Radiation-enhanced Sublimation Research Articles

Radiation-enhanced sublimation of doped graphites

The radiation-enhanced sublimation (RES) of graphite, doped with boron, silicon and titanium was measured during 1 keV D + irradiation by line-of-sight quadrupole mass spectroscopy. The RES yield depends on both the dopant element and the dopant concentration as well as on the graphite microstructure. Generally, with a few exceptions for small concentrations, RES decreases with increasing dopant concentration. The most significant reduction in RES is observed for specimens with 13.8% boron and specimens with 5.0% titanium surface concentrations (< 1000 nm). The RES yields at 1800 K for these specimens are reduced by factors of 2 and 4, respectively, compared to the yield for undoped graphite produced by the same fabrication process as the doped specimens, and by factors of 3 and 5, respectively, compared to the yield for pyrolytic graphite.

Flux dependence of the radiation-enhanced sublimation of graphite due to D + irradiation

The erosion of pyrolytic graphite due to radiation-enhanced sublimation (RES) has been measured as a function of deuterium ion flux, using line-of-sight quadrupole mass spectrometry. Although the measured flux dependence of approximately YRESα φ D −0.1 is in good agreement with previous experimental results, the flux-dependence function of the RES yield was also found to be weakly dependent on temperature - the higher temperature curve being closer to a linear relationship. Assuming an Arrhenius relationship for the RES yield, the derived “effective activation energy” was found to increase slightly with increasing D + flux density.

Plasma wall interaction and energy fluxes in RFX

The graphite first wall of the Reversed Field Pinch (RFP) experiment RFX has been inspected after one year of operation. Arc tracks and thermal erosion have been observed in particular around the two poloidal gaps of the shell. The ionization lengths calculated from the experimental edge profiles of electron temperature and density show that most of the carbon atoms released by thermal erosion, arcing and Radiation Enhanced Sublimation (RES) are locally redeposited, whereas physical sputtering, including self sputtering, results as the most important source of carbon impurity.

Behaviour of boron- and titanium-doped graphite limiters under high heat loads in TEXTOR

Boron-doped graphite (3.5% B in C (S2508, Carbon Lorraine), 20% B in C (GB120, Toyo Tanso)) and titanium-doped graphite (2.1% Ti in C (RGTi, supplied by Efremov Institute, St. Petersburg)) have been tested in TEXTOR as a single main limiter for typically 20–40 discharges. Convective power loads up to about 30 MW/m 2 led to maximum surface temperatures between 2400–2800°C. Severe cracking has occurred on the 20% boron-doped graphite material showing that it is not applicable as plasma facing material under high heat loads. Some large cracks have also been observed on the RGTi limiter, probably caused by the strong anisotropy of the material. The 3.5% boron-doped material shows similar thermomechanical properties as a usual fine-grain graphite (EK98). Large thermal sublimation of boron and titanium has been observed starting at surface temperatures of about 2000°C and 1800°C, respectively. Very thick layers of redeposited boron or titanium (> 1 μm) were formed near the areas of high sublimation. Besides the doping species (boron, titanium) these materials sublime various (metallic) impurities at elevated temperatures. No significant increase of the emitted carbon flux has been observed below surface temperatures of 2000°C showing that radiation enhanced sublimation (RES) is significantly reduced under these conditions compared with expected values extrapolated from beam experiments.

Sputtering by hydrogen ion bombardment and sublimation of boron- and vanadium-doped graphite

For boron- and vanadium-doped graphites, the yields of radiation — enhanced sublimation (RES) and chemical sputtering by hydrogen ion bombardment were measured by means of a weight loss method, by using an electron cyclotron resonance (ECR) hydrogen ion irradiation apparatus. In addition, the thermal sublimation rates of these materials were measured by using the same apparatus. The reduction of the RES yield due to the doping of boron or vanadium was not clearly observed. The chemical sputtering yields for boron- and vanadium-doped graphites, were reduced by the doping, e.g. 5 times and 20% smaller, compared with that for graphite, respectively. The sublimations of the boron and the vanadium were significantly large when the temperature exceeded approximately 1200°C. The weight loss rate of the doped material to the initial content of the doped material, for example, was approximately 5%/day at 1500°C for the boron- or the vanadium-doped graphite.

