The aim of this paper is to determine the atomic oxygen surface loss probability on silica under microwave plasma conditions around 133 Pa (1 Torr). A pulsed induced fluorescence technique where a main long pulse creates the plasma and a shorter one re-excites atoms in the time post-discharge was used. The method and its validity under the present experimental conditions are discussed at large. The oxygen surface loss probability on silica is found to be around 3% under plasma conditions, while it is estimated to be two orders of magnitude lower for a surface not submitted to the plasma.

We present a modelling study of pulsed H 2 /CH 4 microwave plasmas obtained under moderate pressure discharge conditions in a tubular quartz reactor. The transport in the reactor was described using a Nusselt model for a radially quasi-homogeneous plasma. The thermal behaviour of the plasma was modelled by distinguishing a single heavy species energy mode and the electron translation mode. The chemistry was described using a 30 species-130 reaction model. The time variations of the electron energy distribution function, the species concentrations and the gas temperature were determined by solving the coupled set of the electron Boltzmann equation, species kinetics equations and a total energy equation. Some of the results obtained from the present model were compared to measurements previously carried out on the plasmas considered. Good agreement was obtained for the time variations of the gas temperature, the relative concentration of the H-atom and the intensities of the H α and the argon 750 nm emission lines. The effect of the duty cycle on the time-averaged composition and temperatures of the discharge was also studied. Results showed that moderate pressure H 2 /CH 4 pulsed discharges obtained at duty cycles of less than 20% show different behaviour than those obtained at higher duty cycles. In particular, while the plasma reaches the permanent periodic regime in less than 2 pulse-periods, i.e. 60 ms, for duty cycle values of less than 20%, long-time-scale density variations of hydrocarbon species, ions and electrons are obtained when this parameter is greater than 20%. The model was also used to determine if the use of a pulsed regime may bring some improvements in plasma-assisted diamond deposition processes. For this purpose we analysed the variation with duty cycle of the time-averaged populations of the H-atom and CH 3 that represent the key species for diamond deposition. Results showed that pulsed discharges with small duty cycle, of typically less than 20%, lead to a substantial enhancement of the time-averaged dissociation yield. On the other hand, the CH 3 concentration exhibits a strong decrease with the duty cycle. The methyl concentration in the investigated pulsed discharge is generally smaller than in continuous wave discharges obtained in the same reactor. These results indicate that short-pulse discharges would favour the formation of films with higher Raman quality, while long duty cycle pulsed discharges would enable deposition at higher growth rates.

This paper presents an experimental study of an underwater pulsed plasma discharge in pin-to-pin electrode configuration. Time resolved refractive index-based techniques and electrical measurements have been performed in order to study the pre-breakdown and breakdown phenomena in water. A single high voltage pulse with amplitude of a dozen of kV and duration of [0.1-1] ms is applied between two 100 µm diameter platinum tips separated by 2 mm. This novel experimental work reports that different cases of electrical discharge in water occurs for a unique set of experimental conditions such as (i) bush-like channels from the cathode that do not span the electrode gap, (ii) bush-like channels from the cathode leading to breakdown and (iii) filamentary structures from the anode leading to a stronger breakdown. Two breakdown mechanisms, anode and cathode regimes, have been clearly identified and related to the two principal schools of thoughts to explain discharge propagation in liquid.