L. Heatherly

Tensile and Creep Properties of an Oxide Dispersion-Strengthened Ferritic Steel

The tensile and creep properties of two oxide dispersion-strengthened (ODS) steels with nominal compositions of Fe–12Cr–0.25Y2O3 (designated 12Y1) and Fe–12Cr–2.5W–0.4Ti–0.25Y2O3 (12YWT) were investigated. Optical microscopy, transmission electron microscopy, and atom probe field ion microscopy studies indicated that the 12YWT contained a high density of extremely fine Y–Ti–O clusters, compared to the much larger oxide particles in the 12Y1. The fine dispersion of particles gave the 12YWT better tensile and creep properties compared to commercial ODS alloys and ferritic/martensitic steels that would be replaced by the new ODS steel.

Radiation-induced segregation in HT-9 martensitic steel

Abstract Miniature notched-bar specimens of normalized and tempered HT-9 were neutron irradiated to ∼13 dpa and broken at liquid nitrogen temperatures in a UHV chamber. Fracture surfaces were analyzed using scanning Auger electron spectroscopy. Following irradiation at 410°C, the fracture surface contained a small number of large relatively smooth facets, which are thought to be prior austenite grain boundaries. Strong segregation of Ni, Cr, Si, and P was detected at these surfaces, the remainder of the fracture surface showing no evidence of segregation. At irradiation temperatures of 520°C and 565°C, there was relatively little segregation and none was found in thermal controls.

Studies of hydrogen recycle from the walls in tokamaks using a plasma-wall interaction simulator☆

Abstract The recycle of hydrogen from walls and limiters plays a fundamental role in the operation of tokamaks for several reasons: (1) the average proton confinement time in the plasma is short with respect to plasma pulse length, therefore each proton is, on the average, recycled several times to and from the walls; (2) the walls may elastically or inelastically reflect the hydrogen but, according to our measurements, half or more of the hydrogen incident on the walls may be buried in the wall, thermalized, and most will return to the plasma volume on a time scale which is on the order of 10−1 s. We have studied details of the hydrogen recycle phenomena from 304L stainless steel using a plasma-wall interaction simulator. Low energy (30–200 eV) atomic and molecular hydrogen ions were used. The evolution rate from surfaces bombarded with a flux of 6 × 1015ions/cm2 · s has been measured for several conditions and appears to be limited by complex diffusion phenomena and perhaps, in some cases, by surface recombination rates. The evolution rate reveals effects due to diffusion from the highly supercharged near surface region. Several mechanisms were discussed but it is not possible to identify which are responsible for the prompt desorption processes.

Surface impurities and “clean-up” techniques in ORMAK

Abstract The detrimental role played by impurities in thermonuclear devices is now well-known. Experimental studies to identify those impurities residing on the liner, or first wall, of the Oak Ridge Tokamak (ORMAK) have been carried out in the laboratory using Auger Electron (AES) and X-ray Photoelectron (XPS) spectroscopic techniques. Additionally, liner measurements have been made in situ using a small Soft X-ray Appearance Potential Spectrometer (SXAPS). Oxygen, iron and carbon were found to be the major surface impurities and, as confirmed by plasma diagnostics, also the major plasma impurities. Glow discharge cleaning of gold and stainless steel surfaces has been studied using various gases and gas mixtures. Oxygen discharges are very effective and hydrogen moderately effective in removing carbon and hydrocarbon deposits from both types of surfaces. Other parameters involved in the contamination-decontamination process, such as pressure and temperature, have been studied using techniques to controllably contaminate surfaces with hydrocarbons.

Observations of arcing in the ISX tokamak

Abstract Arcing has been proposed as a major source of metal impurities in tokamak plasmas. Arc tracks have been observed in the ISX tokamak on the limiter, the inner-wall surface, and on the samples from the surface analysis station. Linear as well as fern-like arc tracks have been observed. From optical and SEM analysis of the tracks, it was estimated that about 1016–1017 atoms were released per arc. To study the influence of arcing on the tokamak discharge, an experiment was set up to measure electrical and optical signals of arcing in situ. In well controlled tokamak discharges, arcing was observed only during the initial breakdown of the plasma and during the quenching phase at the end of the discharge. In disrupted discharges, each plasma disruption was accompanied by arcing. The pulse-length of one single unipolar arc was measured to be about 50 μs and the current amplitude was typically about 20 A.

