Chemical Erosion Of Carbon Research Articles

Chemical erosion of carbon at ITER relevant plasma fluxes: Results from the linear plasma generator Pilot-PSI

The chemical erosion of carbon was investigated in the linear plasma device Pilot-PSI for ITER divertor relevant hydrogen plasma flux densities 1023<Γ<1025m−2s−1. The erosion was analyzed in situ by optical emission spectroscopy and post mortem by surface profilometry. The experiments indicate a threshold for the absolute carbon erosion rate as a function of plasma temperature Te around 0.7eV, a peak of the surface temperature around 550°C, and no dependence on plasma flux density. The latter implies a flux dependence of the chemical erosion yield as Γ−1. The value of the chemical erosion yield at the surface temperature of maximum erosion and Γ=1×1024m−2s−1 was 0.9% for Te<0.5eV as determined by surface profilometry.

Suppression of carbon dust agglomeration and volatile molecular formation in C–H–N reactive molecular system

Chemical erosion of carbon materials and dust formation in low-temperature and neutral particle-dominated plasmas were investigated using high-pressure inductively coupled plasmas. Experiments were performed with Ar/H2/N2 mixture plasma irradiation to graphite targets. The addition of just a few percent of nitrogen gas to hydrogen led to significant suppression of carbon dust formation on the graphite target. From optical emission spectroscopy, CN band spectra were observed strongly in Ar/H2/N2 plasmas with a decrease of CH and C2 band emission intensity. These results showed that CN bond formation, which caused chemical erosion of carbon by producing volatile CN, HCN, and C2N2 particles, might have been a key suppression mechanism of the carbon particle aggregation.

Molecular dynamics simulation of energy exchange during hydrogen collision with graphite sheet containing a vacancy

Molecular dynamics simulation is applied to investigation of energy exchanges during hydrogen collision with graphite sheet containing a vacancy. The effects of the monovancancy defect on the energy exchanges are discussed in detail. This paper analyzes the energy loss of the incident hydrogen atom, the energy range for the adsorption process, and the energy transfer process for target atom, in the course of a hydrogen atom bombarding the carbon atom at the edge of monovacancy defect in the graphite sheet. The simulation results show that the adsorption process proceeds more easily when the graphite sheet contains a vacancy than when the graphite sheet has perfect crystalline structure. In certain areas of the graphite sheet, adsorption of an incident hydrogen atom can occur in two energy ranges. The sp2 structure as well as overhang configuration occurs when a hydrogen atom is adsorbed. This adsorption process does not reduce the C—C bond energy. It is found that the carbon atom at the edge of monovacancy defect can adsorb an incident hydrogen atom more easily but can not diffuse the gained energy as efficiently as in a perfect graphite sheet. These results are helpful for understanding the chemical erosion of carbon based materials and the ensuing tritium retention in fusion devices.

Mitigation of carbon erosion in beryllium seeded deuterium plasma under bombardment by argon and helium ions in PISCES-B

The influence of argon and helium impurities on chemical erosion of carbon by deuterium and on the mitigation of erosion by the beryllium seeding has been studied in the PISCES-B linear plasma device. Optical spectroscopy has been used to quantify the amounts of Be, Ar and He in the deuterium plasma and to monitor the reduction of the CD band as a measure of carbon chemical erosion of the fine-grain graphite target. The addition of Ar and He has not resulted in an increase of the carbon erosion rate. Moreover, the characteristic time for the Be layer formation was in agreement with the reference exposures. It can be concluded, that for the covered range of experimental parameters the addition of Ar and He to plasma does not affect the formation of the protective Be carbide layer and the associated mitigation of carbon erosion.

Simulation of Be–C interaction dynamics in mixed Be/C layers formed in experiments at PISCES-B

The mitigation of carbon chemical erosion observed in PISCES-B due to the presence of a minor beryllium concentration in plasma can be determined to a large extent by formation of carbide (Be2C) on the plasma-facing surface. To study this, dedicated beryllium seeding experiments in the linear plasma simulator PISCES-B have been simulated using the Monte-Carlo code ERO. The spectroscopy is used to benchmark the simulations against observations. A temporal dependent evolution of the beryllium carbide formation on the carbon surface of the target is introduced on the basis of characteristic time scales. The influence of carbide formation on the effective mitigation time for chemical erosion is investigated.

The breakup of methane under ITER divertor hydrogen plasma conditions for carbon chemical erosion analysis with CH spectroscopy

Methane (CH4) was injected into the high density (ne ∼ 1020 m−3), low temperature (Te ∼ 1 eV) hydrogen plasma in Pilot-PSI to determine the CH A–X photon efficiency in this unexplored plasma regime. The effects of particle transport and particle reflection on the emission of directly excited CH under these plasma conditions were assessed with the 3D Monte Carlo code ERO. The simulations of the inverse photon efficiency showed a difference of ∼20% between full hydrocarbon sticking or no sticking (reflection). In addition it predicts that particle transport may lead to more than a factor of 10 increase. The measured inverse photon efficiency is however constant at 100 ± 30 for 0.1 < Te < 1.0 eV. The constancy is consistent with dissociative recombination of , and to produce excited CH instead of direct excitation. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.

