Characterizing Erosion and Redeposition of Aluminum in DIII-D Divertor Plasmas PDF Download
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Author: Christopher Peter Chrobak
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Languages : en
Pages : 233
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Understanding the erosion and redeposition behavior of tokamak plasma facing materials is important to ensure component lifetime. The goal of this thesis was to characterize the erosion and redeposition of aluminum (Al) when exposed to a tokamak divertor plasma. Al was chosen for its similarities to beryllium (Be), which is intended for use as the first wall material in ITER but is toxic and restricted in DIII-D and most other tokamaks. The Divertor Material Evaluation Station (DiMES) was used to expose a set of Al-coated samples to low-density L-mode plasma discharges in the DIII D divertor. Different plasma conditions were used for each sample (including He and D plasmas), and samples with both ideal (smooth) and practical (rough) surfaces were exposed. Measurements of quantitative emission spectroscopy and film thickness change were compared to simulations of sputtering, ionization, and redeposition of Al to determine the gross erosion rate, redeposition fraction, and spectroscopic emission efficiency. We present the first quantitative spectroscopic measurements of neutral emission anisotropy due to sputtering erosion in a tokamak divertor plasma. We present an ionization-emission model that reproduced the anisotropy by assuming full angular sputtering yield distributions predicted by grazing angle sputtering simulations and ion beam sputtering measurements. Grazing angle ions were expected due to the disappearance of the classical Debye sheath in favor of a thicker magnetic pre-sheath (MPS) at small magnetic field surface inclination angles. The direction of presumed sputtering anisotropy and E×B drift of ions within the MPS was consistent with the direction of deposition patterns found on the samples and within individual pores of the rough surfaces. A model of the erosion-redeposition cycle including re-erosion and material mixing reproduced observed film thickness change measurements. The characteristic Al redeposition length was on the order of its ionization length, 1-3mm at the 1×10^13cm^-3 plasma densities analyzed. Total redeposition fractions ranged from 30% to 76%, increasing with higher plasma temperature due to the higher sheath electric field strength. On rough surfaces on the order of 50% of redepositing material was retained in hidden or shadowed regions, while on smooth surfaces this effect was negligible.
Author: Christopher Peter Chrobak
Publisher:
ISBN:
Category :
Languages : en
Pages : 233
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Book Description
Understanding the erosion and redeposition behavior of tokamak plasma facing materials is important to ensure component lifetime. The goal of this thesis was to characterize the erosion and redeposition of aluminum (Al) when exposed to a tokamak divertor plasma. Al was chosen for its similarities to beryllium (Be), which is intended for use as the first wall material in ITER but is toxic and restricted in DIII-D and most other tokamaks. The Divertor Material Evaluation Station (DiMES) was used to expose a set of Al-coated samples to low-density L-mode plasma discharges in the DIII D divertor. Different plasma conditions were used for each sample (including He and D plasmas), and samples with both ideal (smooth) and practical (rough) surfaces were exposed. Measurements of quantitative emission spectroscopy and film thickness change were compared to simulations of sputtering, ionization, and redeposition of Al to determine the gross erosion rate, redeposition fraction, and spectroscopic emission efficiency. We present the first quantitative spectroscopic measurements of neutral emission anisotropy due to sputtering erosion in a tokamak divertor plasma. We present an ionization-emission model that reproduced the anisotropy by assuming full angular sputtering yield distributions predicted by grazing angle sputtering simulations and ion beam sputtering measurements. Grazing angle ions were expected due to the disappearance of the classical Debye sheath in favor of a thicker magnetic pre-sheath (MPS) at small magnetic field surface inclination angles. The direction of presumed sputtering anisotropy and E×B drift of ions within the MPS was consistent with the direction of deposition patterns found on the samples and within individual pores of the rough surfaces. A model of the erosion-redeposition cycle including re-erosion and material mixing reproduced observed film thickness change measurements. The characteristic Al redeposition length was on the order of its ionization length, 1-3mm at the 1×10^13cm^-3 plasma densities analyzed. Total redeposition fractions ranged from 30% to 76%, increasing with higher plasma temperature due to the higher sheath electric field strength. On rough surfaces on the order of 50% of redepositing material was retained in hidden or shadowed regions, while on smooth surfaces this effect was negligible.
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Languages : en
Pages : 1
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Languages : en
Pages : 16
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The ability to withstand disruptions makes carbon-based materials attractive for use as plasma-facing components in divertors. However, such materials suffer high erosion rates during attached plasma operation which, in high power long pulse machines, would give short component lifetimes and high tritium inventories. The authors present results from recent experiments in DIII-D, in which the Divertor Materials Evaluation System (DiMES) was used to examine erosion and deposition during short exposures to well defined plasma conditions. These studies show that during operation with detached plasmas, produced by gas injection, net erosion is suppressed everywhere in the divertor. Net deposition of carbon with deuterium was observed at the inner and outer strikepoints and in the private-flux region between strikepoints. For these low temperature plasmas (T{sub e}
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Languages : en
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Net erosion rates of carbon target plates have been measured in situ for the DIII-D lower divertor. The principal method of obtaining this data is the DiMES sample probe. Recent experiments have focused on erosion at the outer strike-point (OSP) of two divertor plasma conditions: attached (T[sub e]> 40 eV) ELMing plasmas, and detached (T[sub e]2 eV) ELMing plasmas. For the attached cases, the erosion rates exceed 10 cm/exposure-year, even with incident heat flux
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Languages : en
Pages : 18
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Net erosion rates at the outer strike point of the DIII-D divertor plasma were measured for several materials during quiescent H-mode operation with deuterium plasmas. Materials examined include graphite, beryllium, tungsten, vanadium and molybdenum. For graphite, net erosion rates up to 4 nm/sec were found. Erosion rates for the metals were much smaller than for carbon. Ion fluxes from Langmuir probe measurements were used to predict gross erosion by sputtering. Measured net erosion was much smaller than predicted gross erosion. Transport of metal atoms by the plasma across the divertor surface was also examined. Light atoms were transported farther than heavy atoms as predicted by impurity transport models.
