High-flux Deuterium Plasma Research Articles

Erosion behavior and surface characterization of beryllium under high-flux deuterium plasma bombardment

The erosion of beryllium subject to a high-flux deuterium plasma is studied, including post-bombardment surface characterization. Simulating divertor plasma conditions of the ITER tokamak, the PISCES-B steady-state plasma facility at U.C., San Diego is used to bombard beryllium specimens. The measured sputtering yield is compared with theoretical Monte Carlo calculations using the TRIM.SP code, and with experimental values obtained by others using an ion beam facility. It is found that at elevated specimen temperatures (250° C ≤ T ≤ 720° C), low atomic number impurities of the plasma, such as carbon, nitrogen, and oxygen, form an impurity layer on the beryllium surface. This layer is not eroded away by extended plasma exposure but appears to be continuously regenerated. The presence of this film reduces the apparent sputtering yield of beryllium by up to two orders of magnitude. An in-situ emission spectroscopy technique is used to confirm the effect of deposited contaminants. Depth-profile Auger electron spectroscopy (AES) data shows that relative concentration of carbon, nitrogen, and oxygen in the thin film is each in the range, 10 to 20%. X-ray photoelectron spectroscopy (XPS) data indicates the formation of BeO and a small amount of Be 2C on the surface. X-ray diffraction (XRD) patterns obtained with a small angle of incidence show no evidence for crystalline Be 2C and suggest that the impurity layer may have a disordered or amorphous structure. Importantly, no impurity deposition is observed when the specimen is maintained at room temperature (∼ 40° C). Application of energy dispersive X-ray spectroscopy (EDX) confirms that the low temperature surface is free of impurities except for a trace amount of oxygen. The sputtering yield measured at room temperature agrees with theoretical values within a factor of two.

AFM and STM characterization of surfaces exposed to high flux deuterium plasma

This paper reports the results of scanning tunneling (STM) and atomic force microscopy (AFM) studies of D + irradiated graphite and graphite-silicon mixtures. The microscopes were used for studying surface topography and for measuring the surface roughness. The substrates were exposed at various temperatures (60 and 700°C) to different doses of deuterium ions in simulators of plasma - surface interactions and in the TEXTOR tokamak. Also nuclear reaction analysis (NRA) and Rutherford backscattering spectroscopy were applied for the qualitative and quantitative determination of surface composition. The initial stages of radiation damage, nanometer-sized bubbles/blisters, were found in plasma-eroded surfaces. These structures only appeared in the graphite phase on the multicomponent material. The microroughness of the surfaces was measured. We also used the AFM for probing the thickness of the plasma-modified layers. The results correlate with the presence of deuterium measured by NRA depth-profiling. Moreover, the AFM reveals the co-deposited layers formed on surfaces facing the tokamak plasma. The appearance of these layers is clearly correlated to the amount of co-deposited atoms.

Bulk-boronized graphites for plasma-facing components in ITER

Newly developed bulk-boronized graphites and boronized C-C composites with a total boron concentration ranging from 1 wt% to 30 wt% have been evaluated as plasma-facing component materials for the International Thermonuclear Experimental Reactor (ITER). Bulk-boronized graphites have been bombarded with high-flux deuterium plasmas at temperatures between 200 and 1600 °C. Plasma interaction induced erosion of bulk-boronized graphites is observed to be a factor of 2–3 smaller than that of pyrolytic graphite, in regimes of physical sputtering, chemical sputtering and radiation enhanced sublimation. Postbombardment thermal desorption spectroscopy indicates that bulk-boronized graphites enhance recombinative desorption of deuterium, which leads to a suppression of the formation of deuterocarbon due to chemical sputtering. The tritium inventory in graphite has been found to decrease by an order of magnitude due to 10 wt% bulk-boronization at temperatures above 1000 °C. The critical heat flux to induce cracking for bulk-boronized graphites has been found to be essentially the same as that for non-boronized graphites. Also, 10 wt% bulk-boronization of graphite hinders air oxidation nearly completely at 800° C and reduces the steam oxidation rate by a factor of 2–3 at around 1100 and 1350 °C.

Temperature and composition dependence of the high flux plasma sputtering yield of Cu-Li binary alloys

Because of its very high thermal conductivity, actively cooled copper is an attractive plasma-interactive material for long pulse fusion devices such as ETR and devices with very high wall power loadings, such as reversed-field pinched (RFPs) and the proposed compact ignition torus (CIT). Pure copper however, has an unacceptably low threshold energy for runaway self-sputtering. Low Z materials such as graphite and beryllium are not subject to runaway self-sputtering, but suffer from high light ion erosion rates and very nonuniform redeposition. It has been suggested that strongly segregating alloys such as Cu-Li might be used to provide a low- Z self-sustaining coating while maintaining the desirable redeposition, thermal and mechanical properties of the majority alloy component. High flux deuterium plasma sputtering and ion beam experiments have been performed on Cu-Li alloys to determine if the reduction in copper erosion previously predicted and observed in low flux ion beam experiments occurs at particle fluxes representative of an RFP first wall or tokamak limiter. Partial sputtering yields of the copper and lithium components have been measured as a function of alloy composition and sample temperature using optical plasma emission spectroscopy, weight loss and catcher foil techniques. It is found that the lithium sputtering yield increases with increasing sample temperature while the copper yield decreases by as much as two orders of magnitude. The temperature required to obtain the reduction in copper erosion is found to be a function of bulk lithium concentration. Consequences of these experimental results for anticipated erosion/redeposition properties are calculated, and the Cu-Li alloy is found to compare favorably with conventional low- Z materials.

1037. Investigation of the behaviour of compressed seals at enhanced temperatures: S A Nikolaeva et al, Elektron Tekh Materialy, 4, 1970, 30–36 (in Russian)

Four different high temperature brazes (Zr, 90wt%Ni.10%Ti, 90Cu.10Ti and 70Ag.27Cu.3Ti, nominal composition prior to brazing) were used to join isotropic fine grain graphite and TZM, a high temperature alloy of molybdenum. The general response of the brazes to a high flux deuterium plasma bombardement was examined using samples whose exposed surface intersected the braze line. Only in the case of Zr, which is known for its hydride forming properties, near-surface layers chipped off in the braze region directly exposed to the plasma. However, in graphite-shielded Zr-braze regions no disintegration of the interface was observed. The other brazes showed no visible attack at all. The interaction of a hydrogen plasma and a braze surface was studied in more detail by bombarding specimens in the PISCES-B facility. In this case the graphite was removed completely to enable an exaggerated plasma attack onto the bare braze. Even under these testing conditions the brazes CuTi and NiTi showed no particular sensitivity towards hydrogen.To prove the thermal stability of the brazed joints each quality was heat treated for 1 hr and 10 hrs, respectively, 50° below the softening temperature of the braze in 1 bar of Ar or 96vol%Ar/4%H2. After the heat treatment no changes or damages were observed in the brazes AgCuTi and CuTi, whereas interstices are found in the widened NiTi-interlayer after 10 hrs in both, Ar and Ar/H2. Zr suffers different microstructural changes in particular after treatment in Ar/H2 atmosphere.