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A novel method for measuring the number density of water molecules in low-temperature plasma is developed. The absolute intensities of rotational lines of the (0,0) band of the OH(A 2Σ–X 2П) transition are used. Lines with sufficiently large rotational quantum numbers referring to the so-called “hot” group of molecules produced by electron-impact dissociative excitation of water molecules are chosen for measurements. The excitation rate of a process with a known cross section is determined by measuring the parameters of plasma electrons by means of the probe method. The measured number densities of molecules are compared with those in the initial plasma-forming mixture. The time evolution of the particle densities in plasma is investigated. The problems of the sensitivity and applicability of the absolute spectral method are considered.

The validation of spectroscopic method for localization of the water plume around a coolant leak in ITER plasma chamber based on the excitation of coolant particles in plasma near wall is presented. The sensitivity of OH hydroxyl and Xe as minor admixtures to cooling water detection in different types of plasma discharges relevant to ITER tested in stellarator L-2M and hollow cathode glow discharge. The limit of sensitivity of water detection in hot plasma using OH* hydroxyl band is estimated as 3 × 10 −5 Pa m 3 s −1 and ∼1 × 10 −6 Pa m 3 s −1 for Xe. For glow discharge conditions measured sensitivity of OH* detection is equal to 3 × 10 −5 Pa m 3 s −1 .