Modeling of Surface Composition Dynamics in the ITER Divertor Region

The formation of mixed materials in the ITER divertor region is a critical materials issue that is expected to have a profound effect on the operation of the machine. This is particularly true for the divertor, the component where most of the energy is deposited during ITER operation. The divertor is made up of three tungsten domes and two vertical plates (inboard and outboard) which will be manufactured with composite carbon fiber according to the latest design. In the case of the first wall, the plasma-exposed surface is made out of beryllium; some of this beryllium is eroded from the first wall and finds its way into the divertor region, where it may strike the carbon vertical plates and get implanted or deposited. Having beryllium in the divertor enhances tritium trapping via codeposition, hence reducing the net available fuel for the reactor; on the other hand, there is evidence of carbon chemical erosion suppression. In this paper, the dynamic composition of a carbon surface under simultaneous bombardment by deuterium, tritium, and beryllium ions is studied. Both carbon and beryllium can redeposit on the target once eroded, so the source of ions receives feedback from the eroded material. Simulation results show that both the partial sputtering of Be and C atoms as well as the bombarding energy play an important role in Be accumulation on the vertical target plate, which can reach up to 50% near the surface. The results also show that the contribution of redeposited species may become so large that the DT bombarding flux may drop to 70% of the incident species.

CH spectroscopy for carbon chemical erosion analysis in high density low temperature hydrogen plasma

The CH A−X molecular band is measured upon seeding the hydrogen plasma in the linear plasma generator Pilot-PSI [electron temperature Te=0.1–2.5 eV and electron density ne=(0.5–5)×1020 m−3] with methane. Calculated inverse photon efficiencies for these conditions range from 3 up to &amp;gt;106 due to a steeply decreasing electron excitation cross section. The experiments contradict the calculations and show a constant effective inverse photon efficiency of ∼100 for Te&amp;lt;1 eV. The discrepancy is explained as the CH A level is populated through dissociative recombination of the molecular ions formed by charge exchange. Collisional de-excitation is observed for ne&amp;gt;5×1020 m−3 and 0.1 eV&amp;lt;Te&amp;lt;1 eV. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.

Investigation of detached recombining deuterium plasma and carbon chemical erosion in the toroidal divertor simulator NAGDIS-T

Detached deuterium recombining plasma has been generated in the toroidal divertor simulator. The electron temperature (0.1–0.4 eV) and density (∼10 18 m −3) in the detached plasmas were evaluated with a spectroscopic method using a series of deuterium Balmer line emission from highly excited levels and the Stark broadening of D(2–12). We have investigated the role of volume plasma recombination through Electron–Ion Recombination (EIR) and Molecular Activated Recombination (MAR) processes. Moreover, the carbon erosion in the detached deuterium plasma has been studied with a weight loss method. It is found that deuterium neutrals generated by EIR process could have strong influence on the carbon chemical erosion.

Modelling of chemical erosion mitigation experiments at PISCES-B using the 3D Monte-Carlo code ERO

Mitigation of carbon chemical erosion by seeding of Be in the plasma is observed at the divertor simulator PISCES-B. The 3D Monte-Carlo ERO code is applied to model this effect. The time evolution of the process is modelled based on different surface models. The influence of various effects such as the plasma parameters, target biasing and carbide formation on the Be transport and the mitigation of carbon chemical erosion is investigated.

Hydrocarbon injection for quantification of chemical erosion yields in tokamaks

Chemical erosion of carbon can be described as function of the incident ion flux, the ion energy and the surface temperature. Spectroscopy on the hydrocarbon break-up products CH and C2 is applied to determine in-situ hydrocarbon fluxes and quantify the chemical erosion by means of hydrocarbon injection. Present-day knowledge on critical issues concerning the erosion yield and its determination is presented: Effective inverse photon efficiencies are measured in TEXTOR for different hydrocarbon species and compared with calculations from HYDKIN. The underlying database for the methane break-up, used in different erosion/deposition models, has been put to test. The chemical erosion yield related to higher hydrocarbons is determined to be lower in the JET outer divertor than measured in previous experiments. In ASDEX Upgrade and DIII-D first in-situ calibrations of hydrocarbon fluxes in the detached outer divertor are performed.

