High-speed photographic analysis of microwave plasma torch source behaviourElectronic supplementary information (ESI) available. See DOI: 10.1039/c5ja00483g

Spatiotemporal motion characteristics of the microwave plasma torch (MPT) source were for the first time investigated with a high-speed camera. It was found that the cone shape with a central channel of the Ar MPT, which is beneficial for the introduction of the sample analyzed, is formed by the regular and high-speed rotary motion of the discharge filament(s), and it is also the origin of the unique performance compared with other microwave plasma sources. The rotational frequency of the plasma filament is typically 100 Hz, which is affected by the flow-rates and microwave forward power level. To investigate their relationship, the parametric analysis was used, suggesting that the support gas flow-rate is the major factor, followed by microwave power and the carrier gas flow rate. It was also illustrated that the structural symmetry of the torch is important for the stability of the excitation source. In addition, the excitation process of MgCl 2 aerosol in the MPT was preliminary studied on the microscopic time scale. Knowledge of this spatiotemporal behaviour is relevant to studies of the fundamental properties of the MPT and its applications. Spatiotemporal motion characteristics of the microwave plasma torch (MPT) source were for the first time investigated with a high-speed camera.