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

Abstract

Chemical erosion of carbon materials and dust formation in low-temperature and neutral particle-dominated plasmas were investigated using high-pressure inductively coupled plasmas. Experiments were performed with Ar/H 2 /N 2 mixture plasma irradiation to graphite targets. The addition of just a few percent of nitrogen gas to hydrogen led to significant suppression of carbon dust formation on the graphite target. From optical emission spectroscopy, CN band spectra were observed strongly in Ar/H 2 /N 2 plasmas with a decrease of CH and C 2 band emission intensity. These results showed that CN bond formation, which caused chemical erosion of carbon by producing volatile CN, HCN, and C 2 N 2 particles, might have been a key suppression mechanism of the carbon particle aggregation. Carbon materials are used for the plasma-facing components (PFC) in fusion devices because of their superior thermomechanical properties. However, dust particles are formed by plasma-surface interactions in fusion experimental devices [1][2][3][4][5]. Because carbon dust retains large amounts of hydrogen isotopes, the dust particles in fusion reactors cause safety problems mainly concerning the tritium inventory. 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]. Carbon dust formation in addition to tritium removal efficiency should be studied in nitrogen containing plasmas.Experiments have been performed in Ar/H 2 /N 2 mixture plasma irradiation to an isotropic graphite target (IG-430U, Toyo Tanso Co. Ltd.) at a surface temperature of ∼1020 K. The argon gas flow rate is 101 Pa m 3 /s, and the flow rates of hydrogen and nitrogen gas into the argon plasma are 3.4-5.1 Pa m 3 /s and 0-0.51 Pa m 3 /s, respectively. The working gas pressure is ∼4 kPa. The electron temperature is ∼1 eV. vertor plasma conditions. The irradiation time is set to 180 minutes. The surface temperature is measured through a quartz window with a radiation thermometer. A Scanning Electron Microscope (SEM) was used to observe the generated dust particles on the targets. 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 hydr...

Removal of carbon deposited film and hydrogen retention control by low temperature H–C–N reactive plasmas

“…Nitrogen injection into hydrogen plasmas has been examined as effective methods for suppression of carbon dust growth [1]. In the experiments which are performed with Ar/H 2 /N 2 plasma irradiation to graphite targets, using high-power inductively coupled plasmas (ICPs), we obtained the results that just a few percent of nitrogen addition (N 2 /H 2~2 %) into Ar/H 2 plasmas led to significant suppression of agglomeration of carbon particles to form large size carbon dust particles in the number of carbon dust particles [2]. In this experiment it is considered that volatile C-N bond formation may play an important role to restrain cohesion of carbon particles.…”