AbstractThe monitoring method of discharge cleaning which was based on microwave spectroscopy was developed in order to apply it to fusion devices and availability of this method was experimentally investigated. Water molecules are one of the major residual molecules in a vacuum vessel, and hence the partial pressure of water is a good index of progress in conditioning. Temporal changes in the partial pressure were measured by using this method during discharge cleaning consequently. Three subjects were studied with a Stark modulated microwave spectrometer, namely, proportionality between the spectrum intensity and the partial pressure, resolving power of the spectrometer for isotopic water molecules and applicability of this method to the measurement of radioactive water molecules. Rotational spectra of the light water H2O (22.235 GHz) and the hydrogen isotopic waters HDO (22.307 GHz) and HTO (16.563 GHz) were observed in several plasma devices for these purposes.