Surface impurities studies during the start-up and early operations of ISX-A using a surface analysis station with sample transfer

Abstract A surface analysis station was installed on ISX-A before start-up which provided data on samples of wall material and a variety of other materials exposed at positions between the first wall and the limiter inner radius. Samples were introduced, from air, into an ultra-high vacuum transport system without disturbing the vacuum in ISX-A or the analysis station. An Auger electron spectrometer in the analysis station along with other diagnostics provided information on the initial “clean up” as well as the subsequent routine operation of ISX-A. During the start-up period substantial sulfur contamination was evident but this was greatly reduced as hydrogen discharge cleaning proceeded. Samples of oxidized stainless steel were not cleaned to an oxygen-free status but the surface was reduced to a thin substoichiometric oxide. Carbon was significantly removed but not eliminated entirely. Routine tokamak operations were monitored by a means of a special composite sample which made possible estimates of both the state of cleanliness of the system and its rate of clean-up.

Hydrogen recycle and isotope exchange from dense carbon films

Dense carbon films were prepared by deposition from hydrogen plasmas to which methane was added. The initial hydrogen recycle coefficient from the films ranges from more than two to less than one. The films contain large amounts of hydrogen (up to 50 at. %). They adjust themselves to provide recycling coefficients near unity. Isotope changeover times tend to be long. The reservoir of hydrogen instantly available to the plasma to maintain or stabilize the recycle coefficient and isotopic composition of the plasma is 10/sup 15/ cm/sup -2/ or greater depending on film preparation, temperature, and prior plasma exposure conditions. Simulator observations tend to support and improve the understanding of the observations in TEXTOR and JET; however, they also point out the need for control of film deposition and operating parameters to provide desirable and reproducible properties. The films and the hydrogen isotopes they contain can be removed easily by plasma processes. Since the hydrogen in these films is relatively immobile except in the zone reached by energetic particles, or at temperatures above 400/sup 0/C, dense carbon films may be useful in managing the tritium recovery from near-term fusion experiments.

Surface composition changes of inconel 625 during RG and ECR discharge cleaning of textor at 300°C☆

Abstract Two surface analysis stations equipped for Auger electron spectroscopy were used to monitor changes in surface composition during cleanup experiments in TEXTOR covering a period of about 300 h. Two forms of discharge cleaning combined with baking were very effective in cleaning the interior surfaces of the TEXTOR tokamak. The radiofrequency-assisted glow (RG) discharge causes energetic hydrogen ion bombardment of the surfaces while the electron cyclotron resonance (ECR) plasma exposed the surface to low energy atomic hydrogen. The tokamak vacuum vessel (304N) and the inner liner (Inconel 625) were heated to 300°C. Oxygen levels below 5 at. % (as measured by Auger electron spectroscopy) were obtained. The possible role of site competition in determining surface composition is discussed.

Arcing studies in ISX-B

Abstract Results of experimental measurements of arcing in ISX-B are described. Samples of stainless steel were exposed at the plasma edge and arcing currents to the sample were measured. Currents from several amperes up to 100 A were observed. It was possible to trigger arcs during well-behaved discharges by applying a high voltage to the sample. Elemental analysis of surface features of samples has shown that sulfur segregates to surfaces of arc craters in type 304 stainless steel. Detection of sulfur on surface samples may be related to arcing during cleanup processes in tokamaks.

Studies on microarcing

Introduction of impurities into plasma discharges by unipolar arcs has been observed in various plasma experiments. However, the mechanisms which lead to microarcing are not well understood. To study the conditions under which micro-arcing occurs, as well as the resulting erosion rates, a small scale experiment has been initiated. Electrodes are immersed in a low-temperature RF-plasma. A capacitor is connected between them and biased by an external voltage supply. Currents between 10 and 200 A with pulse duration between 10 and 20 μs are measured when arcs strike across the space charge sheath between the negative electrode and the plasma. These arcs produce craters on the negative electrode with diameters between 20 and 50 μm. Relating the removed material to the integrated current of the arcs yields erosion rates of about 10−8 kg/C for 304 stainless steel. The present results suggest that arcing on the first wall of tokamaks is a transient phenomenon which may be controlled by proper surface conditioning.