Growth of carbon impurity density and adequate divertor wall temperature for fusion experimental reactors

Sudden increase of carbon impurity called carbon bloom has terminated the energy breakeven condition in the present large tokamak. In order to lengthen the burning plasma state in the next device, carbon bloom has to be well suppressed. The temporal evolution of carbon impurity density is analytically examined by using a simple one-point kinetic or zero-dimensional model including the effects of graphite erosions due to oxygen and ion, and gettering for oxygen due to boron or beryllium. The growth of carbon bloom due to radiation-enhanced sublimation is discussed based on the effective self-sputtering of carbon. Even when the self-sputtering yield is less than unity, carbon density is observed to continuously increase with the discharge time if the oxygen gettering action is not perfectly conducted. From the present analysis and data on the erosion of carbon materials, and the evaporation of gettering materials, it is suggested that the divertor wall temperature has to be kept less than approximately 900–1000°C to avoid the continuous growth of the carbon density.

Dynamics of B sublimation from boron doped graphite USB15

The diffusion and release of boron in USB15 at temperatures above 1700 K has been investigated. The boron depth distribution has been measured using the 11B(p, α 0) 8Be nuclear reaction after heating the sample for different times and at different temperatures. After prolonged heating a boron depleted carbon layer develops due to outdiffusion of boron in solid solution and the dissolution of the B 4C precipitates. Additionally, the high temperature erosion of USB15 under 1 keV D + bombardment at a flux of 2 × 10 15 D +/cm 2 s has been studied. The radiation enhanced sublimation (RES) is limited by the boron concentration at the surface. A model which relates the reduction of RES to the surface concentration of boron predicts a stronger suppression of RES for larger ion fluxes.

Temperature-dependent sputtering of doped graphites and boron carbide

Abstract The temperature dependence of the sputtering behaviour of various low-Z materials is presented. The hydrocarbon production under deuterium bombardment is investigated as a function of the target temperature. Carbon doped with beryllium or boron, as well as boron carbide show a strong reduction in the chemical sputtering yield. The reduction of the radiation-enhanced sublimation yield (RES) above 1200 K is small and not uniquely correlated with the dopant concentration. A variety of doped graphites is compared for 1 keV D+ sputtering at 300, 800 and 1470 K. The resulting erosion yields are discussed in terms of dopant concentration and the microstructure of the materials.

ボロン混合黒鉛のプラズマ対向材料としての適合性

Boron-doped graphite and a-C/B : H film are regarded as attractive low Z plasma facing materials, since radiation loss due to oxygen and carbon impurities is largely reduced. It is also suggested that the radiation enhanced sublimation (RES) might be reduced by the boron doping. However, suitable temperature range for this material has not yet been clarified. From the oxidation experiments of the boron-doped graphite, it is observed that B2O3 formed by oxygen gettering evaporates at the temperature over 900°C. From the measurement of RES, it is also shown that the boron evaporation becomes dominant when the temperature exceeds approximately 1000°C. In addition, the reduction of the RES yield is not clearly observed in our experiment. Thus it is proposed that the suitable temperature range of boron-doped graphite should be less than about 900°C. The boron-doped graphite can be used only for walls of relatively low temperature, i.e., the first wall region. To extend this material to the divertor region of the fusion experimental reactor such as ITER, the heat flux must be greatly reduced by expanding the divertor configuration.

Chemical sputtering and radiation enhanced sublimation of graphite due to hydrogen ions

The chemical sputtering yields of metal-deposited graphites were measured by using an ECR ion irradiation apparatus, and compared with that of graphite with a clean surface. The yield of the graphite was significantly reduced by the deposition of metal such as Fe, Ti or Cr onto the graphite, even if the deposition layer was very thin. In particular, the reduction of the yield was largest in the case of a Ti-deposited graphite. From surface analysis such as AES, XPS and SEM, it was found that this reduction was due to the formation of a uniform carbide layer at the surface. It was also observed that the hydrogen retention of the metal-deposited graphite measured by a thermal desorption technique was considerably lower than that of graphite with a clean surface. The yields of the radiation-enhanced sublimation for the Isotropic graphites with different pore sizes were also measured based on weight loss. The yield was very similar to that of pyrolytic carbon. It was observed that the radiation-enhanced sublimation had no dependency with the graphite species, similarly for the chemical sputtering.

