DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 280: DiMES Investigation of Helium Plasma Induced Nanostructured Morphology on Tungsten | ||
| Name: | Matthew Baldwin ( |
Affiliation: | University of California, San Diego |
| Research Area: | Boundary | Presentation time: | Requested | Co-Author(s): | R Doerner, C Wong, P West, D Rudakov |
| Description: | The diverted reactor exhaust in confinement machines like ITER and DEMO will be intense-mixed plasmas of fusion (D, T, He) species characterized by tremendous heat and particle fluxes. In both devices, the divertor walls are to be exposed to such plasma and must operate at high temperature for long durations. Tungsten, with its high-melting point and low-sputtering yield is currently viewed as the leading choice for divertor-wall material in this next generation class of fusion devices, and is supported by an enormous amount of work that has been done to examine its performance in hydrogen isotope plasmas. However, studies of the more realistic scenario, where ionized He is present, are considerably less. Current helium plasma on tungsten experiments on the linear machines NAGDIS II and PISCES-B have lead to the observation of plasma induced nanoscopic tendril like morphology. Such morphology may influence the thermal properties of the surface, have a significant impact on fuel retention and also contribute to in-vessel dust accumulation.
A experiment involving DiMES, outfitted with a high temperature tungsten sample that is exposed to successive He plasma shots will help to elucidate the magnitude of this effect in a tokamac environment. To date, there has been no experiment that capitalizes on the opportunity to explore the formation of these structures in an actual divertor plasma. | ||
| Experimental Approach/Plan: | Exposure of a polished DiMES W target at 900 degrees C to as many detached He or D2/He plasma discharges as possible. | ||
| Background: | The conversion of a hot W surface into tendril like nano-structures has very recently been observed in the divertor plasma simulators NAGDIS-II and PISCES-B. Experiments in LHD also show precursor bubble formation/pitting on hot W surfaces after as little as 1 s of exposure to He plasma. NAGDIS experiments have revealed that He ion bombardment at energies greater than 10 eV leads to the observation of this effect. PISCES investigations show that the formation rate of the nano-structures is a diffusion limited process. Both sets of experiments provide the necessary background to be able to predict the conditions of formation in a tokamac experiment. | ||
| Resource Requirements: | Some modification to DiMES system to facilitate higher temperature (900 degrees C) operation. | ||
| Diagnostic Requirements: | DiMES (RUDAKOV, WEST, WONG) | ||
| Analysis Requirements: | Ex-situ surface analysis provided at UCSD PISCES LAB (BALDWIN, DOERNER) | ||
| Other Requirements: | -- | ||
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