Plasma-Surface Interaction Studies on DIII-D and Their Implications for Next-Step Fusion Experiments

Unique diagnostic and access features of the DIII-D tokamak, including a sample exposure system, have been used to carry out controlled and well-diagnosed plasma-surface interactions (PSI) experiments. An important contribution of the experiments has been the ability to link a given plasma exposure condition to a measured response of the plasma-facing surface and to thus understand the interaction. This has allowed for benchmarking certain aspects of erosion models, particularly near-surface particle transport. DIII-D has empirically quantified some of the PSI effects that will limit the operation availability and lifetime of future fusion devices, namely, net erosion limiting divertor plate lifetime and hydrogenic fuel retention in deposit layers. Cold divertor plasmas obtained with detachment can suppress net carbon divertor erosion, but many low-temperature divertor PSI phenomena remain poorly understood: nondivertor erosion sources, long-range particle transport, global erosion/deposition patterns, the enhancement of carbon erosion with neon impurity seeding, the sputtered carbon velocity distribution, and the apparent suppression of carbon chemical erosion in detachment. Long-term particle and energy fluences have reduced the chemical erosion yield of lower-divertor tiles. Plasma-caused modification of a material’s erosion properties, including material mixing, will occur quickly and be important in long-pulse fusion devices, making prediction of PSI difficult in future devices.

Influence of beryllium plasma impurities on the erosion of graphite

This paper is concerned with the importance of mixed-material effects that could mitigate the problems associated with chemical erosion of carbon in the next step fusion device ITER. It presents experiments on the influence of beryllium (Be) impurities in deuterium (D) plasmas on the erosion properties of graphite. Graphite (C) targets were exposed to a Be seeded steady state D plasma in the PISCES-B divertor simulator. The Be plasma concentrations and the changes in chemical and physical erosion were measured using a visible light spectroscopy system. During all experiments a bias voltage of −50 V was applied to the C target, the target temperatures were in the range from 500 to 1280 K and the Be plasma concentration was in the range from 0.02% to 0.2%. It was found that even at these low Be concentrations, the C targets are covered with a shielding Be layer that eliminates both chemical and physical erosion of the underlying C target. The formation of this shielding Be layer also results in diminishing C and large Be concentrations in the material co-deposited on a collector probe that was shielded from direct plasma impact.

A Detailed Study of Carbon Chemical Erosion in L-Mode Plasmas in the DIII-D Divertor

A series of quiescent L-mode discharges have been used with varying degrees of divertor attachment to study carbon erosion in the DIII-D tokamak divertor. Spectra of atomic and molecular carbon plasma emissions are measured across the divertor. Predictions of carbon emission resulting from chemical and physical sputtering at the outer strikepoint are provided by the WBC transport code. For attached ionizing plasmas (Te ~ 20 eV and carbon surface T ~ 350 K) the CD and C2 carbon radical emissions are consistent with a total chemical sputtering yield, Ychem ~ 0.3%. During sweeps of the inner divertor leg, CD and C2 emissions indicate the main-wall tiles has a six times higher Ychem than the divertor tiles despite identical incident plasma conditions. Emission of atomic (CI) and singly ionized carbon are dominated by physical sputtering with a measured yield Yphys ~ 2% as expected from laboratory data, indicating that chemical sputtering plays a minor role as a divertor carbon source. The Doppler broadening of the CI emission agrees with the physical sputtering model, but the predicted Doppler shift is a factor three larger than the experiment. CD and C2 emission are reduced below detection during divertor detachment (Te ~ 1–2 eV), an indication of the suppression of chemical erosion with Ychem < 10-4 based on expected photon intensity from WBC.

Chapter 10: Plasma-Wall Interaction and First-Wall Materials in ASDEX Upgrade

Experiments dealing with plasma-wall interactions and first-wall materials comprise a significant part of the work program of ASDEX Upgrade. To elucidate carbon chemical erosion under reactor-relevant conditions, dedicated spectroscopic measurements were performed. These investigations are complemented with long-term erosion and deposition probes consisting of various materials, which are mounted at numerous locations inside the vacuum vessel. The codeposition of hydrogen with carbon below the divertor is studied in detail with long-term samples as well as with quartz microbalance measurements, which allow a discharge-resolved measurement of the layer growth. In parallel to the investigations on carbon, the behavior of tungsten plasma facing components (PFCs) and their influence on plasma performance is studied. In several experimental campaigns, the divertor as well as large parts of the PFCs in the main chamber were equipped with tungsten-coated tiles. Surface conditioning by applying a silicon layer (siliconization) was performed as a preexperiment of the tungsten program, and the results are compared to those of surface conditioning with boron (boronization).

