Investigation of Deuterium Plasma Property Variation by Carbon Impurity from Graphite

Abstract

Spatially and temporally varying deuterium plasma properties caused by carbon impurity influx are investigated on the basis of morphological change of graphite that has strong influences on the formation of carbon impurities. The present study aims to provide a new perspective on interpreting plasma-surface interactions by considering the surface morphological change. It is revealed that the consideration of the variations of both the plasma property and the graphite morphology is necessary to understand the deuterium plasma–graphite surface interaction. Electron cyclotron resonance (ECR) plasma was used to graphite target as the main interaction area and to accelerate ions with vertical incidence on the target. Experiments were carried out with a plasma density range of 1–3.5 × 10^(11) cm^(-3), an electron temperature range of 3.5–5.5 eV, and an ion energy range of 17–100 eV, which are experimental conditions similar for the KSTAR scrape-off layer (SOL) plasma. The morphological change of the graphite caused by deuterium plasma irradiation was analyzed according to the energy dose, which is the product of the energy and dose of ions, and therefore the energy dose indicates the total energy transferred on graphite per unit area. The energy dose determines the degree of the morphological change of the graphite as the energy needed for motion of carbon atoms causing the morphological change is proportional to the energy dose. Moreover, energetic deuterium ion actively reacts and bonds with the carbon, resulting in the C-D bond formation on the graphite surface. Accordingly, the energy dose was applied up to the KSTAR SOL plasma steady-state condition to analyze progress of the morphological change of the graphite. As a result, the graphite morphology changed from a plane surface to a conical tip as the energy dose is increased. Further increases in the energy dose enlarged the size and the aspect ratio of the conical tip. The increased sp^(3) character on graphite by the ion irradiation is turned out to induce conical tip formation on the graphite surface by containing a small amount of diamond-like carbon, which has a slightly higher displacement threshold energy than graphitic atoms. The formation of the cone-shaped carbon bundle owing to the gathering of small conical tips occurs when the sheath electric field is stronger than 3.8 × 10^(5) V/m. Therefore, plasma physical properties such as the energy dose, sheath electric field, and plasma chemical property of C-D bond formation play crucial roles in cone-shaped carbon bundle formation. The results indicate that the interpretation of the plasma-based physics and chemistry are necessary to analyze the morphological change of graphite. A sputtering yield model for the conical tip is established. Considering that the morphological change of the graphite surface entails the increment in the local angle of ion incidence and the additional collisions of backscattered ions, the Roths model for a plane surface has been modified. The newly established sputtering yield model reflects analytical anticipation that the morphological change of graphite caused by the energy of the ions increases the physical and chemical sputtering yields compared to those for a plane surface. The measured sputtering yields for the morphologically changed graphite surface indeed turned out to be two times larger than those estimated using Roths model for a plane surface owing to the morphological change of graphite. The results indicate that the interpretation of the plasma physics is necessary to analyze both the morphological change of graphite and the carbon impurity formation. The inflow of deuterated carbon decreases the electron temperature and increases the electron density in space, as the deuterated carbon is more likely to be dissociated or ionized as a result of collisions with electrons than those for deuterium. The spatial variation of deuterium plasma properties was experimentally analyzed by based on a global model that calculates the spatially averaged plasma properties. It is found that increase in the sputtering yield slightly raises a few % of the ratio of deuterated carbon to deuterium plasma, and this slight increment induces the variation of the deuterium plasma properties. The results indicate that the interpretation of the interaction between the carbon impurity and the deuterium plasma is necessary to analyze the deuterium plasma properties. The inflow of deuterated carbon decreases the electron temperature and increases the electron density with time, because the morphological change of graphite with the operation time increases the sputtering yield. Increases in the ion energy and ion flux, i.e., an increase in the energy dose, cause a severe morphological change of graphite, resulting in large variations of the deuterium plasma properties. In addition, the increased ion flux due to the carbon impurity influx into the deuterium plasma causes additional collisions of backscattered ions, resulting in the re-increment of the sputtering yield. Therefore, it is discovered that the variation of the deuterium plasma properties caused by the carbon impurity influx can accelerate graphite wall erosion. In this study, plasma and graphite are analyzed simultaneously in order to examine the interaction between the deuterium plasma and the graphite surface. The morphological change of graphite, in particular, is found to be a major factor to be considered in deuterium plasma–graphite surface interaction. The morphological change of the graphite caused by deuterium plasma irradiation increases the carbon impurity influx into the deuterium plasma, resulting in large variations of the deuterium plasma properties. New discoveries described in this study will greatly contribute to understanding of the KSTAR plasma, which uses deuterium plasma with graphite, and the plasma processing for carbon and hydrogen gases. Moreover, revealed that the variation of the plasma properties according to the operation time can be caused by the by-products, this study is believed to provide important basis for the estimation and control of plasma properties in the related fields of study.