Carbon Erosion and Dust Formation under Heavy Atomic Hydrogen Irradiation

Abstract

Experiments on erosion and dust formation on graphite materials have been performed using high power induction plasmas containing high atomic hydrogen flux (∼10 24 m −2 s −1 ). Chemical sputtering by atomic hydrogen irradiation with an incident energy below 1 eV eroded the graphite targets significantly, and the sputtering yield was roughly estimated to be 0.002-0.005, which is as high as that obtained by ion beam experiments. The transport of the released hydrocarbon along the gas flow results in carbon dust formation on the eroded graphite target and also on the silicon and graphite targets located at the remote position. The dust structure strongly depends on the target surface temperature, and the graphite dust turns into diamond crystals when the surface temperature rises to 1100 K. Carbon materials, such as isotropic graphite, and carbon fiber reinforced composites (CFC) are superior plasma facing components, which are used in a fusion reactor because of their high thermal conductivity and tensile strength. Divertor graphite tiles, however, are eroded significantly by irradiation with high particle and heat flux divertor plasmas. Sputtering erosion and dust formation of carbon materials have significant effects on reactor performance, such as tritium retention, impurity release, degradation of vacuum sealing, and electrical isolation, etc. So far several methods to reduce the divertor plasma heat load have been tested and it is recognized that the detached divertor plasma operation is the most probable one in the fusion reactor. Although the detached divertor works well for heat load reduction, the graphite erosion by irradiation of low energy hydrogen ions and atoms in the detached divertor plasma is not yet understood.In this paper, high power inductively coupled plasmas (ICPs) [1] with a power level of ∼20 kW are used to study plasma surface interactions and dust formation mechanism. High power ICPs have characteristic features, such as a high particle flux (ion flux: 10 20 -10 21 m −2 s −1 , atomic hydrogen flux: 10 23 -10 24 m −2 s −1 ), high heat flux (∼1 MW/m 2 ), and low temperature (∼ 1 eV) under experimental conditions, which include an input power of 15 kW, argon gas flow rate of 60 slpm and hydrogen gas flow rate of 2 slpm. High power ICPs are the irradiation source for both low energy and dominant atomic hydrogen. Although the working gas pressure is high (P ∼ 5 kPa), these features author's e-mail: 1027take@ee.t.kanazawa-u.ac.jp are very helpful in studying the fundamental mechanism of carbon erosion and dust formation in detached plasmas.Experimental results of argon-hydrogen mixture (Ar: 60 slpm, H 2 : 0, 2, 8 slpm) plasma irradiation onto graphite targets are reported. Figure 1 shows a schematic diagram of a plasma irradiation system. One graphite target (IG-430U fabricated by TOYO Tanso Co.) is located under the plasma torch and five other graphite or silicon crystal targets, with diameters of 15 mm, are placed at different positions to determine how the irradiation condition affects grap...

Suppression of Carbon Dust Formation by Nitrogen Injection into Hydrogen Plasmas in Detached Plasma Conditions

“…The suppression of dust formation is an important issue in future fusion reactors. In this study, we report on experiments conducted to investigate the influence of nitrogen injection into argon/hydrogen plasmas on carbon dust formation by using high-power inductively coupled plasmas (ICP) [6,7]. Nitrogen injection has been considered and tested as one of the methods for tritium and co-deposits removal in carbon PFC [8,9].…”

“…The dominant erosion process of the graphite target is due to chemical sputtering by low-energy hydrogen atoms under Ar/H 2 plasma irradiation. Many carbon dust particles are observed on the graphite target eroded by chemical erosion [7]. Figure 1 shows the number density and size of the carbon dust particles, target weight loss, and optical emission intensity of CH (431.4 nm), C 2 (516.5 nm), CN (388.3 nm), and NH (336 nm) band spectra normalized to Ar I (750.4 nm) emission as a function of N 2 injection ratio into hydrogen, where the N 2 injection ratio is defined as the nitrogen gas flow rate normalized by the sum of nitrogen and hydrogen flow rates.…”

“…Figure 2 shows SEM photographs of carbon dust observed on graphite surfaces irradiated by Ar/H 2 /N 2 plasmas at different values of N 2 content. In the Ar/H 2 plasma, most of the carbon dust particles have a spherical shape at a surface temperature of ∼1020 K [7]. With increasing nitrogen gas injection ratio, the carbon dust shape changes into polyhedral particles at an N 2 content of 0.3-0.7% and clusters made of smaller particles at an N 2 content > ∼2%.…”