Impurity Release Research Articles

Radiated power measurement during the thermal quench phase of a density limit disruption

Radiated power measurements taken during the thermal quench of a density limit disruption in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] demonstrate that ten times more power is radiated from the main chamber than is deposited into the divertor floor. This contrasts with the standard picture of the thermal quench of disruptions, where energy is lost along open field lines into the divertor. The observed radiation appears to result from a sudden increase in deuterium outgassing and carbon impurity release from the main chamber walls. The impurities and the associated cold front are observed to mix into the plasma core at a rate about 100 times larger than standard (H-mode) particle diffusion rates. Comparison with other types of disruptions in DIII-D suggests that plasma–main wall interactions may play an important role in the thermal quench of many disruptions.

Direct fabrication of large micropatterned single crystals.

Micropatterning of single crystals for technological applications is a complex, multistep process. Nature provides alternative fabrication strategies, when crystals with exquisite micro-ornamentation directly develop within preorganized frameworks. We report a bio-inspired approach to growing large micropatterned single crystals. Micropatterned templates organically modified to induce the formation of metastable amorphous calcium carbonate were imprinted with calcite nucleation sites. The template-directed deposition and crystallization of the amorphous phase resulted in the fabrication of millimeter-sized single calcite crystals with sub-10-micron patterns and controlled crystallographic orientation. We suggest that in addition to regulating the shape, micropatterned frameworks act as sites for stress and impurity release during the amorphous-to-crystalline transition. The proposed mechanisms may have direct biological relevance and broad implications in materials synthesis.

Recent activities on the compatibility of the ferritic steel wall with the plasma in the JFT-2M tokamak

The compatibility of the low activation ferritic steel with a fusion plasma has been investigated in the JFT-2M tokamak. The program consists of three stages. In the first stage, the reduction of fast ion losses was well demonstrated by ferritic steel plates (FPs) outside the vacuum vessel (VV). In the second stage, 20% of the inner surface of the VV was covered by the FPs. The plasma control, stability, and impurity release were preliminary investigated. No deteriorative effect on the plasma was observed at least in the following conditions: partial covering of 20% and the normalized beta value less than 2.8. First boronization was applied to JFT-2M leading to a remarkable decrease of the oxygen impurity. After the boronization, plasmas with the highest normalized beta in JFT-2M were obtained. Thus encouraging results were gained for this stage. In the third stage, the VV was fully covered by FPs, where the ripple reduction and the plasma stability will be investigated as a full scale testing.

Testing for Compatibility of Reduced Activation Ferritic Steel with Plasma on JFT-2M Partial Coverage of the Vacuum Vessel with Ferritic Steel.

The compatibility of reduced activation ferritic steel (F82H), which is a leading candidate material for the demo reactor (e.g. SSTR), with plasma has been investigated in the JFT-2M tokamak with 3 steps in an AMTEX (Advanced Material Tokamak EXperiment). In the first step, the reduction of fast ion losses was well demonstrated with the ferritic steel outside the vacuum vessel. In the second step, the ferritic steel was installed inside the vacuum vessel in order to perform a preliminary investigation of the effect of the ferro-magnetism on plasma stability and control, and impurity release. For this purpose, ferritic steels of 7 mm thickness were installed to form 2 sets of toroidally uniform belts, which cover 20% of the vacuum vessel. No deteriorative effects were observed regarding mode locking, plasma control, and impurity desorption. The initial boron coating was applied in order to modify the surface of the ferritic steel. The impurity is remarkably reduced and high normalized-beta plasma was obtained. Thus encouraging results were obtained for the third step, where whole vacuum vessel wall will be covered with ferritic steel.

Impurity release and recycling behaviour in TEXTOR-94 with siliconised walls

Siliconisation has been performed for a long period of operation as the routine wall coating procedure of TEXTOR-94. The general aim was to study the behaviour of TEXTOR-94 with a medium- Z plasma-facing material and to compare it with that under freshly boronised as well as `carbon dominated' conditions. This contribution focuses on the characteristics of Si-impurity release from the limiters, and the influence of the Si coating on the release of carbon and oxygen impurities with special emphasis on material mixing effects on the plasma-exposed surfaces.

