Predicting power-voltage characteristics and mode transitions in the COST reference microplasma jet

A 2D cross-field plasma fluid model (CFPM) is applied to He and He/O-2 discharges in the COoperation in Science and Technology (COST) reference microplasma jet to investigate the operating modes, namely alpha-, alpha-gamma, and gamma-mode. The model not only captures the measured spatiotemporal behavior of He excitation to He metastable but also quantitatively predicts measured power-voltage (PV) characteristics for He/O-2 discharges; although not addressed by previous studies, this is a prerequisite for the reliability of the model predictions for the critical-for-applications densities of reactive species. Through a comparison to time-averaged emission profiles and allowed by the dimensionality of the CFPM, the localized, close to the outlet of the discharge channel, onset of gamma-mode for He discharges is predicted and justified. The sheath boundary is defined by the maximum of the electron density derivative and the model results compare well to measurements of time-averaged sheath width. Criteria for the transition between the operating modes are formulated. It is considered that when the production rate of He metastable in the sheaths reaches 10% of its total production rate, transition from alpha- to alpha-gamma mode takes place. When this percentage reaches similar to 50%, i.e. alpha- and gamma- modes have an almost equal contribution to the discharge, the electron temperature becomes maximum. Finally, the sensitivity of PV characteristics on the secondary electron emission coefficients, condition of the electrode surface, and fabrication or assembly mishits of the COST jet, is investigated.