Enhanced carbon influx into TFTR supershots

Under some conditions, a very large influx of carbon into TFTR occurs during neutral beam injection into low recycling plasmas (the supershot regime). These carbon ‘blooms’ result in serious degradation of plasma parameters. The sources of this carbon have been identified as hot spots on the TFTR bumper limiter at or near the last closed flux surface. Two separate temperature thresholds have been identified. One threshold, at about 1650°C, is consistent with radiation enhanced sublimation (RES). The other, at about 2300°C, appears to be thermal sublimation of carbon from the limiter. The carbon influx can be quantitatively accounted for by taking laboratory values for RES rates, making reasonable assumptions about the extent of the blooming area and assuming unity carbon recycling at the limiter. Such high carbon recycling is expected, and it is shown that, in target plasmas at least, it is observed on TFTR. The sources of the carbon blooms are sites which have either loosely attached fragments of limiter material (caused by damage) or surfaces that are nearly perpendicular to the magnetic field lines. Such surfaces may have local power depositions two orders of magnitude higher than usual. The TFTR team modified the limiter during the opening of winter 1989–1990. The modifications greatly reduced the number and magnitude of the blooms, so that they are no longer a problem.

Interactions of Bulk-Boronized Graphites with Deuterium Plasmas in the Pisces-B Facility

Newly developed bulk-boronized graphites and boronized carbon-carbon composites, with a total boron concentration ranging from 3 to 30 wt%, have been bombarded with steady-state deuterium plasmas at temperatures between 200 and 1600°C in the PISCES-B facility. The erosion yield of bulk-boronized graphite is smaller than that of pyrolytic graphite by a factor of 2 to 3 in regimes of chemical sputtering, physical sputtering, and radiation-enhanced sublimation (RES). Plasma bombardment at elevated temperatures does not noticeably alter the near-surface composition of bulk-boronized graphite. A chemical pinning effect of boron on the migration of interstitial carbon atoms is the key to the reduction of erosion due to RES. Post-bombardment thermal desorption spectroscopy indicates that bulk boronization enhances recombinative desorption of deuterium. The enhanced deuterium desorption is responsible for the suppressed chemical sputtering. Deuterium retention in bulk-boronized graphite at temperatures from room temperature to 800°C has been measured, and it is maximized at temperatures around 300°C. The maximized deuterium retention increases by a factor of 2 as the boron concentration changes from 0 to 90%.

Radiation enhanced sublimation of carbon and carbon related materials

Above 1200 K ion bombardment of carbon materials results in an anomalous emission of carbon atoms. The present experimental basis of this effect, called Radiation Enhanced Sublimation (RES), is described. The current model attributes the RES of carbon materials to the diffusion of radiation produced interstitials to the graphite surface from where they evaporate easily. The basic features of this model are presented and compared with experimental observations. The present knowledge on the influence of doping carbon materials with metals on the RES emission of carbon atoms is shown.

Radiation-Enhanced Sublimation of Graphite Materials due to Hydrogen Ions

For isotropic graphite, C/C composite and pyrolytic graphite, yields of Radiation-Enhanced Sublimation (RES) due to hydrogen ion bombardment were measured in the temperature range up to 2000K by weight loss method. Graphite samples were irradiated by H3+ ions with the energy of 4.5keV and fluences from 1019H/cm2 to 1020H/cm2. The RES yields measured showed no clear differences among these three types of isotropic graphite and the pyrolytic carbon. Namely, the RES did not depend on physical properties and/or structure of graphite. The activation energy corresponding to the RES was approximately 0.8eV for the isotropic graphites. The angular dependences of sputtering at various temperature were also examined. The result showed that the physical sputtering yield increased with ion-incident angle, but the chemical sputtering and RES yields were approximately independent of the incident angle.Raman spectra of the samples as-received and irradiated in RES regime were examined. While R-number (=I1360/I1585) before irradiation varied with the kind of graphite samples, the value became almost the same for every graphite sample irradiated at elevated temperature. In the RES regime, the sample irradiated at higher temperature showed better crystallinity than that at lower temperature. The change of surface morphology due to irradiation was also examined by Scanning Electron Microscopy.