Operational limits under different wall conditions on TEXTOR-94

Generally the operational range of tokamaks is limited by the β and the density limit. Experimentally it was found, that the appearance of MARFEs [B. Lipschultz, J. Nucl. Mater. 145–147 (1987) 15] inhibited an increase of the density significantly above the so-called Greenwald density ( n GW) [M. Greenwald et al., Nucl. Fus. 28 (1988) 2199]. In TEXTOR-94 the onset of MARFEs has been investigated under different wall conditions. With fresh siliconization and fresh boronization the appearance of MARFEs can be postponed to higher densities ( N GW = n e /n GW =1.7 ). At those high densities n e =8×10 19 m −3 the Z eff is reduced to 1.3. But as important as the reduction of the global impurity content is the suppressed carbon release on the high-field-side (HFS) just after wall conditioning. Modeling of the MARFE onset by the codes B2-Eirene and TECXY demonstrates that an instability on the HFS due to deuterium recycling and radiation only leads to the formation of MARFEs at higher line-averaged densities in comparison to calculations which include carbon impurities. Complete suppression of MARFEs has been achieved by controlled displacement of the plasma column to the low-field-side (LFS), which reduces the local recycling and impurity release at the inner bumper limiter. This resulted in the highest densities achieved at TEXTOR-94 of N GW=2.1 independent of wall conditions. In this case the maximum achievable density is limited by a radiative collapse.

Behaviour of silicon-doped CFC limiter under high heat load in TEXTOR-94

In order to study the impurity production, recycling and power deposition a Si doped CFC test limiter (NS31) was used in TEXTOR-94. The release of impurities (C, Si, O, Cr, CD radicals) was measured spectroscopically. A reduced methane production was found in the Si doped graphite when compared to a pure graphite limiter. A smaller decrease of the carbon fluxes could also be observed. The limiter contained about 1%–1.5% of Si, but a relative Si flux (Si/D) from the Si doped CFC surface between 0.12% and 0.4% has been measured. A chemical erosion of Si due to formation of SiDx has not been observed. Silicon evaporated from the surface at temperatures above 1500 °C. This led to an increase of Si concentration and total radiation losses from the plasma. Surface analysis shows the formation of microcracks and holes on the plasma exposed limiter surface. The released Si was deposited in the vicinity of the tangency point of the limiter. Whereas a Si depletion was observed in the area of highest power loading with values reaching in and in-between fibres values of 0.03% and 0.02% respectively.

Simulation study of carbon and tungsten deposition on W/C twin test limiter in TEXTOR-94

In order to investigate the impurity release and surface modification on a W/C twin test limiter, made of a half of W and the other half of C, exposed to the edge plasma of TEXTOR-94, simulation calculations of ion-surface interaction are conducted by a Monte Carlo code. According to the calculations, experimentally observed spatial distributions of WI and CII line intensities around the W side of the limiter can be explained by physical sputtering of W, reflection of bombarding C ions and physical sputtering of implanted C. The CII line emission, resulting from thermal C atoms, around the C side of the limiter is suppressed by deposition of W, and the reflection of C ions from W deposited on C causes the CII intensity to decay more slowly than that from C without the deposition. Bombardment with deuterium edge plasmas, containing impurity W, produces a thick W layer on the C side of the limiter, whereas C implanted in the W side is strongly sputtered due to impact of most constituent D ions.

Calculation of Impurity Release from Walls and its Transport in Edge Plasmas

Computer simulation by the plasma-solid interaction code EDDY has been performed in order to investigate the erosion and impurity redeposition processes of both W and C materials under the mutual contamination, i.e., C deposition on W and W deposition on C. In the code, dynamic surface composition change during on bombardment, ionization of released atoms in the plasma, and gyromotion of the ionized atoms in a magnetic field are included. The redeposition of sputtered atoms causes substantially lower effective sputtering yield and lower erosion rate of a W surface, and a decrease in low-energy component of the atoms finally penetrating into a core plasma. W impurities are deposited on a C surface due to the redeposition, whereas C impurities deposited on the W surface are sputtered by impact of most constituent D ions. The W (C) deposition on the C (W) surface substantially increases (slightly decreases) the reflection coefficient for D and impurity ions.

