Electromagnetic Properties of a New Microwave Plasma Torch with Double Resonance Configuration

In order to obtain a stable plasma and improve the performance of the torch for atomic emission spectroscopy(AES), the structure of microwave plasma torch(MPT) was analyzed. The transmission and distribution characteristics of the electromagnetic field of the torch configuration with two or three concentric tubes, as well as the metal spacer between inner and intermediate tubes with different depths were simulated with electromagnetic simulation software and verified by experiments. The results indicate that the inner tube of MPT plays an important role in strengthening the electric field intensity at the opening end of the MPT and redistributing the electromagnetic field in the whole torch by forming a double resonance configuration, and contributes to enhancing the macroscopic stability and the self-sustainment of the plasma. The stability of the plasma is proved to be excellent when the metal spacer between the inner and intermediate tubes is located at a place 20—30 mm away from the top opening of the torch. A proper location of the spacer can also avoid the formation of a static filament plasma or a rotating plasma rooted from the outer wall of the inner tube. With the help of morphological analysis, the underlying reason why MPT possesses a great tolerance to wet aerosols and air introduction was clearly made, that is, the formation region of the plasma formed with MPT is apparently separated from the reaction zone of it.