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An open-air microwave plasma is generated in the resonant cavity based on parallel multislot rectangular waveguides by using an argon pre-ionization plasma jet device. The side photos show that the plasma separates from the argon jet and gradually shifts to the locations with high electric-field strength when the microwave power increases. In the absence of the plasma jet, the early plasma structure is relatively similar to the calculated electric field distribution. As the time goes on, the plasma connects the two waveguides. Small portions of plasma always move up and down between the waveguides. At the microwave power of 5.6 kW, the plasma size is evaluated to be (110-130) mm × (407-50) mm × 87 mm, which is significantly large. As the microwave power increases from 5.6 kW to 8.0 kW, the plasma electron temperature increases from 4400 K to 7600 K. The average electron density along its length is measured by using the Mach-Zehnder interferometer. The electron density is relatively high in the center and two ends near the slots of the plasma. The electron density is as high as 1017 cm-3, and it gradually decreases from 3.03 ×1017 cm-3 in its center to zero at its edge across the width of the plasma. The open-air plasma excited at 6.5 kW is close to local thermodynamic equilibrium.