Calculation of Impurity Release from Walls and its Transport in Edge Plasmas

Calculation of Impurity Release from Walls and its Transport in Edge Plasmas

Activation of impurities in ZnSe crystals, stimulated by a laser shock wave

The effect of a laser shock wave on the photoconductivity spectra and the thermally stimulated conductivity spectra and also on the temperature dependence of the photocurrent and dark current of ZnSe single crystals with different residual impurity levels is investigated. It is shown that a temperature dependence of the dark currents after passage of a shock wave is observed at higher shock-wave pressures for the less defective crystals, while activation of the photocurrent and dark current is due to the release of residual impurities from clusters during passage of the shock wave.

Simulation and experimental studies of impurity release from tungsten exposed to edge plasmas in TEXTOR-94

Particle release from a W test limiter exposed to deuterium edge plasmas, containing impurities C and O, in the tokamak TEXTOR-94 is studied both experimentally and theoretically by means of Monte-Carlo simulations. The simulation model combines the dynamic composition change in the surface layer with the transport of released particles in the plasma. The release of W impurities from the limiter is strongly suppressed by prompt redeposition, whereas low- Z impurities (C and O) and D are much less influenced. Experimentally observed radial distributions of WI and CII line intensities in front of the limiter can be explained by physical sputtering of W and deposited C, respectively, in addition to high-energy reflection of impurity C ions. On the other hand, the CII line emission observed in front of graphite as a limiter is attributed to many low-energy (∼0.1 eV) atoms probably due to chemical sputtering by impact of D ions, which is strongly suppressed by deposition of high- Z impurities, such as W. The observed Dγ line emission indicates an important contribution of high-energy reflection to the release of D atoms from the W limiter, in addition to a large contribution of low-energy re-emission.

Enhancement of productivity of recombinant alpha-amidating enzyme by low temperature culture.

We have produced a recombinant C-terminal alpha-amidating enzyme (799BglIIalpha-AE) derived from Xenopus laevis by culturing a CHO cell line named 3mu-1S. Recently, we demonstrated that culturing 3mu-1S cells at a temperature below 37 degrees C led to the following phenomena: inhibited cell growth with high viability, enhanced cellular productivity (maximally at 32 degrees C), and suppressed medium consumption and release of impurities from the cells. Therefore, it is suggested that the 799BglIIalpha-AE production will be increased by culturing a sufficient number of the cells at a low temperature (especially at 32 degrees C). To assess this effect on batch and perfusion cultures, the culture temperature was shifted from 37 to 32 degrees C in the mid-exponential phase in the case of batch culture and from 37 to 34 degrees C when the cell density became high enough in the case of perfusion culture. Application of the low temperature culture to batch and perfusion cultures was effective in comparison with the culture at 37 degrees C: the productivity per medium and the productivity per time were increased severalfold with enhanced cellular productivity at a low culture temperature. The low temperature culture also increased the relative content of 799BglIIalpha-AE in the supernatant and reduced the glucose consumption. The method presented here would contribute to production of bioactive proteins using other recombinant cell lines.

Experiments with tungsten limiters in TEXTOR-94

The release of tungsten and light impurities from tungsten limiters exposed into the plasma edge of TEXTOR-94 has been measured by spectroscopic methods. Absolute effective tungsten sputtering yields are compared with model calculations on physical sputtering. The agreement is reasonable: however the observed strong decrease of tungsten release with increasing density cannot be fully explained. Erosion areas are clearly separated from carbon deposition zones. Surface analysis found neither carbon nor deuterium on the shiny metallic areas: A very sharp transition from “clean” metallic areas to carbon deposition zones within about 2–4 mm is found, instead. The carbon deposit is about 200–300 nm thick and contains deuterium with a D/C ratio of 0.05–0.1.

Safety approach for plasma anomaly events in fusion reactor: Code development and assessments

A safety assessment method for plasma transients, including thermal response of in-vessel components, was provided. We developed a plasma physics model for safety analysis which has been implemented in a safety analysis code (SAFALY). The SAFALY code consists of a zero-dimensional plasma dynamics model and a 1.5-dimensional thermal behavior model of in-vessel components in the thickness direction. The code can treat hydraulic accidents using the results from a hydraulic code and analyze a passive plasma shutdown due to the impurity release from the wall. The abnormal events in the International Thermonuclear Experimental Reactor (ITER) calculated here are: (i) loss of plasma control, and (ii) large ex-vessel loss of coolant accident (LOCA) in the first wall and large ex-vessel LOCA in the divertor. The plasma transient and thermal response of in-vessel components in such events have been assessed using the code.

