G.J. Thomas

Helium and hydrogen re-emission during implantation of molybdenum, vanadium and stainless steel☆

Abstract He re-emission has been measured during 300 keV He+ implantation from 400°C to 1200°C in V and Mo between −170°C and 700°C in 316 SS. H re-emission has been measured during 150 keV H+ implantation of Mo, V, and 316 SS between −90°C and 115 °C. In general the He re-emission is low until a critical dose is reached, at which time the re-emission abruptly rises to higher values. Both the critical dose and re-emission are strongly temperature dependent. The H re-emission generally rises smoothly to an equilibrium value which is both temperature and material dependent. Only at the lowest temperature does the H re-emission in Mo bear a qualitative resemblance to He re-emission.

Surface deformation in He and H implanted metals

Abstract The surface deformations produced by high-dose He and H implantations in Mo, 316 stainless steel, and V have been studied using scanning electron microscopy. It was found that He implantation produces surface deformation in all three materials at temperatures ranging from −170°C to 1200°C. In contrast, H implantation produced blistering only in Mo and stainless steel implanted at low temperatures.

Low-energy helium implantation of aluminum

A series of 20 keV He/sup +/ implantations was conducted on well-annealed MARZ grade aluminum at fluxes of 6 x 10/sup 14/ and 6 x 10/sup 13/ He/sup +//cm/sup 2/ sec. Three distinct, temperature dependent He release mechanisms were found by He re-emission measurements during implantation, and by subsequent SEM and TEM investigations. At 0.08 of the melting temperature (T/sub m/) gas re-emission rose smoothly after a critical dose of 3 x 10/sup 17/ He/sup +//cm/sup 2/, with extensive blistering. The intermediate temperature range (approximately 0.3 T/sub m/) was characterized by repeated flake exfoliation and bursts of He after a dose of 3 x 10/sup 17/ He/sup +//cm/sup 2/. Rapid He evolution, with hole formation was found above 0.7 T/sub m/. No significant differences in either gas re-emission or surface deformation were found between the two fluxes employed.

Helium implantation effects in SAP and aluminum

Abstract A series of 300 keV He implantations of Al and SAP 930 have been conducted at temperatures between 150 and 773 K. The He re-emission was monitored during implantation and the samples were examined with a scanning electron microscope after implantation. Both Al and SAP 930 were found to blister after a critical He dose was reached at temperatures above 473 K, both underwent flaking below that temperature, with blistering re-appearing in SAP 930 at an implantation temperature of 150 K. The surface deformation and He re-emission are strongly dependent on microstructural effects in the intermediate temperature regime.

Impurity deposition on ALT-I limiter heads☆

Abstract Detailed spatial profiles of impurities deposited on the surface of the ALT-I pumped limiter heads used in TEXTOR have been measured using proton induced X-ray emission in the external ion beam analysis system. The TiC coated, ATJ graphite ALT-I limiter head was analyzed after each of two plasma exposure periods. Measurements were also made on the uncoated ATJ limiter head. For limiter operation at plasma radii ranging from 40 to 46 cm we find large accumulations of metal impurities on the ion and electron sides of the limiter. The composition implies that the source of these impurities is predominantly the stainless steel main limiters in TEXTOR. Surface concentrations of iron as high as 10 18 /cm 2 have been measured. On each of the limiter front faces a relative maximum in metal concentration occurs near the tangency point and relative minima are found near the toroidal extremities. An analysis of impurity transport in the edge region of TEXTOR using Redep is also presented. The calculations predict impurity atom migration patterns that strongly resemble the measured metal atom deposition profiles. The analysis provides considerable insight into impurity transport in the tokamak edge. Benchmark calculations of this type are valuable for the design of future limiter or divertor systems.

In-situ observations of ion implanted surfaces

Abstract A scanning electron microscope facility is described which enables direct observations of sample surfaces during implantation. This technique enables measurements of the growth rates of surface features caused by He implantation of metals. Results from 300 keV He + implantation of Ti samples indicate that exfoliation or flaking of the sample surface is a distinctly different process from blistering, and proceeds at radically different rates. In addition, it was observed that exfoliation could be significantly reduced by cold-working.

Microstructure of low energy deuterium implanted stainless steel

Transmission electron microscopy has been used to determine the defect structure produced in stainless steel by deuterium ions of energies from 0.3 to 5 keV. These energies are typical of neutral atoms incident on the first wall of current tokamak devices. At room temperature, high defect densities were found at our lowest dose of 1 x 10/sup 15/ D/sup +//cm/sup 2/. With increasing dose, the defect structures coarsened, and lattice strain effects in the implanted region became noticeable. Following low temperature implantation, no gas bubbles were observed for deuterium concentrations as high as approximately 10 atom percent.

Surface analysis of sputtered stainless steel

Abstract The behaviour of 316 stainless steel during sputtering by Ar and H ions in the temperature range 25–500°C was studied by in situ surface analysis using a scanning electron microscope and Auger electron spectroscopy. The main differences between Ar and H sputtering are in the sputtering rates and in the relative amounts of surface impurities. Similar development of surface topography occurred during Ar and H sputtering. A lower limit to the H dose required to sputter through the impurity layers on stainless steel was obtained.

Geometry effects on particle removal with the pump limiter ALT-I

Abstract In the course of the ALT-I pump limiter experiments on TEXTOR, limiter heads of different shapes, exhaust channels and materials have been applied and their performance has been compared. Particle removal rates up to 10 Torr l s have been observed in the case of TiC surfaces. Values with the graphite head at the same radial position are typically a factor of 2–3 lower. The removal rate for a “detached plasma” discharge is 30–50% reduced as compared to a normal one. The removal efficiency reaches values in excess of 50%. The results suggest a guiding rule for the design of pump limiters with high removal efficiency: The effective pumping speed at the neutralizer plate should be large compared to the conductance for neutral particles backstreaming from the neutralizer plate to the main plasma.

The effects of gas transport properties on blister formation in He+-implanted glass

Abstract Blister formation in He + -implanted glasses is correlated with the measured helium gas diffusivity. A scries of glasses with diffusivities from ∼3 × 10 −7 to ∼5 × 10 −12 cm 2 sec −1 was implanted under nearly identical conditions with 150 keV He + ions at a flux of 15 μA cm −2 and a nominal sample temperature of 110°C. Glasses with D less than ∼1 × 10 −9 cm 2 sec −1 were fully blistered, whereas those with D greater than ∼3 × 10 −8 cm 2 sec −1 showed no surface deformation. Glasses with diffusivities between ∼3 × 10 −8 and ∼1 × 10 −9 cm 2 sec −1 had local regions with low density coverage of relatively large blisters. The critical concentration of implanted helium, estimated by comparing experimental data with results from a simple theoretical model, is ∼1 × 10 19 cm −3 , consistent with high pressure solubility measurements. Reemission data at low fluence are qualitatively in agreement with analytical calculations. Implications for CTR technology are discussed.