Process of Radiation-Enhanced Sublimation of Graphite due to Hydrogen Ions

For radiation-enhanced sublimation (RES) of graphite, a simple model is proposed. The model shows that the produced interstitials which escape the recombination with vacancies can reach the surface and evaporate. Only a small fraction of interstitials contribute to the RES yield. The dependences on energy, mass and incident angle of ion are explained based on this mode. The changes of surface morphology and structure due to the RES are investigated. It is found that the interstitials and vacancies are completely mixed in the surface and thus the structure change becomes the same for every graphite material. From this result, it is suggested that the RES yield becomes the same even if the graphite texture is different. In order to suppress the RES, thus it is important to reduce the heat flux to the graphite wall in a fusion reactor.

Plasma-surface interactions of graphite as nuclear fusion material

As nuclear fusion devices are being scaled to larger size with substantially increased plasma stored energy, the engineering tasks associated with plasma-surface interactions of the plasma facing material referred to as the first wall have become increasingly important. In large tokamaks, the energy confinement of the plasma has been substantially improved by the use of graphite or carbon material. Graphite, however, has several disadvantages, e.g. large capacity to absorb gases due to its porous structure and a large erosion rate due to bombardment by hydrogen ions. Therefore, various problems must be solved before graphite can be used as the first wall material of next generation fusion devices. In this paper, we first explain plasma-surface interactions. In particular, the problem associated with impurities is described. We than show the results of our characterization study for several kinds of graphite as the first wall material. These include (1) surface and vacuum properties of graphites, i.e. gas desorption and micro surface area (effective surface area), and (2) interactions of graphite with hydrogen ions, i.e. chemical sputtering, radiation-enhanced sublimation and hydrogen retention. Required conditioning of, and modifications to graphite intended for use as the first wall material are also discussed.

7507. Radiation-enhanced sublimation of graphite in PISCES experiments: R E Nygren et al,J Vac Sci Technol, A8, 1990, 1778–1782

7507. Radiation-enhanced sublimation of graphite in PISCES experiments: R E Nygren et al,J Vac Sci Technol, A8, 1990, 1778–1782

Radiation enhanced sublimation of boron containing carbon materials

The radiation enhanced sublimation (RES) of boron containing graphites as well as amorphous boron containing carbon films (a-C/B : H films) has been investigated with the result that it cannot be completely suppressed. However, the onset temperature for RES is shifted to higher values compared to carbon resulting in a reduced yield in a limited temperature range to about 1800 K. The reduction of RES of boron containing carbon materials depends on the boron content as well as on the microstructure of the bulk material. Above 1800 K, the thermal sublimation of boron leads to a recovery of RES up to values as observed for pure carbon.

Carbon erosion due to simultaneous H + and e − impact

Experiments were performed at the University of Toronto and KFA Jülich to investigate carbon erosion due to simultaneous impact by H + ions and electrons. The effect of electrons was found to be negligible over the H + ions-only case, for temperatures corresponding to peak chemical erosion (~800 K) and radiation-enhanced sublimation (1600–2000 K).

Enhanced carbon and beryllium influxes during high performance operation in JET

During high-power operation in JET, a sudden and large increase in the flux of impurities (carbon and beryllium) entering the discharge is often observed shortly after the start of additional heating, especially at the low plasma density required for high ion temperature and fusion yield. This influx leads to a degradation of the plasma performance due to an increase of the impurity concentration and associated deuterium dilution, reduced beam penetration and a reduction in the ion temperature. There is good evidence that these influxes result from a small fraction of the total area of the carbon (beryllium) surfaces in contact with the plasma (tile edges particularly) reaching temperatures where radiation-enhanced sublimation and thermal sublimation for carbon and evaporation for beryllium become important. In this paper we report on measurements of the phenomena observed during inner-wall and carbon or beryllium belt limiter discharges both with and without beryllium gettering.