Reduction of divertor carbon sources in DIII-D

The evolution of carbon release from the DIII-D lower divertor tiles is studied using atomic and molecular spectroscopy. Newly installed virgin graphite tiles in 1992 are found to have had a chemical erosion yield, Y chem⩽ 3–5%, consistent with both laboratory results and similar experiments in other totamaks. The average Y chem measured in the DIII-D lower divertor decreased approximately a factor of ten between 1992 and 2000. The presumed cause of this reduction is the cumulative effect of >30 wall-conditioning boronizations and 10 5 s of plasma exposure, although the relative importance of these two mechanisms is unknown. This result indicates that a substantial reduction in carbon chemical erosion, and its relative importance as a source of carbon, can be obtained by long-term in situ wall conditioning techniques. The total carbon source sputtered into the DIII-D lower divertor has also apparently decreased over the same period. However, there has been no significant decrease in the average core carbon contamination with the decreasing lower divertor carbon source.

Investigation of carbon chemical erosion with increasing plasma flux and density

The possible reduction of the chemical erosion yield of carbon Ychem withincreasing hydrogenic ion flux is a critical issue for the use ofcarbon based plasma facing materials in future high flux confinementdevices. The PISCES linear plasma device is used to assess carbonYchem through nearly an order of magnitude scan in flux (⩽ 1023 s-1 m-2) and plasma density, while controlling other exposureparameters that are known to affect Ychem; namely, incident ion energyand surface temperature. Two independent techniques are used tomeasure Ychem, CD band emission spectroscopy and sample massloss. Both techniques are carefully benchmarked using a combination ofmethane injection experiments and modelling. The key results of thisstudy are that the CD molecular band photon efficiency is sensitive tothe plasma density at fixed electron temperature and that theredeposition efficiency of hydrocarbons increases with density,thereby decreasing the measured net erosion. After taking these trendsinto account, no measurable flux dependence of Ychem is found fromeither of the two measurement techniques, with Ychem ∼ 3-5% for anincident deuterium ion energy of 30 eV. Independent scans of incidentenergy and surface temperature confirm previously measured (by ionbeams with relatively low fluxes) Ychem dependences on theseparameters. These results together suggest that apparent reductions ofYchem in plasma devices with increasing flux are more likely to beattributed to either increasing redeposition efficiency with density,changing dissociation/excitation/transport properties of thehydrocarbons near the surface, decreasing incident energy or somecombination thereof.

Chemical Erosion of Carbon in the Divertor of ASDEX Upgrade

Chemical erosion of carbon via hydrocarbon formation is investigated by spectroscopic particle flux measurements and with Langmuir probes in the outer Divertor II of the ASDEX Upgrade tokamak. The measurements are made about 10 cm above the strike point of the separatrix at the vertical target plate for hydrogen ion flux densities up to 1023 m-2 s-1 and up to reactor-relevant values of the power per major plasma radius, P/R ≤ 12 MW/m. The erosion yields obtained exhibit a pronounced hydrogen/deuterium flux dependence and an isotope effect, resulting in lower yields for higher fluxes and a factor of 2 higher yields for deuterium in comparison to hydrogen. Surface temperatures are relatively low and do not exceed 100°C at the position of the measurements documenting the advantageous power spreading features of carbon in combination with the Divertor II vertical target geometry. The interpretation of the measured CH molecular line emission in terms of underlying hydrocarbon molecular fluxes from the wall is hampered by the uncertainties in the atomic data and by the formation of heavier hydrocarbons. The simple model used for the interpretation of the CH band intensity in terms of flux is supported by the behaviour of carbon ion (C+, C2+) radiation. Owing to the high electron densities, carbon is found to be a very effective coolant for the divertor plasma.

The amount and distribution of deuterium retained in the jet divertor after the C and Be phases in 1994-1995

Abstract The amounts of D on the graphite and beryllium divertor tiles used in JET campaigns during 1994–1995 have been measured and compared. Deposits primarily of carbon containing very high D concentrations were present in the cold shadowed areason both sets of tiles. Plasma-surface interactions involving chemical erosion of carbon and D diffusion into Be are evident.

Sputtering of boron-doped graphite USB15—Investigation of the origin of low chemical erosion

The changes in surface composition of USB15—a boron doped graphite containing 15 wt.% of boron—during bombardment with D ions were determined by in situ Auger electron spectroscopy at temperatures from 300 up to 1000 K. For energies above 100 eV no strong increase of the boron surface concentration could be observed even around 800 K, i.e., at the maximum for chemical erosion of pure graphite. Chemical factor analysis of the carbon Auger peak in this energy-regime results in a much larger carbidic fraction of carbon atoms than suspected from the boron content of 15%. Thus, boron influences much more carbon atoms in their chemical reactivity with deuterium ions than is expected for the stoichiometric B4C precipitates. For ion energies below 100 eV a strong increase of boron surface concentration with decreasing ion energy at room temperature was observed. The chemical erosion of carbon in this energy regime is not suppressed by boron doping and indicates a different, surface related release process of hydrocarbon molecules.