W. Bauer

Surface processes in plasma wall interactions

Abstract Plasma surface interactions play a critical role in all tokamak experiments. Phenomena such as impurity influx and radiation, H isotope recycling and first wall modification have become widely recognised in the past year of tokamak operation. It is now generally recognised that impurities are introduced through sputtering, arcing desorption and other mechanisms. In future DT devices, helium blistering may assume an important role. Examples and the relative importance of these various surface processes are discussed. Another critical area for tokamak operation involves hydrogen isotope trapping at the first wall. Recent experiments on several tokamaks indicate that trapping and subsequent re-emission of hydrogen isotopes on walls during and between discharges play an important role in fuel balance, performance and eventually in the tritium inventory. Existing data and understanding of H/D trapping and re-emission phenomena are reviewed.

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.

Deuterium permeation through 309S stainless steel with thin, characterized oxides☆

Abstract Results from deuterium permeation through 309S stainless steel have been combined with oxide growth and characterization information to establish the effects of surface oxides on permeation. Permeation results for 0.25 mm thick diaphragms were obtained using an all-metal, ion-pumped vacuum system and a quadrupole mass spectrometer. The temperature range covered was 250 to 450°C with an upstream deuterium pressure of 100 torr. Analysis by sputter-through Auger electron spectroscopy (AFS) revealed that some oxides were reduced or altered by the permeation process. The study included oxide thicknessess of 100 to 2500 A grown by various procedures. Other characterization techniques employed were proton-induced X-ray spectroscopy (PIX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The oxides investigated were found to have only a minor effect on the measured permeabilities.

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.

Erosion and redeposition experiments in the PISCES facility

The modification of surfaces during exposure to plasma bombardment is a critical issue in the development of limiter and wall materials for fusion confinement experiments. Controlled studies of the erosion and redeposition of materials during high flux and fluence plasma exposure are now possible in the PISCES facility. PISCES is a continuously operating plasma device which has achieved hydrogen plasma densities of over 1013 cm−3 and electron temperatures of 5 to 24 eV over large areas. Ion fluxes of 1017 to 1019 cm−2 s−1 and fluences of up to 1023 cm−2 have been used to bombard biased samples inserted into th plasma. The plasma parameters can be selected to produce simple sputtering, or redeposition by the ionization and recycling of the sputtered target materials. Collaborative studies on the performance of Cu and Cu-Li alloys (with ANL), stainless steel (with SNLL), and graphite (with IPP at Garching, and SNLL) have been undertaken. Surface topography modification is always observed after a sufficient fluence is achieved. The net erosion rate is significantly lower during redeposition than one would expect from classical sputtering yields. The transport and deposition of different materials by the plasma to the samples during redeposition conditions results in greatly modified surface composition and morphology. Chemical sputtering of graphite during low energy, high flux (>1018 cm−2 s−1) plasma bombardment is observed. Chemically formed hydrocarbons are relatively easily redeposited compared to sputtered carbon. The performance of these materials, the surface morphology evolution, and the characteristics of the redeposited materials are discussed.

Alpha transport and blistering in tokamaks

The particle flux and angular distribution of 3.5 MeV alpha particles impinging on the first wall from uncontained banana orbits in an axisymmetric tokamak reactor have been calculated. The resulting helium concentration profiles in the first wall can give rise to surface exfoliation under specified conditions. The major mitigating factor is the simultaneous surface recession due to sputtering by the D-T charge-exchange neutral flux. For the parameters used in these calculations blistering in high-sputtering-rate materials such as beryllium is unlikely, whereas in low-sputtering-rate materials such as niobium helium-induced surface deformation is quite probable.

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.

He pumping in the presence of an H beam

Abstract In a tokamak, one of the major questions concerning the applicability of passive pumping of helium ions accelerated across the plasma sheath involves the effect on helium retention of the concurrent bombardment of the surface with D/T ions. We have investigated helium trapping and critical fluence behavior during simultaneous 2 keV hydrogen and 4 keV helium bombardment in nickel and beryllium. In nickel, the helium trapping behavior appears unaffected by simultaneous H bombardment, reaching saturation trapping levels of ~1.0 × 10 17 helium cm −2 . In contrast, similar measurements on beryllium indicate that the simultaneous hydrogen bombardment significantly reduces He trapping.

Deuterium profiles in titanium and alloys

Abstract Deuterium concentration profiles in Ti, T1-6A1, and Ti-6A1-4V have been investigated after 10 keV D3+ implantation at room temperature. Profiles were determined using the D(3He,α)p nuclear reaction and also by secondary ion mass spectrometry (SIMS). The amount of retained D in the near-surface region (~.5 μm) was monitored as a function of time and was found to be strongly microstructure dependent. Annealed Ti retained the most D while the Ti-6A1-4V alloy retained the least D in this region. The D remained in the bulk of the sample during implant and subsequent aging at room temperature.