Effective Decomposition of Water Vapor in Radio-Frequency Plasma with Carbon Deposition on Vessel Wall

In a thermonuclear fusion reactor, a fuel cycle system that recovers and reuses unburnt hydrogen isotopes is indispensable. Oxygen (O) and carbon (C) impurities are chemically combined with the unburnt hydrogen isotopes and exhausted from the plasma vessel of the fusion reactor. The impurities must be decomposed in the fuel cycle system to recover the hydrogen isotopes, especially tritium. In this study, a flow-type reactor using a radio-frequency (RF) plasma was applied to decompose water vapor (H2O) molecules. The effect of C deposition on the vessel wall (stainless steel) was confirmed experimentally to promote reactions relating to the decomposition. At RF powers of 30 - 150 W, the decomposition ratio was around 30% with an argon gas mixed with 5% H2O at the pressure of 100 Pa (5 Pa for H2O) and the flow rate of 20 sccm (1 sccm for H2O). The decomposition was enhanced by C depositions on the vessel wall; the decomposition ratio was largely increased to be 75% at the RF power of 150 W. Reasons for the increased ratio may be a reduction of O atoms and molecules and a production of carbon monoxide through the interaction with C depositions. The reduction of O products promoted the decomposition of H2O in the plasma because the recombination of O and H can be suppressed.