Effect of Transverse DC Magnetic Field on the Impedance Matching and Morphology of a Microwave Atmospheric Pressure Plasma Jet

Impedance matching plays a vital role in a microwave atmospheric pressure plasma jet (MW-APPJ). However, real-time tuning of MW-APPJ impedance matching has rarely been investigated. In this study, we have found that the dissipated microwave power and length of a MW-APPJ are effected by the application of a DC-bias magnetic field. By placing a MW-APPJ in a gap between two electromagnets, the consumed microwave power varied with the insertion position of the MW-APPJ. When the MW-APPJ was fixed at the center of the electromagnets, the consumed microwave power decreased as the DC magnetic field intensities increased, from 12 to 750 mT. The variation of consumed microwave power was much higher at lower gas velocities, which indicates that the DC magnetic field has a relatively strong effect on microwave plasma with lower gas velocity. The length of the plasma jet also decreased with increasing DC magnetic field, which was mainly due to the variation of the consumed microwave power. To explain these interesting phenomena, the effective permittivity of microwave plasma and the equivalent circuit model of a generator under DC-bias magnetic field have been analyzed. The effective permittivity of the plasma is related to its cyclotron frequency, which is a function of the DC-bias magnetic field. The variation of the effective permittivity changes the load impedance of the plasma generator. Therefore, the reflection coefficient of the plasma generator is adjusted and the length of the plasma jet varies with the change in consumed microwave power.