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Languages : en
Pages : 18
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The DiMES (Divertor Material Evaluation Studies) mechanism allows insertion of material samples to the lower divertor floor of the DIII-D tokamak. The main purpose of these studies is to measure erosion rates and redeposition mechanisms under tokamak divertor plasma conditions in order to obtain a physical understanding of the erosion/redeposition processes and to determine its implications for fusion power plant plasma facing components. Thin metal films of Be, W, V, and Mo, were deposited on a Si depth-marked graphite sample and exposed to the steady-state outer strike point on DIII-D.A variety of surface analysis techniques are used to determine the erosion/redeposition of the metals and the carbon after 5--15 seconds of exposure. These short exposure times ensure controlled exposure conditions and the extensive array of DIII-D divertor diagnostics provide a well characterized plasma for modeling efforts. Erosion rates and redeposition lengths are found to decrease with the atomic number of the metallic species, as expected. Under these conditions, the peak net erosion rate for carbon is (approximately) 4 nm/s, with the erosion following the ion flux profile. Comparisons of the measured carbon erosion with REDEP code calculations show good agreement for both the absolute net erosion rate and its spatial variation. Measured erosion rates of the metals are smaller than predicted for sputtering from a bare metal surface, apparently due to effects of carbon deposition on the metal surface. Visible spectroscopic measurements of singly ionized Be have determined that the erosion process reaches steady-state during the exposure.
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Languages : en
Pages : 20
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A strong effect of a moderately elevated surface temperature on net carbon deposition and deuterium co-deposition in the DIII-D divertor was observed under detached conditions. A DiMES sample with a gap 2 mm wide and 18 mm deep was exposed to lower-single-null (LSN) L-mode plasmas first at room temperature, and then at 200 C. At the elevated temperature, deuterium co-deposition in the gap was reduced by an order of magnitude. At the plasma-facing surface of the heated sample net carbon erosion was measured at a rate of 3 nm/s, whereas without heating net deposition is normally observed under detachment. In a related experiment three sets of molybdenum mirrors recessed 2 cm below the divertor floor were exposed to identical LSN ELMy H-mode discharges. The first set of mirrors exposed at ambient temperature exhibited net carbon deposition at a rate of up to 3.7 nm/s and suffered a significant drop in reflectivity. In contrast, two other mirror sets exposed at elevated temperatures between 90 C and 175 C exhibited virtually no carbon deposition.
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Languages : en
Pages : 31
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The three-dimensional Monte Carlo code ERO has been used to simulate dedicated DIII-D experiments in which Mo and W samples with different sizes were exposed to controlled and well-diagnosed divertor plasma conditions to measure the gross and net erosion rates. Experimentally, the net erosion rate is significantly reduced due to the high local redeposition probability of eroded high-Z materials, which according to the modelling is mainly controlled by the electric field and plasma density within the Chodura sheath. Similar redeposition ratios were obtained from ERO modelling with three different sheath models for small angles between the magnetic field and the material surface, mainly because of their similar mean ionization lengths. The modelled redeposition ratios are close to the measured value. Decreasing the potential drop across the sheath can suppress both gross and net erosion because sputtering yield is decreased due to lower incident energy while the redeposition ratio is not reduced owing to the higher electron density in the Chodura sheath. Taking into account material mixing in the ERO surface model, the net erosion rate of high-Z materials is shown to be strongly dependent on the carbon impurity concentration in the background plasma; higher carbon concentration can suppress net erosion. As a result, the principal experimental results such as net erosion rate and profile and redeposition ratio are well reproduced by the ERO simulations.
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Languages : en
Pages : 16
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Languages : en
Pages : 48
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We are in the process of assembling a database of edge and divertor plasma parameters suitable for use in benchmarking tious 2D models of the scrape-off- layer (SOL) plasma. Also, we are using the Braams B2 code to derive transport coefficients for the edge plssma. In parallel, work is starting on an upgrade to the B2 code that includes padlel current flow and EXB drifts. These efforts are directed at increasing the confidence level of models of the tokamak edge plasma so that we can predict the effect of planned upgrades to DIII-D (e.g., the Advanced Divertor Program) and the performance of next generation machines such as CIT or ITER, where initial design studies show that plasma conditions at the divertor targets can have a large impact on the lifetime and cost of the machine. This report summarizes our recent progress in characterizing the DIII-D SOL plasma and in modeling these data with the the B2 code. Section I contains a brief description of the diagnostics available for characterizing the SOL plasma. In Section II we present our measurements of the SOL parameters for H-mode plasmas. This includes data showing how the divertor plasma parameters (n{sub e}(r), T{sub e}(r), and Q(r)) vary from ohmic to L-mode to H-mode, and power balance for quasi-stationary H-mode plasmas. Section III covers divertor-target heat-flux asymmetries for double and single null operation with forward and reversed toroidal field. In Section IV we show the scaling of L-mode parameters with neutral beam power, and Section V concludes with a summary of the results obtained from the Braams B2 SOL simulation code.