Spring Shock!: Impact of Spring Snowmelt on Lakes and Streams

Acids from atmospheric pollution may be deposited to the environment through dry deposition (by acidic aerosols or gas-phase acids) or by wet deposition. Wet deposition includes acid rain, acid sleet, acid fog, and acid snow. In poorly buffered watersheds acid precipitation causes acidification. In many areas of the U.S. and Europe, year-round or chronic acidification has eradicated native fish species in lakes and streams (1, 2). Some streams that are not chronically acid may become acidic during the spring spawning season. This acute or episodic acidification (spring shock) can result in a decreased rate of sport fish hatching and reduced viability of the fry that do manage to hatch (3). One reason for the toxicity of acidification is the increased solubility of metals, especially aluminum, at low pH. Acute acidification during snowmelt or storms is a complex process and may be due to a number of factors. One of these may be early or preferential release of acid impurities during the initial stages of s...

Effect of culture temperature on a recombinant CHO cell line producing a C-terminal α-amidating enzyme.

In order to seek an efficient method for producing a recombinant protein by using animal cell culture, we investigated various effects of the culture temperature on a recombinant CHO cell line (3µ-1S), producing a C-terminal α-amidating enzyme (799BglIIα-AE) originating from Xenopus laevis. The results revealed that a low culture temperature (below 37 °C) led to the following phenomena: [1] inhibited cell growth, [2] enhanced cellular productivity of the recombinant protein, [3] maintained high cell viability, [4] suppressed medium consumption, and [5] suppressed release of impurities from the cells. These findings indicate that a quite simple method, the culture at low temperature, will contribute to the total improvement of the industrial process for the production of the recombinant protein, 799BglIIα-AE.

Rate Limiting Mechanism of Transition Metal Gettering in Multicrystalline Silicon

The authors have performed studies on multicrystalline silicon used for solar cells in the as-grown state and after a series of processing and gettering steps. The principal goal of this work is to determine the rate limiting step for metal impurity gettering from multicrystalline silicon with an emphasis on the release of impurities from structural defects. Synchrotron-based x-ray fluorescence mapping was used to monitor the release process. Copper and nickel impurities were found to reside primarily at dislocations in the as-grown state of the material. Short annealing treatments rapidly dissolved the impurity agglomerates. Based on these results and modeling of the dissolution process, copper and nickel is in the form of small agglomerates (< 10 nm) clustered together over micron-scale regions in the as-grown material. Aluminum gettering further disintegrated the agglomerates to below the sensitivity of the system, 2--5 nm in radii. No significant barrier to release of copper or nickel from dislocations was observed.

Release of metal impurities from structural defects in polycrystalline silicon

Metal impurity release from structural defects in polycrystalline silicon was studied following thermal treatments, and, in addition, a correlation between impurity distributions and structural defects was established. Impurities were mapped with synchrotron-based x-ray fluorescence in the as-grown state, after rapid thermal annealing and following aluminum gettering treatments. The goal of this work was to determine if impurity release from structural defects limits gettering of metal impurities. The results reveal that nickel and copper metal impurities are primarily found at dislocations in as-grown crystals, and the release of these impurities from defects occurs rapidly with no apparent barrier to dissolution. Gettering treatments dissolved metal impurity precipitates to &amp;lt;2–5 nm in radii; however, the material performance was not greatly enhanced.

Transport studies of high-Z elements in neon edge radiation cooled discharges in TEXTOR-94

High-Z materials as tungsten are intended to be used in future fusion reactors due to their low sputtering rates and high melting points. In this context the important question is whether the use of high-Z materials is compatible with the concept of a cold radiative boundary. To investigate the local release and transport behaviour of the high-Z impurities, Mo and W test limiters were used in auxiliary heated discharges under different radiation scenarios with neon seeding. In addition, laser blow-off of tungsten as well as xenon gas puffing were performed. In some particular discharge conditions impurity accumulation was observed in the plasma core, which in the case of ohmic discharges led to minor disruptions. The connection between the accumulation and the evolution of the current density profiles is discussed. A transport analysis is made, in order to compare the observations with the neoclassical theory. It is shown that with neon the impurity - impurity driven fluxes enhance the high-Z concentration in the plasma core. However, if the source of the high-Z elements is significantly reduced, by increasing the plasma density, a development of an accumulation instability can be avoided.