Structure of the plasma jet of a pulsed microwave plasma torch of nozzle type

The structure of a microwave torch plasma jet was investigated by taking pictures of the jet with a digital camera and processing them next with the aid of software for astronomical imaging and data visualization. The commonly accepted idea of the structure of such a plasma jet is that the jet consists of two regions: a core and an envelope around it. It can be assumed that the core is that region where the microwave power dissipation occurs. Its extent depends on the intensity and geometry of the electric field, which is concentrated close to the nozzle tip. The remaining region of the jet is filled with recombining (afterglow) plasma, not held in position by the electric field, and forming an envelope around the core. The results reported here helped to clarify that picture. The plasma jet was produced in argon at atmospheric pressure by a pulsed 2.45 GHz microwave field applicator of nozzle type. The gas flow rate was equal to 3 l/min and the maximum peak power amounted to 1 000 W. Photographs taken with short exposure times showed that for a turbulent jet the core, rather than being immersed in the envelope, is surrounded by erratically wandering tongues of excited gas leaving the core region. Thus, the regular envelope does actually not exist as such, but is observed in long-exposure photographs or by the naked eye as a superposition of images of the excited gas tongues. The electron concentration in the core of the microwave torch plasma jet was estimated based on the core radius. The value obtained, 4×10²¹ m^-3, has the correct order of magnitude for the discharge under consideration.