and will be a useful tool for impurity behavior study.

Divertor enrichment of recycling impurity species (He, N2, Ne, Ar, Kr) in ASDEX Upgrade H-modes

Particle balance calculations are done for the seeding species N2, Ne, Ar, Kr as well as He with the aim of obtaining a realistic description of the divertor and core plasma impurity content. Experimental time traces of main plasma impurity densities are fitted by a single, time-independent parameter v z,ineff . This parameter represents the product of the impurity inward pinch in the pedestal, used for the description of gross fueling here, and the enrichment factor between the sub-divertor gas reservoir and the upstream separatrix. v z,ineff depends strongly on the first ionization energy as well as on the charge Z or mass of the impurity. The prevailing dependence of the enrichment values on the ionization energy suggests the importance of the relative impurity and deuterium ionization lengths in the divertor. Regression analysis of v z,ineff yields an expression for the impurity concentrations in the core and the divertor of ASDEX Upgrade which allows the prediction of the corresponding impurity densities and their divertor enrichment within a factor of 2 with only engineering parameters as input. A simple wall model has been introduced to take into account wall storage and release of impurities, e.g. for conditions of pre-loaded walls due to seeding in previous discharges. Wall effects are observed for all species considered, but wall storage turns out to be more important for N and He compared to Ne, Ar, Kr. Similar enrichment values are obtained for ELMy H-modes and EDA/QCE no-ELM regimes. A factor of approximately 1.4 reduction in enrichment is observed for divertor conditions for pronounced detachment with Ar and N2. The obtained analytical model for the core and sub-divertor impurity densities is well suited for integration into a discharge flight simulator or a real time state observer.

Synthetic dimension-induced pseudo Jahn-Teller effect in one-dimensional confined fermions

We demonstrate the failure of the adiabatic Born-Oppenheimer approximation to describe the ground state of a quantum impurity within an ultracold Fermi gas despite substantial mass differences between the bath and impurity species. Increasing repulsion leads to the appearance of nonadiabatic couplings between the fast bath and slow impurity degrees of freedom, which reduce the parity symmetry of the latter according to the pseudo Jahn-Teller effect. The presence of this mechanism is associated to a conical intersection involving the impurity position and the inverse of the interaction strength, which acts as a synthetic dimension. We elucidate the presence of these effects via a detailed ground-state analysis involving the comparison of fully correlated simulations with effective models. Our study suggests ultracold atomic ensembles as potent emulators of complex molecular phenomena. Published by the American Physical Society 2024

Comparative study of morphological and anatomical features of Bidens L. Species

Introduction. Determining closely related impure species when confirming the authenticity of medicinal plant raw materials (mprms) is a very difficult and controversial task, especially for crushed and powdered mprms. One of such objects is the three-part beggarticks, which is prone to polymorphism and occupies the same area with the nodding beggarticks, the Spanish needles and the devil's beggarticks. Objective: to conduct a comparative pharmacognostic analysis of species of the genus Bidens L., which can be collected together with the pharmacopoeial species – three-part beggarticks. Material and methods. The objects of study were dried herbs of three-part beggarticks (Bidens tripartita L.), nodding beggarticks (Bidens cernua L.), Spanish needles (Bidens bipinnata L.), and devil's beggarticks (Bidens frondosa L.), collected in the Moscow region in 2020–2021. In this work, we studied the anatomical and morphological structure of leaves, as the most preferred part from a diagnostic point of view. The study of anatomical and morphological characteristics was carried out in accordance with the General Pharmacopoeial Monograph of the Russian Federation State Pharmacopoeia XIV 1.5.1.0003.15 “Leaves”. Results. A comparative description of the anatomical-diagnostic external and microscopic characteristics of the Bidens leaves has been obtained. Bidens tripartita and Bidens frondosa are higher than others. In contrast to the impure species, the Bidens tripartita has the largest deeply trifid, irregular serrated leaves, more pronounced tortuosity of the epidermal cells of the upper side of the leaf, multicellular thin-walled caterpillar-like and simple thick-walled hairs are combined, and the secretory canals in the mesophyll located along the vein. Conclusion. Among the studied Bidens species, each has characteristic features of its morphological and anatomical structure. These specific features can be used to establish the authenticity of the mprms and distinguish it from impure Bidens species, which require further research.

Neoclassical transport of tungsten ion bundles in total-f neoclassical gyrokinetic simulations of a whole-volume JET-like plasma

The application of a bundling technique to model the diverse charge states of tungsten impurity species in total-f gyrokinetic simulations is demonstrated. The gyrokinetic bundling method strategically groups tungsten ions of similar charge, optimizing computational efficiency. The initial radial configuration of these bundles and their respective charges are derived from a coronal approximation and the quasi-neutrality of the plasma. A low-density JET H-mode like plasma is simulated using the neoclassical version of XGC across the entire plasma volume, spanning from the magnetic axis to the divertor. An accumulation of tungsten is observed at the pedestal top, as a result of low-Z tungsten ions moving inward from the scrape-off-layer into the core region and high-Z tungsten ions moving outward from the core into the pedestal. This organization of the fluxes cannot be captured by a single tungsten-ion simulation. Large up-down poloidal asymmetries of tungsten form in the pedestal and strongly influence the direction of neoclassical fluxes. The temperature screening effect and its correlation with asymmetries are analyzed.

Benign termination of runaway electron beams on ASDEX Upgrade and TCV

This paper discusses the development of a benign termination scenario for runaway electron (RE) beams on ASDEX Upgrade and TCV. A systematic study revealed that a low electron density (n e) companion plasma was required to achieve a large MHD instability, which expelled the confined REs over a large wetted area and allowed for the conversion of magnetic energy to radiation. Control of the companion plasma ne was achieved via neutral pressure regulation and was agnostic to material injection method. The neutral pressure required for recombination was found to be dependent on impurity species, quantity and RE current. On TCV, n e increased at neutral pressures above 1 Pa, indicating that higher collisionality between the REs and neutrals may lead to an upper pressure limit. The conversion of magnetic energy to radiated energy was measured on both machines and a decrease in efficiency was observed at high neutral pressure on TCV. The benign termination technique was able to prevent any significant increase in maximum heat flux on AUG from 200 to 600 kA of RE current, highlighting the ability of this approach to handle fully formed RE beams.

Density functional theory study of Al, Ga and in impurities in diamond

As a consequence of its high atomic number density, diamond can incorporate a relatively limited range of impurities as distributed point-defects, chiefly N, B and H. A few other species can be grown-in, and other impurity species incorporated via implantation and annealing. For applications including electronic, electrical and quantum devices, the presence of states deep within the wide band-gap is of importance, and the list of potential colour centres available for exploitation continues to grow. Although B can be grown into diamond at high concentration, study of other group-13 elements is rather limited. In this paper we present the results of modelling of Al, Ga and In. We find all species readily form complexes with vacancies, and exhibit electronic structures that parallel those of the SiV complex. We report electronic structures, electrical levels, optical transitions and hyperfine interactions of the colour centres, as well as reflect upon the thermodynamics of the complexes. We suggest that co-implanting group-13 elements with nitrogen would give rise to the defect charge states with potential for quantum applications.